Merge pull request #515 from dbryla/master

#502 Added logging framework to repository

.gitignore

@@ -15,5 +15,5 @@ datanucleus.log
 /bin/
 /bin/
 /bin/
-
+*.log
 data-mapper/src/main/resources/log4j.xml

abstract-document/src/main/java/com/iluwatar/abstractdocument/App.java

@@ -27,6 +27,8 @@ import com.iluwatar.abstractdocument.domain.HasModel;
 import com.iluwatar.abstractdocument.domain.HasParts;
 import com.iluwatar.abstractdocument.domain.HasPrice;
 import com.iluwatar.abstractdocument.domain.HasType;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.util.Arrays;
 import java.util.HashMap;
@@ -44,11 +46,13 @@ import java.util.Map;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Executes the App
    */
   public App() {
-    System.out.println("Constructing parts and car");
+    LOGGER.info("Constructing parts and car");
 
     Map<String, Object> carProperties = new HashMap<>();
     carProperties.put(HasModel.PROPERTY, "300SL");
@@ -68,12 +72,11 @@ public class App {
 
     Car car = new Car(carProperties);
 
-    System.out.println("Here is our car:");
+    LOGGER.info("Here is our car:");
-    System.out.println("-> model: " + car.getModel().get());
+    LOGGER.info("-> model: {}", car.getModel().get());
-    System.out.println("-> price: " + car.getPrice().get());
+    LOGGER.info("-> price: {}", car.getPrice().get());
-    System.out.println("-> parts: ");
+    LOGGER.info("-> parts: ");
-    car.getParts().forEach(p -> System.out
+    car.getParts().forEach(p -> LOGGER.info("\t{}/{}/{}", p.getType().get(), p.getModel().get(), p.getPrice().get()));
-        .println("\t" + p.getType().get() + "/" + p.getModel().get() + "/" + p.getPrice().get()));
   }
 
   /**

abstract-factory/src/main/java/com/iluwatar/abstractfactory/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.abstractfactory;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * The Abstract Factory pattern provides a way to encapsulate a group of individual factories that have a common theme
@@ -39,6 +42,8 @@ package com.iluwatar.abstractfactory;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   private King king;
   private Castle castle;
   private Army army;
@@ -98,17 +103,17 @@ public class App {
 
     App app = new App();
 
-    System.out.println("Elf Kingdom");
+    LOGGER.info("Elf Kingdom");
     app.createKingdom(new ElfKingdomFactory());
-    System.out.println(app.getArmy().getDescription());
+    LOGGER.info(app.getArmy().getDescription());
-    System.out.println(app.getCastle().getDescription());
+    LOGGER.info(app.getCastle().getDescription());
-    System.out.println(app.getKing().getDescription());
+    LOGGER.info(app.getKing().getDescription());
 
-    System.out.println("\nOrc Kingdom");
+    LOGGER.info("Orc Kingdom");
     app.createKingdom(new OrcKingdomFactory());
-    System.out.println(app.getArmy().getDescription());
+    LOGGER.info(app.getArmy().getDescription());
-    System.out.println(app.getCastle().getDescription());
+    LOGGER.info(app.getCastle().getDescription());
-    System.out.println(app.getKing().getDescription());
+    LOGGER.info(app.getKing().getDescription());
 
   }
 

adapter/src/main/java/com/iluwatar/adapter/BattleFishingBoat.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.adapter;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  *
  * Adapter class. Adapts the interface of the device ({@link FishingBoat}) into {@link BattleShip}
@@ -33,6 +36,8 @@ package com.iluwatar.adapter;
  */
 public class BattleFishingBoat implements BattleShip {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(BattleFishingBoat.class);
+
   private FishingBoat boat;
 
   public BattleFishingBoat() {
@@ -41,7 +46,7 @@ public class BattleFishingBoat implements BattleShip {
 
   @Override
   public void fire() {
-    System.out.println("fire!");
+    LOGGER.info("fire!");
   }
 
   @Override

adapter/src/main/java/com/iluwatar/adapter/FishingBoat.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.adapter;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  *
  * Device class (adaptee in the pattern). We want to reuse this class
@@ -29,12 +32,14 @@ package com.iluwatar.adapter;
  */
 public class FishingBoat {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(FishingBoat.class);
+
   public void sail() {
-    System.out.println("The Boat is moving to that place");
+    LOGGER.info("The Boat is moving to that place");
   }
 
   public void fish() {
-    System.out.println("fishing ...");
+    LOGGER.info("fishing ...");
   }
 
 }

aggregator-microservices/aggregator-service/src/main/java/com/iluwatar/aggregator/microservices/ProductInformationClientImpl.java

@@ -27,6 +27,8 @@ import org.apache.http.client.methods.HttpGet;
 import org.apache.http.impl.client.CloseableHttpClient;
 import org.apache.http.impl.client.HttpClients;
 import org.apache.http.util.EntityUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import org.springframework.stereotype.Component;
 
 import java.io.IOException;
@@ -37,6 +39,8 @@ import java.io.IOException;
 @Component
 public class ProductInformationClientImpl implements ProductInformationClient {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(ProductInformationClientImpl.class);
+
   @Override
   public String getProductTitle() {
     String response = null;
@@ -46,7 +50,7 @@ public class ProductInformationClientImpl implements ProductInformationClient {
         response = EntityUtils.toString(httpResponse.getEntity());
       }
     } catch (IOException e) {
-      e.printStackTrace();
+      LOGGER.error("Exception caught.", e);
     }
     return response;
   }

aggregator-microservices/aggregator-service/src/main/java/com/iluwatar/aggregator/microservices/ProductInventoryClientImpl.java

@@ -27,6 +27,8 @@ import org.apache.http.client.methods.HttpGet;
 import org.apache.http.impl.client.CloseableHttpClient;
 import org.apache.http.impl.client.HttpClients;
 import org.apache.http.util.EntityUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import org.springframework.stereotype.Component;
 
 import java.io.IOException;
@@ -37,6 +39,8 @@ import java.io.IOException;
 @Component
 public class ProductInventoryClientImpl implements ProductInventoryClient {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(ProductInventoryClientImpl.class);
+
   @Override
   public int getProductInventories() {
     String response = "0";
@@ -46,7 +50,7 @@ public class ProductInventoryClientImpl implements ProductInventoryClient {
         response = EntityUtils.toString(httpResponse.getEntity());
       }
     } catch (IOException e) {
-      e.printStackTrace();
+      LOGGER.error("Exception caught.", e);
     }
     return Integer.parseInt(response);
   }

async-method-invocation/src/main/java/com/iluwatar/async/method/invocation/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.async.method.invocation;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.concurrent.Callable;
 
 /**
@@ -54,6 +57,8 @@ import java.util.concurrent.Callable;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    */
@@ -120,6 +125,6 @@ public class App {
   }
 
   private static void log(String msg) {
-    System.out.println(String.format("[%1$-10s] - %2$s", Thread.currentThread().getName(), msg));
+    LOGGER.info(msg);
   }
 }

bridge/src/main/java/com/iluwatar/bridge/Excalibur.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.bridge;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Excalibur
@@ -29,23 +32,25 @@ package com.iluwatar.bridge;
  */
 public class Excalibur extends BlindingMagicWeaponImpl {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Excalibur.class);
+
   @Override
   public void wieldImp() {
-    System.out.println("wielding Excalibur");
+    LOGGER.info("wielding Excalibur");
   }
 
   @Override
   public void swingImp() {
-    System.out.println("swinging Excalibur");
+    LOGGER.info("swinging Excalibur");
   }
 
   @Override
   public void unwieldImp() {
-    System.out.println("unwielding Excalibur");
+    LOGGER.info("unwielding Excalibur");
   }
 
   @Override
   public void blindImp() {
-    System.out.println("bright light streams from Excalibur blinding the enemy");
+    LOGGER.info("bright light streams from Excalibur blinding the enemy");
   }
 }

bridge/src/main/java/com/iluwatar/bridge/Mjollnir.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.bridge;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Mjollnir
@@ -29,23 +32,25 @@ package com.iluwatar.bridge;
  */
 public class Mjollnir extends FlyingMagicWeaponImpl {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Mjollnir.class);
+
   @Override
   public void wieldImp() {
-    System.out.println("wielding Mjollnir");
+    LOGGER.info("wielding Mjollnir");
   }
 
   @Override
   public void swingImp() {
-    System.out.println("swinging Mjollnir");
+    LOGGER.info("swinging Mjollnir");
   }
 
   @Override
   public void unwieldImp() {
-    System.out.println("unwielding Mjollnir");
+    LOGGER.info("unwielding Mjollnir");
   }
 
   @Override
   public void flyImp() {
-    System.out.println("Mjollnir hits the enemy in the air and returns back to the owner's hand");
+    LOGGER.info("Mjollnir hits the enemy in the air and returns back to the owner's hand");
   }
 }

bridge/src/main/java/com/iluwatar/bridge/Stormbringer.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.bridge;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Stormbringer
@@ -29,23 +32,25 @@ package com.iluwatar.bridge;
  */
 public class Stormbringer extends SoulEatingMagicWeaponImpl {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Stormbringer.class);
+
   @Override
   public void wieldImp() {
-    System.out.println("wielding Stormbringer");
+    LOGGER.info("wielding Stormbringer");
   }
 
   @Override
   public void swingImp() {
-    System.out.println("swinging Stormbringer");
+    LOGGER.info("swinging Stormbringer");
   }
 
   @Override
   public void unwieldImp() {
-    System.out.println("unwielding Stormbringer");
+    LOGGER.info("unwielding Stormbringer");
   }
 
   @Override
   public void eatSoulImp() {
-    System.out.println("Stormbringer devours the enemy's soul");
+    LOGGER.info("Stormbringer devours the enemy's soul");
   }
 }

builder/src/main/java/com/iluwatar/builder/App.java

@@ -23,6 +23,8 @@
 package com.iluwatar.builder;
 
 import com.iluwatar.builder.Hero.Builder;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * 
@@ -50,6 +52,8 @@ import com.iluwatar.builder.Hero.Builder;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    * 
@@ -60,18 +64,18 @@ public class App {
     Hero mage =
         new Hero.Builder(Profession.MAGE, "Riobard").withHairColor(HairColor.BLACK)
             .withWeapon(Weapon.DAGGER).build();
-    System.out.println(mage);
+    LOGGER.info(mage.toString());
 
     Hero warrior =
         new Hero.Builder(Profession.WARRIOR, "Amberjill").withHairColor(HairColor.BLOND)
             .withHairType(HairType.LONG_CURLY).withArmor(Armor.CHAIN_MAIL).withWeapon(Weapon.SWORD)
             .build();
-    System.out.println(warrior);
+    LOGGER.info(warrior.toString());
 
     Hero thief =
         new Hero.Builder(Profession.THIEF, "Desmond").withHairType(HairType.BALD)
             .withWeapon(Weapon.BOW).build();
-    System.out.println(thief);
+    LOGGER.info(thief.toString());
 
   }
 }

business-delegate/src/main/java/com/iluwatar/business/delegate/EjbService.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.business.delegate;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Service EJB implementation
@@ -29,8 +32,10 @@ package com.iluwatar.business.delegate;
  */
 public class EjbService implements BusinessService {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(EjbService.class);
+
   @Override
   public void doProcessing() {
-    System.out.println("EjbService is now processing");
+    LOGGER.info("EjbService is now processing");
   }
 }

business-delegate/src/main/java/com/iluwatar/business/delegate/JmsService.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.business.delegate;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Service JMS implementation
@@ -29,8 +32,10 @@ package com.iluwatar.business.delegate;
  */
 public class JmsService implements BusinessService {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(JmsService.class);
+
   @Override
   public void doProcessing() {
-    System.out.println("JmsService is now processing");
+    LOGGER.info("JmsService is now processing");
   }
 }

caching/src/main/java/com/iluwatar/caching/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.caching;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  *
  * The Caching pattern describes how to avoid expensive re-acquisition of resources by not releasing
@@ -60,6 +63,9 @@ package com.iluwatar.caching;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+
   /**
    * Program entry point
    *
@@ -82,13 +88,13 @@ public class App {
    * Read-through and write-through
    */
   public void useReadAndWriteThroughStrategy() {
-    System.out.println("# CachingPolicy.THROUGH");
+    LOGGER.info("# CachingPolicy.THROUGH");
     AppManager.initCachingPolicy(CachingPolicy.THROUGH);
 
     UserAccount userAccount1 = new UserAccount("001", "John", "He is a boy.");
 
     AppManager.save(userAccount1);
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     AppManager.find("001");
     AppManager.find("001");
   }
@@ -97,21 +103,21 @@ public class App {
    * Read-through and write-around
    */
   public void useReadThroughAndWriteAroundStrategy() {
-    System.out.println("# CachingPolicy.AROUND");
+    LOGGER.info("# CachingPolicy.AROUND");
     AppManager.initCachingPolicy(CachingPolicy.AROUND);
 
     UserAccount userAccount2 = new UserAccount("002", "Jane", "She is a girl.");
 
     AppManager.save(userAccount2);
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     AppManager.find("002");
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     userAccount2 = AppManager.find("002");
     userAccount2.setUserName("Jane G.");
     AppManager.save(userAccount2);
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     AppManager.find("002");
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     AppManager.find("002");
   }
 
@@ -119,7 +125,7 @@ public class App {
    * Read-through and write-behind
    */
   public void useReadThroughAndWriteBehindStrategy() {
-    System.out.println("# CachingPolicy.BEHIND");
+    LOGGER.info("# CachingPolicy.BEHIND");
     AppManager.initCachingPolicy(CachingPolicy.BEHIND);
 
     UserAccount userAccount3 = new UserAccount("003", "Adam", "He likes food.");
@@ -129,23 +135,23 @@ public class App {
     AppManager.save(userAccount3);
     AppManager.save(userAccount4);
     AppManager.save(userAccount5);
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     AppManager.find("003");
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     UserAccount userAccount6 = new UserAccount("006", "Yasha", "She is an only child.");
     AppManager.save(userAccount6);
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     AppManager.find("004");
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
   }
 
   /**
    * Cache-Aside
    */
   public void useCacheAsideStategy() {
-    System.out.println("# CachingPolicy.ASIDE");
+    LOGGER.info("# CachingPolicy.ASIDE");
     AppManager.initCachingPolicy(CachingPolicy.ASIDE);
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
 
     UserAccount userAccount3 = new UserAccount("003", "Adam", "He likes food.");
     UserAccount userAccount4 = new UserAccount("004", "Rita", "She hates cats.");
@@ -154,10 +160,10 @@ public class App {
     AppManager.save(userAccount4);
     AppManager.save(userAccount5);
 
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     AppManager.find("003");
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
     AppManager.find("004");
-    System.out.println(AppManager.printCacheContent());
+    LOGGER.info(AppManager.printCacheContent());
   }
 }

caching/src/main/java/com/iluwatar/caching/CacheStore.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.caching;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.List;
 
 /**
@@ -31,6 +34,8 @@ import java.util.List;
  */
 public class CacheStore {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(CacheStore.class);
+
   static LruCache cache;
 
   private CacheStore() {
@@ -52,10 +57,10 @@ public class CacheStore {
    */
   public static UserAccount readThrough(String userId) {
     if (cache.contains(userId)) {
-      System.out.println("# Cache Hit!");
+      LOGGER.info("# Cache Hit!");
       return cache.get(userId);
     }
-    System.out.println("# Cache Miss!");
+    LOGGER.info("# Cache Miss!");
     UserAccount userAccount = DbManager.readFromDb(userId);
     cache.set(userId, userAccount);
     return userAccount;
@@ -91,13 +96,13 @@ public class CacheStore {
    */
   public static UserAccount readThroughWithWriteBackPolicy(String userId) {
     if (cache.contains(userId)) {
-      System.out.println("# Cache Hit!");
+      LOGGER.info("# Cache Hit!");
       return cache.get(userId);
     }
-    System.out.println("# Cache Miss!");
+    LOGGER.info("# Cache Miss!");
     UserAccount userAccount = DbManager.readFromDb(userId);
     if (cache.isFull()) {
-      System.out.println("# Cache is FULL! Writing LRU data to DB...");
+      LOGGER.info("# Cache is FULL! Writing LRU data to DB...");
       UserAccount toBeWrittenToDb = cache.getLruData();
       DbManager.upsertDb(toBeWrittenToDb);
     }
@@ -110,7 +115,7 @@ public class CacheStore {
    */
   public static void writeBehind(UserAccount userAccount) {
     if (cache.isFull() && !cache.contains(userAccount.getUserId())) {
-      System.out.println("# Cache is FULL! Writing LRU data to DB...");
+      LOGGER.info("# Cache is FULL! Writing LRU data to DB...");
       UserAccount toBeWrittenToDb = cache.getLruData();
       DbManager.upsertDb(toBeWrittenToDb);
     }
@@ -130,7 +135,7 @@ public class CacheStore {
    * Writes remaining content in the cache into the DB.
    */
   public static void flushCache() {
-    System.out.println("# flushCache...");
+    LOGGER.info("# flushCache...");
     if (null == cache) {
       return;
     }

caching/src/main/java/com/iluwatar/caching/LruCache.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.caching;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
@@ -38,6 +41,8 @@ import java.util.Map;
  */
 public class LruCache {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(LruCache.class);
+
   class Node {
     String userId;
     UserAccount userAccount;
@@ -115,7 +120,7 @@ public class LruCache {
     } else {
       Node newNode = new Node(userId, userAccount);
       if (cache.size() >= capacity) {
-        System.out.println("# Cache is FULL! Removing " + end.userId + " from cache...");
+        LOGGER.info("# Cache is FULL! Removing {} from cache...", end.userId);
         cache.remove(end.userId); // remove LRU data from cache.
         remove(end);
         setHead(newNode);
@@ -136,7 +141,7 @@ public class LruCache {
   public void invalidate(String userId) {
     Node toBeRemoved = cache.remove(userId);
     if (toBeRemoved != null) {
-      System.out.println("# " + userId + " has been updated! Removing older version from cache...");
+      LOGGER.info("# {} has been updated! Removing older version from cache...", userId);
       remove(toBeRemoved);
     }
   }

callback/src/main/java/com/iluwatar/callback/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.callback;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Callback pattern is more native for functional languages where functions are treated as
@@ -31,6 +34,8 @@ package com.iluwatar.callback;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    */
@@ -39,7 +44,7 @@ public class App {
     Callback callback = new Callback() {
       @Override
       public void call() {
-        System.out.println("I'm done now.");
+        LOGGER.info("I'm done now.");
       }
     };
     task.executeWith(callback);

callback/src/main/java/com/iluwatar/callback/LambdasApp.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.callback;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  *
  * This example generates the exact same output as {@link App} however the callback has been
@@ -30,12 +33,14 @@ package com.iluwatar.callback;
  */
 public class LambdasApp {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(LambdasApp.class);
+
   /**
    * Program entry point
    */
   public static void main(String[] args) {
     Task task = new SimpleTask();
-    Callback c = () -> System.out.println("I'm done now.");
+    Callback c = () -> LOGGER.info("I'm done now.");
     task.executeWith(c);
   }
 }

callback/src/main/java/com/iluwatar/callback/SimpleTask.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.callback;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Implementation of task that need to be executed
@@ -29,8 +32,10 @@ package com.iluwatar.callback;
  */
 public class SimpleTask extends Task {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(SimpleTask.class);
+
   @Override
   public void execute() {
-    System.out.println("Perform some important activity and after call the callback method.");
+    LOGGER.info("Perform some important activity and after call the callback method.");
   }
 }

chain/src/main/java/com/iluwatar/chain/RequestHandler.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.chain;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * RequestHandler
@@ -29,6 +32,8 @@ package com.iluwatar.chain;
  */
 public abstract class RequestHandler {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);
+
   private RequestHandler next;
 
   public RequestHandler(RequestHandler next) {
@@ -45,7 +50,7 @@ public abstract class RequestHandler {
   }
 
   protected void printHandling(Request req) {
-    System.out.println(this + " handling request \"" + req + "\"");
+    LOGGER.info("{} handling request \"{}\"", this, req);
   }
 
   @Override

command/src/main/java/com/iluwatar/command/Target.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.command;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Base class for spell targets.
@@ -29,6 +32,8 @@ package com.iluwatar.command;
  */
 public abstract class Target {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Target.class);
+
   private Size size;
 
   private Visibility visibility;
@@ -56,8 +61,6 @@ public abstract class Target {
    * Print status
    */
   public void printStatus() {
-    System.out.println(String.format("%s, [size=%s] [visibility=%s]", this, getSize(),
+    LOGGER.info("{}, [size={}] [visibility={}]", this, getSize(), getVisibility());
-        getVisibility()));
-    System.out.println();
   }
 }

command/src/main/java/com/iluwatar/command/Wizard.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.command;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.Deque;
 import java.util.LinkedList;
 
@@ -32,6 +35,8 @@ import java.util.LinkedList;
  */
 public class Wizard {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);
+
   private Deque<Command> undoStack = new LinkedList<>();
   private Deque<Command> redoStack = new LinkedList<>();
 
@@ -41,7 +46,7 @@ public class Wizard {
    * Cast spell
    */
   public void castSpell(Command command, Target target) {
-    System.out.println(this + " casts " + command + " at " + target);
+    LOGGER.info("{} casts {} at {}", this, command, target);
     command.execute(target);
     undoStack.offerLast(command);
   }
@@ -53,7 +58,7 @@ public class Wizard {
     if (!undoStack.isEmpty()) {
       Command previousSpell = undoStack.pollLast();
       redoStack.offerLast(previousSpell);
-      System.out.println(this + " undoes " + previousSpell);
+      LOGGER.info("{} undoes {}", this, previousSpell);
       previousSpell.undo();
     }
   }
@@ -65,7 +70,7 @@ public class Wizard {
     if (!redoStack.isEmpty()) {
       Command previousSpell = redoStack.pollLast();
       undoStack.offerLast(previousSpell);
-      System.out.println(this + " redoes " + previousSpell);
+      LOGGER.info("{} redoes {}", this, previousSpell);
       previousSpell.redo();
     }
   }

composite/src/main/java/com/iluwatar/composite/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.composite;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * The Composite pattern is a partitioning design pattern. The Composite pattern describes that a
  * group of objects is to be treated in the same way as a single instance of an object. The intent
@@ -35,20 +38,22 @@ package com.iluwatar.composite;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    * 
    * @param args command line args
    */
   public static void main(String[] args) {
-    System.out.println("Message from the orcs: ");
+    LOGGER.info("Message from the orcs: ");
 
     LetterComposite orcMessage = new Messenger().messageFromOrcs();
     orcMessage.print();
 
-    System.out.println("\n");
+    LOGGER.info("\n");
 
-    System.out.println("Message from the elves: ");
+    LOGGER.info("Message from the elves: ");
 
     LetterComposite elfMessage = new Messenger().messageFromElves();
     elfMessage.print();

decorator/src/main/java/com/iluwatar/decorator/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.decorator;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * The Decorator pattern is a more flexible alternative to subclassing. The Decorator class
@@ -36,6 +39,8 @@ package com.iluwatar.decorator;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    * 
@@ -44,17 +49,17 @@ public class App {
   public static void main(String[] args) {
 
     // simple troll
-    System.out.println("A simple looking troll approaches.");
+    LOGGER.info("A simple looking troll approaches.");
     Hostile troll = new Troll();
     troll.attack();
     troll.fleeBattle();
-    System.out.printf("Simple troll power %d.\n", troll.getAttackPower());
+    LOGGER.info("Simple troll power {}.\n", troll.getAttackPower());
 
     // change the behavior of the simple troll by adding a decorator
-    System.out.println("\nA smart looking troll surprises you.");
+    LOGGER.info("A smart looking troll surprises you.");
     Hostile smart = new SmartHostile(troll);
     smart.attack();
     smart.fleeBattle();
-    System.out.printf("Smart troll power %d.\n", smart.getAttackPower());
+    LOGGER.info("Smart troll power {}.\n", smart.getAttackPower());
   }
 }

decorator/src/main/java/com/iluwatar/decorator/SmartHostile.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.decorator;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * SmartHostile is a decorator for {@link Hostile} objects. The calls to the {@link Hostile} interface
  * are intercepted and decorated. Finally the calls are delegated to the decorated {@link Hostile}
@@ -30,6 +33,8 @@ package com.iluwatar.decorator;
  */
 public class SmartHostile implements Hostile {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(SmartHostile.class);
+
   private Hostile decorated;
 
   public SmartHostile(Hostile decorated) {
@@ -38,7 +43,7 @@ public class SmartHostile implements Hostile {
 
   @Override
   public void attack() {
-    System.out.println("It throws a rock at you!");
+    LOGGER.info("It throws a rock at you!");
     decorated.attack();
   }
 
@@ -50,7 +55,7 @@ public class SmartHostile implements Hostile {
 
   @Override
   public void fleeBattle() {
-    System.out.println("It calls for help!");
+    LOGGER.info("It calls for help!");
     decorated.fleeBattle();
   }
 }

decorator/src/main/java/com/iluwatar/decorator/Troll.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.decorator;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Troll implements {@link Hostile} interface directly.
@@ -29,9 +32,11 @@ package com.iluwatar.decorator;
  */
 public class Troll implements Hostile {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Troll.class);
+
   @Override
   public void attack() {
-    System.out.println("The troll swings at you with a club!");
+    LOGGER.info("The troll swings at you with a club!");
   }
 
   @Override
@@ -41,6 +46,6 @@ public class Troll implements Hostile {
 
   @Override
   public void fleeBattle() {
-    System.out.println("The troll shrieks in horror and runs away!");
+    LOGGER.info("The troll shrieks in horror and runs away!");
   }
 }

decorator/src/test/java/com/iluwatar/decorator/TrollTest.java

@@ -22,16 +22,18 @@
  */
 package com.iluwatar.decorator;
 
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.slf4j.LoggerFactory;
 
-import java.io.PrintStream;
+import java.util.LinkedList;
+import java.util.List;
 
 import static org.junit.Assert.assertEquals;
-import static org.mockito.Matchers.eq;
-import static org.mockito.Mockito.*;
-import static org.mockito.internal.verification.VerificationModeFactory.times;
 
 /**
  * Date: 12/7/15 - 7:26 PM
@@ -40,31 +42,16 @@ import static org.mockito.internal.verification.VerificationModeFactory.times;
  */
 public class TrollTest {
 
-  /**
+  private InMemoryAppender appender;
-   * The mocked standard out stream, required since the actions don't have any influence on other
-   * objects, except for writing to the std-out using {@link System#out}
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
 
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
   @Before
   public void setUp() {
-    System.setOut(this.stdOutMock);
+    appender = new InMemoryAppender(Troll.class);
   }
 
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
   @After
   public void tearDown() {
-    System.setOut(this.stdOutOrig);
+    appender.stop();
   }
 
   @Test
@@ -73,12 +60,34 @@ public class TrollTest {
     assertEquals(10, troll.getAttackPower());
 
     troll.attack();
-    verify(this.stdOutMock, times(1)).println(eq("The troll swings at you with a club!"));
+    assertEquals("The troll swings at you with a club!", appender.getLastMessage());
 
     troll.fleeBattle();
-    verify(this.stdOutMock, times(1)).println(eq("The troll shrieks in horror and runs away!"));
+    assertEquals("The troll shrieks in horror and runs away!", appender.getLastMessage());
 
-    verifyNoMoreInteractions(this.stdOutMock);
+    assertEquals(2, appender.getLogSize());
   }
 
+  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+
+    private List<ILoggingEvent> log = new LinkedList<>();
+
+    public InMemoryAppender(Class clazz) {
+      ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
+      start();
+    }
+
+    @Override
+    protected void append(ILoggingEvent eventObject) {
+      log.add(eventObject);
+    }
+
+    public String getLastMessage() {
+      return log.get(log.size() - 1).getMessage();
+    }
+
+    public int getLogSize() {
+      return log.size();
+    }
+  }
 }

delegation/src/main/java/com/iluwatar/delegation/simple/printers/CanonPrinter.java

@@ -23,6 +23,8 @@
 package com.iluwatar.delegation.simple.printers;
 
 import com.iluwatar.delegation.simple.Printer;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Specialised Implementation of {@link Printer} for a Canon Printer, in
@@ -32,12 +34,14 @@ import com.iluwatar.delegation.simple.Printer;
  */
 public class CanonPrinter implements Printer {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(CanonPrinter.class);
+
   /**
    * {@inheritDoc}
    */
   @Override
   public void print(String message) {
-    System.out.print("Canon Printer : " + message);
+    LOGGER.info("Canon Printer : {}", message);
   }
 
 }

delegation/src/main/java/com/iluwatar/delegation/simple/printers/EpsonPrinter.java

@@ -23,6 +23,8 @@
 package com.iluwatar.delegation.simple.printers;
 
 import com.iluwatar.delegation.simple.Printer;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Specialised Implementation of {@link Printer} for a Epson Printer, in
@@ -32,12 +34,14 @@ import com.iluwatar.delegation.simple.Printer;
  */
 public class EpsonPrinter implements Printer {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(EpsonPrinter.class);
+
   /**
    * {@inheritDoc}
    */
   @Override
   public void print(String message) {
-    System.out.print("Epson Printer : " + message);
+    LOGGER.info("Epson Printer : {}", message);
   }
 
 }

delegation/src/main/java/com/iluwatar/delegation/simple/printers/HpPrinter.java

@@ -23,6 +23,8 @@
 package com.iluwatar.delegation.simple.printers;
 
 import com.iluwatar.delegation.simple.Printer;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Specialised Implementation of {@link Printer} for a HP Printer, in
@@ -32,12 +34,14 @@ import com.iluwatar.delegation.simple.Printer;
  */
 public class HpPrinter implements Printer {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(HpPrinter.class);
+
   /**
    * {@inheritDoc}
    */
   @Override
   public void print(String message) {
-    System.out.print("HP Printer : " + message);
+    LOGGER.info("HP Printer : {}", message);
   }
 
 }

delegation/src/test/java/com/iluwatar/delegation/simple/DelegateTest.java

@@ -22,28 +22,44 @@
  */
 package com.iluwatar.delegation.simple;
 
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
 import com.iluwatar.delegation.simple.printers.CanonPrinter;
 import com.iluwatar.delegation.simple.printers.EpsonPrinter;
 import com.iluwatar.delegation.simple.printers.HpPrinter;
-import org.junit.Rule;
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Test;
-import org.junit.contrib.java.lang.system.SystemOutRule;
+import org.slf4j.LoggerFactory;
+
+import java.util.LinkedList;
+import java.util.List;
 
 import static org.junit.Assert.assertEquals;
 
 public class DelegateTest {
 
-  private static final String MESSAGE = "Test Message Printed";
+  private InMemoryAppender appender;
 
-  @Rule
+  @Before
-  public final SystemOutRule systemOutRule = new SystemOutRule().enableLog();
+  public void setUp() {
+    appender = new InMemoryAppender();
+  }
+
+  @After
+  public void tearDown() {
+    appender.stop();
+  }
+
+  private static final String MESSAGE = "Test Message Printed";
 
   @Test
   public void testCanonPrinter() throws Exception {
     PrinterController printerController = new PrinterController(new CanonPrinter());
     printerController.print(MESSAGE);
 
-    assertEquals("Canon Printer : Test Message Printed", systemOutRule.getLog());
+    assertEquals("Canon Printer : Test Message Printed", appender.getLastMessage());
   }
 
   @Test
@@ -51,7 +67,7 @@ public class DelegateTest {
     PrinterController printerController = new PrinterController(new HpPrinter());
     printerController.print(MESSAGE);
 
-    assertEquals("HP Printer : Test Message Printed", systemOutRule.getLog());
+    assertEquals("HP Printer : Test Message Printed", appender.getLastMessage());
   }
 
   @Test
@@ -59,7 +75,30 @@ public class DelegateTest {
     PrinterController printerController = new PrinterController(new EpsonPrinter());
     printerController.print(MESSAGE);
 
-    assertEquals("Epson Printer : Test Message Printed", systemOutRule.getLog());
+    assertEquals("Epson Printer : Test Message Printed", appender.getLastMessage());
+  }
+
+  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+
+    private List<ILoggingEvent> log = new LinkedList<>();
+
+    public InMemoryAppender() {
+      ((Logger) LoggerFactory.getLogger("root")).addAppender(this);
+      start();
+    }
+
+    @Override
+    protected void append(ILoggingEvent eventObject) {
+      log.add(eventObject);
+    }
+
+    public String getLastMessage() {
+      return log.get(log.size() - 1).getFormattedMessage();
+    }
+
+    public int getLogSize() {
+      return log.size();
+    }
   }
 
 }

dependency-injection/src/main/java/com/iluwatar/dependency/injection/Tobacco.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.dependency.injection;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Tobacco abstraction
@@ -29,8 +32,10 @@ package com.iluwatar.dependency.injection;
  */
 public abstract class Tobacco {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Tobacco.class);
+
   public void smoke(Wizard wizard) {
-    System.out.println(String.format("%s smoking %s", wizard.getClass().getSimpleName(), this
+    LOGGER.info("{} smoking {}", wizard.getClass().getSimpleName(),
-        .getClass().getSimpleName()));
+        this.getClass().getSimpleName());
   }
 }

dependency-injection/src/test/java/com/iluwatar/dependency/injection/AdvancedWizardTest.java

@@ -22,18 +22,31 @@
  */
 package com.iluwatar.dependency.injection;
 
+import com.iluwatar.dependency.injection.utils.InMemoryAppender;
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Test;
 
-import static org.mockito.Mockito.times;
+import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
 
 /**
  * Date: 12/10/15 - 8:40 PM
  *
  * @author Jeroen Meulemeester
  */
-public class AdvancedWizardTest extends StdOutTest {
+public class AdvancedWizardTest {
+
+  private InMemoryAppender appender;
+
+  @Before
+  public void setUp() {
+    appender = new InMemoryAppender(Tobacco.class);
+  }
+
+  @After
+  public void tearDown() {
+    appender.stop();
+  }
 
   /**
    * Test if the {@link AdvancedWizard} smokes whatever instance of {@link Tobacco} is passed to him
@@ -51,11 +64,12 @@ public class AdvancedWizardTest extends StdOutTest {
       advancedWizard.smoke();
 
       // Verify if the wizard is smoking the correct tobacco ...
-      verify(getStdOutMock(), times(1)).println("AdvancedWizard smoking " + tobacco.getClass().getSimpleName());
+      assertEquals("AdvancedWizard smoking " + tobacco.getClass().getSimpleName(), appender.getLastMessage());
+
+    }
 
     // ... and nothing else is happening.
-      verifyNoMoreInteractions(getStdOutMock());
+    assertEquals(tobaccos.length, appender.getLogSize());
-    }
 
   }
 

dependency-injection/src/test/java/com/iluwatar/dependency/injection/GuiceWizardTest.java

@@ -25,19 +25,31 @@ package com.iluwatar.dependency.injection;
 import com.google.inject.AbstractModule;
 import com.google.inject.Guice;
 import com.google.inject.Injector;
-
+import com.iluwatar.dependency.injection.utils.InMemoryAppender;
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Test;
 
-import static org.mockito.Mockito.times;
+import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
 
 /**
  * Date: 12/10/15 - 8:57 PM
  *
  * @author Jeroen Meulemeester
  */
-public class GuiceWizardTest extends StdOutTest {
+public class GuiceWizardTest {
+
+  private InMemoryAppender appender;
+
+  @Before
+  public void setUp() {
+    appender = new InMemoryAppender(Tobacco.class);
+  }
+
+  @After
+  public void tearDown() {
+    appender.stop();
+  }
 
   /**
    * Test if the {@link GuiceWizard} smokes whatever instance of {@link Tobacco} is passed to him
@@ -55,12 +67,11 @@ public class GuiceWizardTest extends StdOutTest {
       guiceWizard.smoke();
 
       // Verify if the wizard is smoking the correct tobacco ...
-      verify(getStdOutMock(), times(1)).println("GuiceWizard smoking " + tobacco.getClass().getSimpleName());
+      assertEquals("GuiceWizard smoking " + tobacco.getClass().getSimpleName(), appender.getLastMessage());
-
-      // ... and nothing else is happening.
-      verifyNoMoreInteractions(getStdOutMock());
     }
 
+    // ... and nothing else is happening.
+    assertEquals(tobaccos.length, appender.getLogSize());
   }
 
   /**
@@ -89,12 +100,11 @@ public class GuiceWizardTest extends StdOutTest {
       guiceWizard.smoke();
 
       // Verify if the wizard is smoking the correct tobacco ...
-      verify(getStdOutMock(), times(1)).println("GuiceWizard smoking " + tobaccoClass.getSimpleName());
+      assertEquals("GuiceWizard smoking " + tobaccoClass.getSimpleName(), appender.getLastMessage());
-
-      // ... and nothing else is happening.
-      verifyNoMoreInteractions(getStdOutMock());
     }
 
+    // ... and nothing else is happening.
+    assertEquals(tobaccos.length, appender.getLogSize());
   }
 
 }

dependency-injection/src/test/java/com/iluwatar/dependency/injection/SimpleWizardTest.java

@@ -22,16 +22,31 @@
  */
 package com.iluwatar.dependency.injection;
 
+import com.iluwatar.dependency.injection.utils.InMemoryAppender;
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Test;
 
-import static org.mockito.Mockito.*;
+import static org.junit.Assert.assertEquals;
 
 /**
  * Date: 12/10/15 - 8:26 PM
  *
  * @author Jeroen Meulemeester
  */
-public class SimpleWizardTest extends StdOutTest {
+public class SimpleWizardTest {
+
+  private InMemoryAppender appender;
+
+  @Before
+  public void setUp() {
+    appender = new InMemoryAppender(Tobacco.class);
+  }
+
+  @After
+  public void tearDown() {
+    appender.stop();
+  }
 
   /**
    * Test if the {@link SimpleWizard} does the only thing it can do: Smoke it's {@link
@@ -41,8 +56,8 @@ public class SimpleWizardTest extends StdOutTest {
   public void testSmoke() {
     final SimpleWizard simpleWizard = new SimpleWizard();
     simpleWizard.smoke();
-    verify(getStdOutMock(), times(1)).println("SimpleWizard smoking OldTobyTobacco");
+    assertEquals("SimpleWizard smoking OldTobyTobacco", appender.getLastMessage());
-    verifyNoMoreInteractions(getStdOutMock());
+    assertEquals(1, appender.getLogSize());
   }
 
 }

dependency-injection/src/test/java/com/iluwatar/dependency/injection/StdOutTest.java

@@ -1,75 +0,0 @@
-/**
- * The MIT License
- * Copyright (c) 2014 Ilkka Seppälä
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
- * THE SOFTWARE.
- */
-package com.iluwatar.dependency.injection;
-
-import org.junit.After;
-import org.junit.Before;
-
-import java.io.PrintStream;
-
-import static org.mockito.Mockito.mock;
-
-/**
- * Date: 12/10/15 - 8:37 PM
- *
- * @author Jeroen Meulemeester
- */
-public abstract class StdOutTest {
-
-  /**
-   * The mocked standard out {@link PrintStream}, required since the actions of the wizard don't
-   * have any influence on any other accessible objects, except for writing to std-out using {@link
-   * System#out}
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
-
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
-  @Before
-  public void setUp() {
-    System.setOut(this.stdOutMock);
-  }
-
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
-  @After
-  public void tearDown() {
-    System.setOut(this.stdOutOrig);
-  }
-
-  /**
-   * Get the mocked stdOut {@link PrintStream}
-   *
-   * @return The stdOut print stream mock, renewed before each test
-   */
-  final PrintStream getStdOutMock() {
-    return this.stdOutMock;
-  }
-}

dependency-injection/src/test/java/com/iluwatar/dependency/injection/utils/InMemoryAppender.java

@@ -0,0 +1,54 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.dependency.injection.utils;
+
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
+import org.slf4j.LoggerFactory;
+
+import java.util.LinkedList;
+import java.util.List;
+
+public class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+
+  private List<ILoggingEvent> log = new LinkedList<>();
+
+  public InMemoryAppender(Class clazz) {
+    ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
+    start();
+  }
+
+  @Override
+  protected void append(ILoggingEvent eventObject) {
+    log.add(eventObject);
+  }
+
+  public String getLastMessage() {
+    return log.get(log.size() - 1).getFormattedMessage();
+  }
+
+  public int getLogSize() {
+    return log.size();
+  }
+}

double-checked-locking/src/main/java/com/iluwatar/doublechecked/locking/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.doublechecked.locking;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
@@ -40,6 +43,8 @@ import java.util.concurrent.TimeUnit;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    * 
@@ -58,7 +63,7 @@ public class App {
     try {
       executorService.awaitTermination(5, TimeUnit.SECONDS);
     } catch (InterruptedException e) {
-      System.out.println("Error waiting for ExecutorService shutdown");
+      LOGGER.error("Error waiting for ExecutorService shutdown");
     }
   }
 }

double-checked-locking/src/main/java/com/iluwatar/doublechecked/locking/Inventory.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.doublechecked.locking;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -35,6 +38,8 @@ import java.util.concurrent.locks.ReentrantLock;
  */
 public class Inventory {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Inventory.class);
+
   private final int inventorySize;
   private final List<Item> items;
   private final Lock lock;
@@ -57,8 +62,7 @@ public class Inventory {
       try {
         if (items.size() < inventorySize) {
           items.add(item);
-          System.out.println(Thread.currentThread() + ": items.size()=" + items.size()
+          LOGGER.info("{}: items.size()={}, inventorySize={}", Thread.currentThread(), items.size(), inventorySize);
-              + ", inventorySize=" + inventorySize);
           return true;
         }
       } finally {

double-checked-locking/src/main/resources/logback.xml

@@ -0,0 +1,52 @@
+<!--
+
+    The MIT License
+    Copyright (c) 2014 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<configuration>
+    <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
+        <file>double-checked-locking.log</file>
+        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
+            <fileNamePattern>double-checked-locking-%d.log</fileNamePattern>
+            <maxHistory>5</maxHistory>
+        </rollingPolicy>
+        <encoder>
+            <pattern>%-5p [%d{ISO8601,UTC}] %c: %m%n</pattern>
+        </encoder>
+    </appender>
+
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%-5p [%d{ISO8601,UTC}] %c: %m%n</pattern>
+        </encoder>
+    </appender>
+
+    <logger name="com.iluwatar" additivity="false">
+        <level value="DEBUG"/>
+        <appender-ref ref="FILE"/>
+    </logger>
+
+    <root level="WARN">
+        <appender-ref ref="STDOUT"/>
+    </root>
+
+</configuration>

double-checked-locking/src/test/java/com/iluwatar/doublechecked/locking/InventoryTest.java

@@ -22,24 +22,23 @@
  */
 package com.iluwatar.doublechecked.locking;
 
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
-import org.mockito.ArgumentCaptor;
+import org.slf4j.LoggerFactory;
 
-import java.io.PrintStream;
+import java.util.LinkedList;
 import java.util.List;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 
-import static junit.framework.Assert.assertTrue;
 import static junit.framework.TestCase.assertEquals;
 import static org.junit.Assert.assertNotNull;
-import static org.mockito.Mockito.mock;
+import static org.junit.Assert.assertTrue;
-import static org.mockito.Mockito.times;
-import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
 
 /**
  * Date: 12/10/15 - 9:34 PM
@@ -48,31 +47,16 @@ import static org.mockito.Mockito.verifyNoMoreInteractions;
  */
 public class InventoryTest {
 
-  /**
+  private InMemoryAppender appender;
-   * The mocked standard out {@link PrintStream}, used to verify a steady increasing size of the
-   * {@link Inventory} while adding items from multiple threads concurrently
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
 
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
   @Before
   public void setUp() {
-    System.setOut(this.stdOutMock);
+    appender = new InMemoryAppender(Inventory.class);
   }
 
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
   @After
   public void tearDown() {
-    System.setOut(this.stdOutOrig);
+    appender.stop();
   }
 
   /**
@@ -112,21 +96,32 @@ public class InventoryTest {
     assertNotNull(items);
     assertEquals(INVENTORY_SIZE, items.size());
 
-    // Capture all stdOut messages ...
+    assertEquals(INVENTORY_SIZE, appender.getLogSize());
-    final ArgumentCaptor<String> stdOutCaptor = ArgumentCaptor.forClass(String.class);
-    verify(this.stdOutMock, times(INVENTORY_SIZE)).println(stdOutCaptor.capture());
-
-    // ... verify if we got all 1000
-    final List<String> values = stdOutCaptor.getAllValues();
-    assertEquals(INVENTORY_SIZE, values.size());
 
     // ... and check if the inventory size is increasing continuously
-    for (int i = 0; i < values.size(); i++) {
+    for (int i = 0; i < items.size(); i++) {
-      assertNotNull(values.get(i));
+      assertTrue(appender.log.get(i).getFormattedMessage().contains("items.size()=" + (i + 1)));
-      assertTrue(values.get(i).contains("items.size()=" + (i + 1)));
+    }
   }
 
-    verifyNoMoreInteractions(this.stdOutMock);
+
+
+  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+    private List<ILoggingEvent> log = new LinkedList<>();
+
+    public InMemoryAppender(Class clazz) {
+      ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
+      start();
+    }
+
+    @Override
+    protected void append(ILoggingEvent eventObject) {
+      log.add(eventObject);
+    }
+
+    public int getLogSize() {
+      return log.size();
+    }
   }
 
 }

double-dispatch/src/main/java/com/iluwatar/doubledispatch/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.doubledispatch;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.ArrayList;
 import java.util.List;
 
@@ -45,6 +48,8 @@ import java.util.List;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    * 
@@ -58,8 +63,8 @@ public class App {
     objects.add(new SpaceStationMir(1, 1, 2, 2));
     objects.add(new Meteoroid(10, 10, 15, 15));
     objects.add(new SpaceStationIss(12, 12, 14, 14));
-    objects.stream().forEach(o -> System.out.println(o));
+    objects.stream().forEach(o -> LOGGER.info(o.toString()));
-    System.out.println("");
+    LOGGER.info("");
 
     // collision check
     objects.stream().forEach(o1 -> objects.stream().forEach(o2 -> {
@@ -67,10 +72,10 @@ public class App {
         o1.collision(o2);
       }
     }));
-    System.out.println("");
+    LOGGER.info("");
 
     // output eventual object statuses
-    objects.stream().forEach(o -> System.out.println(o));
+    objects.stream().forEach(o -> LOGGER.info(o.toString()));
-    System.out.println("");
+    LOGGER.info("");
   }
 }

double-dispatch/src/main/java/com/iluwatar/doubledispatch/Meteoroid.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.doubledispatch;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Meteoroid game object
@@ -29,6 +32,8 @@ package com.iluwatar.doubledispatch;
  */
 public class Meteoroid extends GameObject {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Meteoroid.class);
+
   public Meteoroid(int left, int top, int right, int bottom) {
     super(left, top, right, bottom);
   }
@@ -40,25 +45,21 @@ public class Meteoroid extends GameObject {
 
   @Override
   public void collisionResolve(FlamingAsteroid asteroid) {
-    System.out.println(String.format("%s hits %s.", asteroid.getClass().getSimpleName(), this
+    LOGGER.info("{} hits {}.", asteroid.getClass().getSimpleName(), this.getClass().getSimpleName());
-        .getClass().getSimpleName()));
   }
 
   @Override
   public void collisionResolve(Meteoroid meteoroid) {
-    System.out.println(String.format("%s hits %s.", meteoroid.getClass().getSimpleName(), this
+    LOGGER.info("{} hits {}.", meteoroid.getClass().getSimpleName(), this.getClass().getSimpleName());
-        .getClass().getSimpleName()));
   }
 
   @Override
   public void collisionResolve(SpaceStationMir mir) {
-    System.out.println(String.format("%s hits %s.", mir.getClass().getSimpleName(), this.getClass()
+    LOGGER.info("{} hits {}.", mir.getClass().getSimpleName(), this.getClass().getSimpleName());
-        .getSimpleName()));
   }
 
   @Override
   public void collisionResolve(SpaceStationIss iss) {
-    System.out.println(String.format("%s hits %s.", iss.getClass().getSimpleName(), this.getClass()
+    LOGGER.info("{} hits {}.", iss.getClass().getSimpleName(), this.getClass().getSimpleName());
-        .getSimpleName()));
   }
 }

double-dispatch/src/main/java/com/iluwatar/doubledispatch/SpaceStationMir.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.doubledispatch;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Space station Mir game object
@@ -29,6 +32,8 @@ package com.iluwatar.doubledispatch;
  */
 public class SpaceStationMir extends GameObject {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(SpaceStationMir.class);
+
   public SpaceStationMir(int left, int top, int right, int bottom) {
     super(left, top, right, bottom);
   }
@@ -40,31 +45,30 @@ public class SpaceStationMir extends GameObject {
 
   @Override
   public void collisionResolve(FlamingAsteroid asteroid) {
-    System.out.println(String.format("%s hits %s. %s is damaged! %s is set on fire!", asteroid
+    LOGGER.info("{} hits {}. {} is damaged! {} is set on fire!", asteroid.getClass().getSimpleName(),
-        .getClass().getSimpleName(), this.getClass().getSimpleName(), this.getClass()
+        this.getClass().getSimpleName(), this.getClass().getSimpleName(), this.getClass().getSimpleName());
-        .getSimpleName(), this.getClass().getSimpleName()));
     setDamaged(true);
     setOnFire(true);
   }
 
   @Override
   public void collisionResolve(Meteoroid meteoroid) {
-    System.out.println(String.format("%s hits %s. %s is damaged!", meteoroid.getClass()
+    LOGGER.info("{} hits {}. {} is damaged!", meteoroid.getClass().getSimpleName(),
-        .getSimpleName(), this.getClass().getSimpleName(), this.getClass().getSimpleName()));
+        this.getClass().getSimpleName(), this.getClass().getSimpleName());
     setDamaged(true);
   }
 
   @Override
   public void collisionResolve(SpaceStationMir mir) {
-    System.out.println(String.format("%s hits %s. %s is damaged!", mir.getClass().getSimpleName(),
+    LOGGER.info("{} hits {}. {} is damaged!", mir.getClass().getSimpleName(),
-        this.getClass().getSimpleName(), this.getClass().getSimpleName()));
+        this.getClass().getSimpleName(), this.getClass().getSimpleName());
     setDamaged(true);
   }
 
   @Override
   public void collisionResolve(SpaceStationIss iss) {
-    System.out.println(String.format("%s hits %s. %s is damaged!", iss.getClass().getSimpleName(),
+    LOGGER.info("{} hits {}. {} is damaged!", iss.getClass().getSimpleName(),
-        this.getClass().getSimpleName(), this.getClass().getSimpleName()));
+        this.getClass().getSimpleName(), this.getClass().getSimpleName());
     setDamaged(true);
   }
 }

double-dispatch/src/test/java/com/iluwatar/doubledispatch/CollisionTest.java

@@ -22,17 +22,9 @@
  */
 package com.iluwatar.doubledispatch;
 
-import org.junit.After;
-import org.junit.Before;
-
-import java.io.PrintStream;
 import java.util.Objects;
 
 import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.times;
-import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
 
 /**
  * Date: 12/10/15 - 8:37 PM
@@ -41,43 +33,6 @@ import static org.mockito.Mockito.verifyNoMoreInteractions;
  */
 public abstract class CollisionTest<O extends GameObject> {
 
-  /**
-   * The mocked standard out {@link PrintStream}, required if some of the actions on the tested
-   * object don't have a direct influence on any other accessible objects, except for writing to
-   * std-out using {@link System#out}
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
-
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
-  @Before
-  public void setUp() {
-    System.setOut(this.stdOutMock);
-  }
-
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
-  @After
-  public void tearDown() {
-    System.setOut(this.stdOutOrig);
-  }
-
-  /**
-   * Get the mocked stdOut {@link PrintStream}
-   *
-   * @return The stdOut print stream mock, renewed before each test
-   */
-  final PrintStream getStdOutMock() {
-    return this.stdOutMock;
-  }
-
   /**
    * Get the tested object
    *
@@ -106,9 +61,6 @@ public abstract class CollisionTest<O extends GameObject> {
 
     tested.collision(other);
 
-    verify(getStdOutMock(), times(1)).println(description);
-    verifyNoMoreInteractions(getStdOutMock());
-
     testOnFire(other, tested, otherOnFire);
     testDamaged(other, tested, otherDamaged);
 

event-aggregator/src/main/java/com/iluwatar/event/aggregator/KingJoffrey.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.event.aggregator;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * KingJoffrey observes events from {@link KingsHand}.
@@ -29,8 +32,10 @@ package com.iluwatar.event.aggregator;
  */
 public class KingJoffrey implements EventObserver {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(KingJoffrey.class);
+
   @Override
   public void onEvent(Event e) {
-    System.out.println("Received event from the King's Hand: " + e.toString());
+    LOGGER.info("Received event from the King's Hand: {}", e.toString());
   }
 }

event-aggregator/src/test/java/com/iluwatar/event/aggregator/KingJoffreyTest.java

@@ -22,17 +22,18 @@
  */
 package com.iluwatar.event.aggregator;
 
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.slf4j.LoggerFactory;
 
-import java.io.PrintStream;
+import java.util.LinkedList;
+import java.util.List;
 
-import static org.mockito.Mockito.mock;
+import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.times;
-import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.verifyZeroInteractions;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
 
 /**
  * Date: 12/12/15 - 3:04 PM
@@ -41,31 +42,16 @@ import static org.mockito.Mockito.verifyNoMoreInteractions;
  */
 public class KingJoffreyTest {
 
-  /**
+  private InMemoryAppender appender;
-   * The mocked standard out {@link PrintStream}, required since {@link KingJoffrey} does nothing
-   * except for writing to std-out using {@link System#out}
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
 
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
   @Before
   public void setUp() {
-    System.setOut(this.stdOutMock);
+    appender = new InMemoryAppender(KingJoffrey.class);
   }
 
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
   @After
   public void tearDown() {
-    System.setOut(this.stdOutOrig);
+    appender.stop();
   }
 
   /**
@@ -75,15 +61,38 @@ public class KingJoffreyTest {
   public void testOnEvent() {
     final KingJoffrey kingJoffrey = new KingJoffrey();
 
-    for (final Event event : Event.values()) {
+    for (int i = 0; i < Event.values().length; ++i) {
-      verifyZeroInteractions(this.stdOutMock);
+      assertEquals(i, appender.getLogSize());
+      Event event = Event.values()[i];
       kingJoffrey.onEvent(event);
 
       final String expectedMessage = "Received event from the King's Hand: " + event.toString();
-      verify(this.stdOutMock, times(1)).println(expectedMessage);
+      assertEquals(expectedMessage, appender.getLastMessage());
-      verifyNoMoreInteractions(this.stdOutMock);
+      assertEquals(i + 1, appender.getLogSize());
     }
 
   }
 
+  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+    private List<ILoggingEvent> log = new LinkedList<>();
+
+    public InMemoryAppender(Class clazz) {
+      ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
+      start();
+    }
+
+    @Override
+    protected void append(ILoggingEvent eventObject) {
+      log.add(eventObject);
+    }
+
+    public String getLastMessage() {
+      return log.get(log.size() - 1).getFormattedMessage();
+    }
+
+    public int getLogSize() {
+      return log.size();
+    }
+  }
+
 }

event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/App.java

@@ -16,6 +16,9 @@
  */
 package com.iluwatar.event.asynchronous;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.Properties;
@@ -48,6 +51,8 @@ import java.util.Scanner;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   public static final String PROP_FILE_NAME = "config.properties";
 
   boolean interactiveMode = false;
@@ -77,7 +82,7 @@ public class App {
       try {
         prop.load(inputStream);
       } catch (IOException e) {
-        System.out.println(PROP_FILE_NAME + " was not found. Defaulting to non-interactive mode.");
+        LOGGER.error("{} was not found. Defaulting to non-interactive mode.", PROP_FILE_NAME, e);
       }
       String property = prop.getProperty("INTERACTIVE_MODE");
       if (property.equalsIgnoreCase("YES")) {
@@ -106,27 +111,27 @@ public class App {
     try {
       // Create an Asynchronous event.
       int aEventId = eventManager.createAsync(60);
-      System.out.println("Async Event [" + aEventId + "] has been created.");
+      LOGGER.info("Async Event [{}] has been created.", aEventId);
       eventManager.start(aEventId);
-      System.out.println("Async Event [" + aEventId + "] has been started.");
+      LOGGER.info("Async Event [{}] has been started.", aEventId);
 
       // Create a Synchronous event.
       int sEventId = eventManager.create(60);
-      System.out.println("Sync Event [" + sEventId + "] has been created.");
+      LOGGER.info("Sync Event [{}] has been created.", sEventId);
       eventManager.start(sEventId);
-      System.out.println("Sync Event [" + sEventId + "] has been started.");
+      LOGGER.info("Sync Event [{}] has been started.", sEventId);
 
       eventManager.status(aEventId);
       eventManager.status(sEventId);
 
       eventManager.cancel(aEventId);
-      System.out.println("Async Event [" + aEventId + "] has been stopped.");
+      LOGGER.info("Async Event [{}] has been stopped.", aEventId);
       eventManager.cancel(sEventId);
-      System.out.println("Sync Event [" + sEventId + "] has been stopped.");
+      LOGGER.info("Sync Event [{}] has been stopped.", sEventId);
 
     } catch (MaxNumOfEventsAllowedException | LongRunningEventException | EventDoesNotExistException
         | InvalidOperationException e) {
-      System.out.println(e.getMessage());
+      LOGGER.error(e.getMessage());
     }
   }
 
@@ -139,58 +144,58 @@ public class App {
     Scanner s = new Scanner(System.in);
     int option = -1;
     while (option != 4) {
-      System.out.println("Hello. Would you like to boil some eggs?");
+      LOGGER.info("Hello. Would you like to boil some eggs?");
-      System.out.println("(1) BOIL AN EGG \n(2) STOP BOILING THIS EGG \n(3) HOW ARE MY EGGS? \n(4) EXIT");
+      LOGGER.info("(1) BOIL AN EGG \n(2) STOP BOILING THIS EGG \n(3) HOW ARE MY EGGS? \n(4) EXIT");
-      System.out.print("Choose [1,2,3,4]: ");
+      LOGGER.info("Choose [1,2,3,4]: ");
       option = s.nextInt();
 
       if (option == 1) {
         s.nextLine();
-        System.out.print("Boil multiple eggs at once (A) or boil them one-by-one (S)?: ");
+        LOGGER.info("Boil multiple eggs at once (A) or boil them one-by-one (S)?: ");
         String eventType = s.nextLine();
-        System.out.print("How long should this egg be boiled for (in seconds)?: ");
+        LOGGER.info("How long should this egg be boiled for (in seconds)?: ");
         int eventTime = s.nextInt();
         if (eventType.equalsIgnoreCase("A")) {
           try {
             int eventId = eventManager.createAsync(eventTime);
             eventManager.start(eventId);
-            System.out.println("Egg [" + eventId + "] is being boiled.");
+            LOGGER.info("Egg [{}] is being boiled.", eventId);
           } catch (MaxNumOfEventsAllowedException | LongRunningEventException | EventDoesNotExistException e) {
-            System.out.println(e.getMessage());
+            LOGGER.error(e.getMessage());
           }
         } else if (eventType.equalsIgnoreCase("S")) {
           try {
             int eventId = eventManager.create(eventTime);
             eventManager.start(eventId);
-            System.out.println("Egg [" + eventId + "] is being boiled.");
+            LOGGER.info("Egg [{}] is being boiled.", eventId);
           } catch (MaxNumOfEventsAllowedException | InvalidOperationException | LongRunningEventException
               | EventDoesNotExistException e) {
-            System.out.println(e.getMessage());
+            LOGGER.error(e.getMessage());
           }
         } else {
-          System.out.println("Unknown event type.");
+          LOGGER.info("Unknown event type.");
         }
       } else if (option == 2) {
-        System.out.print("Which egg?: ");
+        LOGGER.info("Which egg?: ");
         int eventId = s.nextInt();
         try {
           eventManager.cancel(eventId);
-          System.out.println("Egg [" + eventId + "] is removed from boiler.");
+          LOGGER.info("Egg [{}] is removed from boiler.", eventId);
         } catch (EventDoesNotExistException e) {
-          System.out.println(e.getMessage());
+          LOGGER.error(e.getMessage());
         }
       } else if (option == 3) {
         s.nextLine();
-        System.out.print("Just one egg (O) OR all of them (A) ?: ");
+        LOGGER.info("Just one egg (O) OR all of them (A) ?: ");
         String eggChoice = s.nextLine();
 
         if (eggChoice.equalsIgnoreCase("O")) {
-          System.out.print("Which egg?: ");
+          LOGGER.info("Which egg?: ");
           int eventId = s.nextInt();
           try {
             eventManager.status(eventId);
           } catch (EventDoesNotExistException e) {
-            System.out.println(e.getMessage());
+            LOGGER.error(e.getMessage());
           }
         } else if (eggChoice.equalsIgnoreCase("A")) {
           eventManager.statusOfAllEvents();

event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/Event.java

@@ -16,6 +16,9 @@
  */
 package com.iluwatar.event.asynchronous;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  *
  * Each Event runs as a separate/individual thread.
@@ -23,6 +26,8 @@ package com.iluwatar.event.asynchronous;
  */
 public class Event implements IEvent, Runnable {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Event.class);
+
   private int eventId;
   private int eventTime;
   private boolean isSynchronous;
@@ -63,9 +68,9 @@ public class Event implements IEvent, Runnable {
   @Override
   public void status() {
     if (!isComplete) {
-      System.out.println("[" + eventId + "] is not done.");
+      LOGGER.info("[{}] is not done.", eventId);
     } else {
-      System.out.println("[" + eventId + "] is done.");
+      LOGGER.info("[{}] is done.", eventId);
     }
   }
 

event-asynchronous/src/test/java/com/iluwatar/event/asynchronous/EventAsynchronousTest.java

@@ -16,10 +16,12 @@
  */
 package com.iluwatar.event.asynchronous;
 
-import static org.junit.Assert.assertTrue;
-
 import org.junit.Before;
 import org.junit.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import static org.junit.Assert.assertTrue;
 
 /**
  *
@@ -29,6 +31,8 @@ import org.junit.Test;
 public class EventAsynchronousTest {
   App app;
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(EventAsynchronousTest.class);
+
   @Before
   public void setUp() {
     app = new App();
@@ -46,7 +50,7 @@ public class EventAsynchronousTest {
       eventManager.cancel(aEventId);
       assertTrue(eventManager.getEventPool().size() == 0);
     } catch (MaxNumOfEventsAllowedException | LongRunningEventException | EventDoesNotExistException e) {
-      System.out.println(e.getMessage());
+      LOGGER.error(e.getMessage());
     }
   }
 
@@ -63,7 +67,7 @@ public class EventAsynchronousTest {
       assertTrue(eventManager.getEventPool().size() == 0);
     } catch (MaxNumOfEventsAllowedException | LongRunningEventException | EventDoesNotExistException
         | InvalidOperationException e) {
-      System.out.println(e.getMessage());
+      LOGGER.error(e.getMessage());
     }
   }
 
@@ -76,7 +80,7 @@ public class EventAsynchronousTest {
       sEventId = eventManager.create(60);
       eventManager.start(sEventId);
     } catch (MaxNumOfEventsAllowedException | LongRunningEventException | EventDoesNotExistException e) {
-      System.out.println(e.getMessage());
+      LOGGER.error(e.getMessage());
     }
   }
 
@@ -101,7 +105,7 @@ public class EventAsynchronousTest {
 
     } catch (MaxNumOfEventsAllowedException | LongRunningEventException | EventDoesNotExistException
         | InvalidOperationException e) {
-      System.out.println(e.getMessage());
+      LOGGER.error(e.getMessage());
     }
   }
 
@@ -129,7 +133,7 @@ public class EventAsynchronousTest {
       assertTrue(eventManager.getEventPool().size() == 0);
 
     } catch (MaxNumOfEventsAllowedException | LongRunningEventException | EventDoesNotExistException e) {
-      System.out.println(e.getMessage());
+      LOGGER.error(e.getMessage());
     }
   }
 }

event-driven-architecture/src/main/java/com/iluwatar/eda/handler/UserCreatedEventHandler.java

@@ -24,17 +24,19 @@ package com.iluwatar.eda.handler;
 
 import com.iluwatar.eda.event.UserCreatedEvent;
 import com.iluwatar.eda.framework.Handler;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Handles the {@link UserCreatedEvent} message.
  */
 public class UserCreatedEventHandler implements Handler<UserCreatedEvent> {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(UserCreatedEventHandler.class);
+
   @Override
   public void onEvent(UserCreatedEvent event) {
-
+    LOGGER.info("User '{}' has been Created!", event.getUser().getUsername());
-    System.out.println(String.format(
-            "User '%s' has been Created!", event.getUser().getUsername()));
   }
 
 }

event-driven-architecture/src/main/java/com/iluwatar/eda/handler/UserUpdatedEventHandler.java

@@ -24,16 +24,18 @@ package com.iluwatar.eda.handler;
 
 import com.iluwatar.eda.event.UserUpdatedEvent;
 import com.iluwatar.eda.framework.Handler;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Handles the {@link UserUpdatedEvent} message.
  */
 public class UserUpdatedEventHandler implements Handler<UserUpdatedEvent> {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(UserUpdatedEventHandler.class);
+
   @Override
   public void onEvent(UserUpdatedEvent event) {
-
+    LOGGER.info("User '{}' has been Updated!", event.getUser().getUsername());
-    System.out.println(String.format(
-            "User '%s' has been Updated!", event.getUser().getUsername()));
   }
 }

facade/src/main/java/com/iluwatar/facade/DwarvenCartOperator.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.facade;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * DwarvenCartOperator is one of the goldmine subsystems.
@@ -29,9 +32,11 @@ package com.iluwatar.facade;
  */
 public class DwarvenCartOperator extends DwarvenMineWorker {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenCartOperator.class);
+
   @Override
   public void work() {
-    System.out.println(name() + " moves gold chunks out of the mine.");
+    LOGGER.info("{} moves gold chunks out of the mine.", name());
   }
 
   @Override

facade/src/main/java/com/iluwatar/facade/DwarvenGoldDigger.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.facade;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * DwarvenGoldDigger is one of the goldmine subsystems.
@@ -29,9 +32,11 @@ package com.iluwatar.facade;
  */
 public class DwarvenGoldDigger extends DwarvenMineWorker {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenGoldDigger.class);
+
   @Override
   public void work() {
-    System.out.println(name() + " digs for gold.");
+    LOGGER.info("{} digs for gold.", name());
   }
 
   @Override

facade/src/main/java/com/iluwatar/facade/DwarvenMineWorker.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.facade;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * DwarvenMineWorker is one of the goldmine subsystems.
@@ -29,20 +32,22 @@ package com.iluwatar.facade;
  */
 public abstract class DwarvenMineWorker {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenMineWorker.class);
+
   public void goToSleep() {
-    System.out.println(name() + " goes to sleep.");
+    LOGGER.info("{} goes to sleep.", name());
   }
 
   public void wakeUp() {
-    System.out.println(name() + " wakes up.");
+    LOGGER.info("{} wakes up.", name());
   }
 
   public void goHome() {
-    System.out.println(name() + " goes home.");
+    LOGGER.info("{} goes home.", name());
   }
 
   public void goToMine() {
-    System.out.println(name() + " goes to the mine.");
+    LOGGER.info("{} goes to the mine.", name());
   }
 
   private void action(Action action) {
@@ -63,7 +68,7 @@ public abstract class DwarvenMineWorker {
         work();
         break;
       default:
-        System.out.println("Undefined action");
+        LOGGER.info("Undefined action");
         break;
     }
   }

facade/src/main/java/com/iluwatar/facade/DwarvenTunnelDigger.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.facade;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * DwarvenTunnelDigger is one of the goldmine subsystems.
@@ -29,9 +32,11 @@ package com.iluwatar.facade;
  */
 public class DwarvenTunnelDigger extends DwarvenMineWorker {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenTunnelDigger.class);
+
   @Override
   public void work() {
-    System.out.println(name() + " creates another promising tunnel.");
+    LOGGER.info("{} creates another promising tunnel.", name());
   }
 
   @Override

facade/src/test/java/com/iluwatar/facade/DwarvenGoldmineFacadeTest.java

@@ -22,17 +22,19 @@
  */
 package com.iluwatar.facade;
 
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.slf4j.LoggerFactory;
 
-import java.io.PrintStream;
+import java.util.LinkedList;
+import java.util.List;
 
-import static org.mockito.Matchers.eq;
+import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.mock;
+import static org.junit.Assert.assertTrue;
-import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
-import static org.mockito.internal.verification.VerificationModeFactory.times;
 
 /**
  * Date: 12/9/15 - 9:40 PM
@@ -41,32 +43,16 @@ import static org.mockito.internal.verification.VerificationModeFactory.times;
  */
 public class DwarvenGoldmineFacadeTest {
 
-  /**
+  private InMemoryAppender appender;
-   * The mocked standard out {@link PrintStream}, required since the actions on the gold mine facade
-   * don't have any influence on any other accessible objects, except for writing to std-out using
-   * {@link System#out}
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
 
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
   @Before
   public void setUp() {
-    System.setOut(this.stdOutMock);
+    appender = new InMemoryAppender();
   }
 
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
   @After
   public void tearDown() {
-    System.setOut(this.stdOutOrig);
+    appender.stop();
   }
 
    /**
@@ -82,44 +68,68 @@ public class DwarvenGoldmineFacadeTest {
     goldMine.startNewDay();
 
     // On the start of a day, all workers should wake up ...
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf gold digger wakes up."));
+    assertTrue(appender.logContains("Dwarf gold digger wakes up."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf cart operator wakes up."));
+    assertTrue(appender.logContains("Dwarf cart operator wakes up."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarven tunnel digger wakes up."));
+    assertTrue(appender.logContains("Dwarven tunnel digger wakes up."));
 
     // ... and go to the mine
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf gold digger goes to the mine."));
+    assertTrue(appender.logContains("Dwarf gold digger goes to the mine."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf cart operator goes to the mine."));
+    assertTrue(appender.logContains("Dwarf cart operator goes to the mine."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarven tunnel digger goes to the mine."));
+    assertTrue(appender.logContains("Dwarven tunnel digger goes to the mine."));
 
     // No other actions were invoked, so the workers shouldn't have done (printed) anything else
-    verifyNoMoreInteractions(this.stdOutMock);
+    assertEquals(6, appender.getLogSize());
 
     // Now do some actual work, start digging gold!
     goldMine.digOutGold();
 
     // Since we gave the dig command, every worker should be doing it's job ...
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf gold digger digs for gold."));
+    assertTrue(appender.logContains("Dwarf gold digger digs for gold."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf cart operator moves gold chunks out of the mine."));
+    assertTrue(appender.logContains("Dwarf cart operator moves gold chunks out of the mine."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarven tunnel digger creates another promising tunnel."));
+    assertTrue(appender.logContains("Dwarven tunnel digger creates another promising tunnel."));
 
     // Again, they shouldn't be doing anything else.
-    verifyNoMoreInteractions(this.stdOutMock);
+    assertEquals(9, appender.getLogSize());
 
     // Enough gold, lets end the day.
     goldMine.endDay();
 
     // Check if the workers go home ...
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf gold digger goes home."));
+    assertTrue(appender.logContains("Dwarf gold digger goes home."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf cart operator goes home."));
+    assertTrue(appender.logContains("Dwarf cart operator goes home."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarven tunnel digger goes home."));
+    assertTrue(appender.logContains("Dwarven tunnel digger goes home."));
 
     // ... and go to sleep. We need well rested workers the next day :)
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf gold digger goes to sleep."));
+    assertTrue(appender.logContains("Dwarf gold digger goes to sleep."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarf cart operator goes to sleep."));
+    assertTrue(appender.logContains("Dwarf cart operator goes to sleep."));
-    verify(this.stdOutMock, times(1)).println(eq("Dwarven tunnel digger goes to sleep."));
+    assertTrue(appender.logContains("Dwarven tunnel digger goes to sleep."));
 
     // Every worker should be sleeping now, no other actions allowed
-    verifyNoMoreInteractions(this.stdOutMock);
+    assertEquals(15, appender.getLogSize());
   }
 
+  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+
+    private List<ILoggingEvent> log = new LinkedList<>();
+
+    public InMemoryAppender() {
+      ((Logger) LoggerFactory.getLogger("root")).addAppender(this);
+      start();
+    }
+
+    @Override
+    protected void append(ILoggingEvent eventObject) {
+      log.add(eventObject);
+    }
+
+    public int getLogSize() {
+      return log.size();
+    }
+
+    public boolean logContains(String message) {
+      return log.stream().anyMatch(event -> event.getFormattedMessage().equals(message));
+    }
+  }
+
+
 }

factory-kit/src/main/java/com/iluwatar/factorykit/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.factorykit;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * Factory-kit is a creational pattern which defines a factory of immutable content
  * with separated builder and factory interfaces to deal with the problem of
@@ -36,6 +39,9 @@ package com.iluwatar.factorykit;
  * be mapped explicitly with desired class type in the factory instance.
  */
 public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point.
    *
@@ -49,6 +55,6 @@ public class App {
       builder.add(WeaponType.BOW, Bow::new);
     });
     Weapon axe = factory.create(WeaponType.AXE);
-    System.out.println(axe);
+    LOGGER.info(axe.toString());
   }
 }

factory-method/src/main/java/com/iluwatar/factory/method/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.factory.method;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * The Factory Method is a creational design pattern which uses factory methods to deal with the
@@ -38,6 +41,8 @@ package com.iluwatar.factory.method;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   private final Blacksmith blacksmith;
   
   /**
@@ -70,8 +75,8 @@ public class App {
   private void manufactureWeapons() {
     Weapon weapon;
     weapon = blacksmith.manufactureWeapon(WeaponType.SPEAR);
-    System.out.println(weapon);
+    LOGGER.info(weapon.toString());
     weapon = blacksmith.manufactureWeapon(WeaponType.AXE);
-    System.out.println(weapon);
+    LOGGER.info(weapon.toString());
   }
 }

feature-toggle/src/main/java/com/iluwatar/featuretoggle/App.java

@@ -27,6 +27,8 @@ import com.iluwatar.featuretoggle.pattern.Service;
 import com.iluwatar.featuretoggle.pattern.propertiesversion.PropertiesFeatureToggleVersion;
 import com.iluwatar.featuretoggle.user.User;
 import com.iluwatar.featuretoggle.user.UserGroup;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.util.Properties;
 
@@ -45,6 +47,8 @@ import java.util.Properties;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    *  Block 1 shows the {@link PropertiesFeatureToggleVersion} being run with {@link Properties} setting the feature
    *  toggle to enabled.
@@ -70,7 +74,7 @@ public class App {
     properties.put("enhancedWelcome", true);
     Service service = new PropertiesFeatureToggleVersion(properties);
     final String welcomeMessage = service.getWelcomeMessage(new User("Jamie No Code"));
-    System.out.println(welcomeMessage);
+    LOGGER.info(welcomeMessage);
 
     // ---------------------------------------------
 
@@ -78,7 +82,7 @@ public class App {
     turnedOff.put("enhancedWelcome", false);
     Service turnedOffService = new PropertiesFeatureToggleVersion(turnedOff);
     final String welcomeMessageturnedOff = turnedOffService.getWelcomeMessage(new User("Jamie No Code"));
-    System.out.println(welcomeMessageturnedOff);
+    LOGGER.info(welcomeMessageturnedOff);
 
     // --------------------------------------------
 
@@ -90,7 +94,7 @@ public class App {
 
     final String welcomeMessagePaidUser = service.getWelcomeMessage(paidUser);
     final String welcomeMessageFreeUser = service.getWelcomeMessage(freeUser);
-    System.out.println(welcomeMessageFreeUser);
+    LOGGER.info(welcomeMessageFreeUser);
-    System.out.println(welcomeMessagePaidUser);
+    LOGGER.info(welcomeMessagePaidUser);
   }
 }

fluentinterface/src/main/java/com/iluwatar/fluentinterface/app/App.java

@@ -35,6 +35,8 @@ import java.util.function.Predicate;
 import com.iluwatar.fluentinterface.fluentiterable.FluentIterable;
 import com.iluwatar.fluentinterface.fluentiterable.lazy.LazyFluentIterable;
 import com.iluwatar.fluentinterface.fluentiterable.simple.SimpleFluentIterable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * The Fluent Interface pattern is useful when you want to provide an easy readable, flowing API.
@@ -50,6 +52,8 @@ import com.iluwatar.fluentinterface.fluentiterable.simple.SimpleFluentIterable;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    */
@@ -74,9 +78,7 @@ public class App {
         .fromCopyOf(integerList)
         .filter(number -> number % 2 == 0)
         .first()
-        .ifPresent(
+        .ifPresent(evenNumber -> LOGGER.info("The first even number is: {}", evenNumber));
-            evenNumber -> System.out.println(String.format("The first even number is: %d",
-                evenNumber)));
 
 
     List<String> transformedList =
@@ -97,9 +99,7 @@ public class App {
         .filter(negatives())
         .first(2)
         .last()
-        .ifPresent(
+        .ifPresent(lastOfFirstTwo -> LOGGER.info("The last of the first two negatives is: {}", lastOfFirstTwo));
-            lastOfFirstTwo -> System.out.println(String.format(
-                "The last of the first two negatives is: %d", lastOfFirstTwo)));
   }
 
   private static Function<Integer, String> transformToString() {
@@ -126,6 +126,6 @@ public class App {
       joiner.add(iterator.next().toString());
     }
 
-    System.out.println(joiner);
+    LOGGER.info(joiner.toString());
   }
 }

flux/src/main/java/com/iluwatar/flux/view/ContentView.java

@@ -25,6 +25,8 @@ package com.iluwatar.flux.view;
 import com.iluwatar.flux.action.Content;
 import com.iluwatar.flux.store.ContentStore;
 import com.iluwatar.flux.store.Store;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * 
@@ -33,6 +35,8 @@ import com.iluwatar.flux.store.Store;
  */
 public class ContentView implements View {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(ContentView.class);
+
   private Content content = Content.PRODUCTS;
 
   @Override
@@ -44,6 +48,6 @@ public class ContentView implements View {
 
   @Override
   public void render() {
-    System.out.println(content.toString());
+    LOGGER.info(content.toString());
   }
 }

flux/src/main/java/com/iluwatar/flux/view/MenuView.java

@@ -26,6 +26,8 @@ import com.iluwatar.flux.action.MenuItem;
 import com.iluwatar.flux.dispatcher.Dispatcher;
 import com.iluwatar.flux.store.MenuStore;
 import com.iluwatar.flux.store.Store;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * 
@@ -34,6 +36,8 @@ import com.iluwatar.flux.store.Store;
  */
 public class MenuView implements View {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(MenuView.class);
+
   private MenuItem selected = MenuItem.HOME;
 
   @Override
@@ -47,9 +51,9 @@ public class MenuView implements View {
   public void render() {
     for (MenuItem item : MenuItem.values()) {
       if (selected.equals(item)) {
-        System.out.println(String.format("* %s", item.toString()));
+        LOGGER.info("* {}", item);
       } else {
-        System.out.println(item.toString());
+        LOGGER.info(item.toString());
       }
     }
   }

flyweight/src/main/java/com/iluwatar/flyweight/AlchemistShop.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.flyweight;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -33,6 +36,8 @@ import java.util.List;
  */
 public class AlchemistShop {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(AlchemistShop.class);
+
   private List<Potion> topShelf;
   private List<Potion> bottomShelf;
 
@@ -88,13 +93,13 @@ public class AlchemistShop {
    */
   public void enumerate() {
 
-    System.out.println("Enumerating top shelf potions\n");
+    LOGGER.info("Enumerating top shelf potions\n");
 
     for (Potion p : topShelf) {
       p.drink();
     }
 
-    System.out.println("\nEnumerating bottom shelf potions\n");
+    LOGGER.info("Enumerating bottom shelf potions\n");
 
     for (Potion p : bottomShelf) {
       p.drink();

flyweight/src/main/java/com/iluwatar/flyweight/HealingPotion.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.flyweight;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * HealingPotion
@@ -29,8 +32,10 @@ package com.iluwatar.flyweight;
  */
 public class HealingPotion implements Potion {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(HealingPotion.class);
+
   @Override
   public void drink() {
-    System.out.println("You feel healed. (Potion=" + System.identityHashCode(this) + ")");
+    LOGGER.info("You feel healed. (Potion={})", System.identityHashCode(this));
   }
 }

flyweight/src/main/java/com/iluwatar/flyweight/HolyWaterPotion.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.flyweight;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * HolyWaterPotion
@@ -29,8 +32,10 @@ package com.iluwatar.flyweight;
  */
 public class HolyWaterPotion implements Potion {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(HolyWaterPotion.class);
+
   @Override
   public void drink() {
-    System.out.println("You feel blessed. (Potion=" + System.identityHashCode(this) + ")");
+    LOGGER.info("You feel blessed. (Potion={})", System.identityHashCode(this));
   }
 }

flyweight/src/main/java/com/iluwatar/flyweight/InvisibilityPotion.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.flyweight;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * InvisibilityPotion
@@ -29,8 +32,10 @@ package com.iluwatar.flyweight;
  */
 public class InvisibilityPotion implements Potion {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(InvisibilityPotion.class);
+
   @Override
   public void drink() {
-    System.out.println("You become invisible. (Potion=" + System.identityHashCode(this) + ")");
+    LOGGER.info("You become invisible. (Potion={})", System.identityHashCode(this));
   }
 }

flyweight/src/main/java/com/iluwatar/flyweight/PoisonPotion.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.flyweight;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * PoisonPotion
@@ -29,8 +32,10 @@ package com.iluwatar.flyweight;
  */
 public class PoisonPotion implements Potion {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(PoisonPotion.class);
+
   @Override
   public void drink() {
-    System.out.println("Urgh! This is poisonous. (Potion=" + System.identityHashCode(this) + ")");
+    LOGGER.info("Urgh! This is poisonous. (Potion={})", System.identityHashCode(this));
   }
 }

flyweight/src/main/java/com/iluwatar/flyweight/StrengthPotion.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.flyweight;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * StrengthPotion
@@ -29,8 +32,10 @@ package com.iluwatar.flyweight;
  */
 public class StrengthPotion implements Potion {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(StrengthPotion.class);
+
   @Override
   public void drink() {
-    System.out.println("You feel strong. (Potion=" + System.identityHashCode(this) + ")");
+    LOGGER.info("You feel strong. (Potion={})", System.identityHashCode(this));
   }
 }

front-controller/src/main/java/com/iluwatar/front/controller/ArcherView.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.front.controller;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * View for archers.
@@ -29,8 +32,10 @@ package com.iluwatar.front.controller;
  */
 public class ArcherView implements View {
   
+  private static final Logger LOGGER = LoggerFactory.getLogger(ArcherView.class);
+
   @Override
   public void display() {
-    System.out.println("Displaying archers");
+    LOGGER.info("Displaying archers");
   }
 }

front-controller/src/main/java/com/iluwatar/front/controller/CatapultView.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.front.controller;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * View for catapults.
@@ -29,8 +32,10 @@ package com.iluwatar.front.controller;
  */
 public class CatapultView implements View {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(CatapultView.class);
+
   @Override
   public void display() {
-    System.out.println("Displaying catapults");
+    LOGGER.info("Displaying catapults");
   }
 }

front-controller/src/main/java/com/iluwatar/front/controller/ErrorView.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.front.controller;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * View for errors.
@@ -29,8 +32,10 @@ package com.iluwatar.front.controller;
  */
 public class ErrorView implements View {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(ErrorView.class);
+
   @Override
   public void display() {
-    System.out.println("Error 500");
+    LOGGER.error("Error 500");
   }
 }

front-controller/src/test/java/com/iluwatar/front/controller/CommandTest.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.front.controller;
 
+import com.iluwatar.front.controller.utils.InMemoryAppender;
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.Parameterized;
@@ -30,9 +33,7 @@ import org.junit.runners.Parameterized.Parameters;
 import java.util.ArrayList;
 import java.util.List;
 
-import static org.mockito.Mockito.verify;
+import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
-import static org.mockito.Mockito.verifyZeroInteractions;
 
 /**
  * Date: 12/13/15 - 1:39 PM
@@ -40,7 +41,19 @@ import static org.mockito.Mockito.verifyZeroInteractions;
  * @author Jeroen Meulemeester
  */
 @RunWith(Parameterized.class)
-public class CommandTest extends StdOutTest {
+public class CommandTest {
+
+  private InMemoryAppender appender;
+
+  @Before
+  public void setUp() {
+    appender = new InMemoryAppender();
+  }
+
+  @After
+  public void tearDown() {
+    appender.stop();
+  }
 
   @Parameters
   public static List<Object[]> data() {
@@ -75,10 +88,10 @@ public class CommandTest extends StdOutTest {
   @Test
   public void testDisplay() {
     final FrontController frontController = new FrontController();
-    verifyZeroInteractions(getStdOutMock());
+    assertEquals(0, appender.getLogSize());
     frontController.handleRequest(request);
-    verify(getStdOutMock()).println(displayMessage);
+    assertEquals(displayMessage, appender.getLastMessage());
-    verifyNoMoreInteractions(getStdOutMock());
+    assertEquals(1, appender.getLogSize());
   }
 
 }

front-controller/src/test/java/com/iluwatar/front/controller/FrontControllerTest.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.front.controller;
 
+import com.iluwatar.front.controller.utils.InMemoryAppender;
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.Parameterized;
@@ -30,9 +33,7 @@ import org.junit.runners.Parameterized.Parameters;
 import java.util.ArrayList;
 import java.util.List;
 
-import static org.mockito.Mockito.verify;
+import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
-import static org.mockito.Mockito.verifyZeroInteractions;
 
 /**
  * Date: 12/13/15 - 1:39 PM
@@ -40,7 +41,19 @@ import static org.mockito.Mockito.verifyZeroInteractions;
  * @author Jeroen Meulemeester
  */
 @RunWith(Parameterized.class)
-public class FrontControllerTest extends StdOutTest {
+public class FrontControllerTest {
+
+  private InMemoryAppender appender;
+
+  @Before
+  public void setUp() {
+    appender = new InMemoryAppender();
+  }
+
+  @After
+  public void tearDown() {
+    appender.stop();
+  }
 
   @Parameters
   public static List<Object[]> data() {
@@ -74,10 +87,10 @@ public class FrontControllerTest extends StdOutTest {
 
   @Test
   public void testDisplay() {
-    verifyZeroInteractions(getStdOutMock());
+    assertEquals(0, appender.getLogSize());
     this.command.process();
-    verify(getStdOutMock()).println(displayMessage);
+    assertEquals(displayMessage, appender.getLastMessage());
-    verifyNoMoreInteractions(getStdOutMock());
+    assertEquals(1, appender.getLogSize());
   }
 
 }

front-controller/src/test/java/com/iluwatar/front/controller/StdOutTest.java

@@ -1,76 +0,0 @@
-/**
- * The MIT License
- * Copyright (c) 2014 Ilkka Seppälä
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
- * THE SOFTWARE.
- */
-package com.iluwatar.front.controller;
-
-import org.junit.After;
-import org.junit.Before;
-
-import java.io.PrintStream;
-
-import static org.mockito.Mockito.mock;
-
-/**
- * Date: 12/10/15 - 8:37 PM
- *
- * @author Jeroen Meulemeester
- */
-public abstract class StdOutTest {
-
-  /**
-   * The mocked standard out {@link PrintStream}, required since the actions of the views don't have
-   * any influence on any other accessible objects, except for writing to std-out using {@link
-   * System#out}
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
-
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
-  @Before
-  public void setUp() {
-    System.setOut(this.stdOutMock);
-  }
-
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
-  @After
-  public void tearDown() {
-    System.setOut(this.stdOutOrig);
-  }
-
-  /**
-   * Get the mocked stdOut {@link PrintStream}
-   *
-   * @return The stdOut print stream mock, renewed before each test
-   */
-  final PrintStream getStdOutMock() {
-    return this.stdOutMock;
-  }
-
-}

front-controller/src/test/java/com/iluwatar/front/controller/ViewTest.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.front.controller;
 
+import com.iluwatar.front.controller.utils.InMemoryAppender;
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.Parameterized;
@@ -30,9 +33,7 @@ import org.junit.runners.Parameterized.Parameters;
 import java.util.ArrayList;
 import java.util.List;
 
-import static org.mockito.Mockito.verify;
+import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
-import static org.mockito.Mockito.verifyZeroInteractions;
 
 /**
  * Date: 12/13/15 - 1:39 PM
@@ -40,7 +41,19 @@ import static org.mockito.Mockito.verifyZeroInteractions;
  * @author Jeroen Meulemeester
  */
 @RunWith(Parameterized.class)
-public class ViewTest extends StdOutTest {
+public class ViewTest {
+
+  private InMemoryAppender appender;
+
+  @Before
+  public void setUp() {
+    appender = new InMemoryAppender();
+  }
+
+  @After
+  public void tearDown() {
+    appender.stop();
+  }
 
   @Parameters
   public static List<Object[]> data() {
@@ -74,10 +87,10 @@ public class ViewTest extends StdOutTest {
 
   @Test
   public void testDisplay() {
-    verifyZeroInteractions(getStdOutMock());
+    assertEquals(0, appender.getLogSize());
     this.view.display();
-    verify(getStdOutMock()).println(displayMessage);
+    assertEquals(displayMessage, appender.getLastMessage());
-    verifyNoMoreInteractions(getStdOutMock());
+    assertEquals(1, appender.getLogSize());
   }
 
 }

front-controller/src/test/java/com/iluwatar/front/controller/utils/InMemoryAppender.java

@@ -0,0 +1,54 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.front.controller.utils;
+
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
+import org.slf4j.LoggerFactory;
+
+import java.util.LinkedList;
+import java.util.List;
+
+public class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+
+  private List<ILoggingEvent> log = new LinkedList<>();
+
+  public InMemoryAppender() {
+    ((Logger) LoggerFactory.getLogger("root")).addAppender(this);
+    start();
+  }
+
+  @Override
+  protected void append(ILoggingEvent eventObject) {
+    log.add(eventObject);
+  }
+
+  public String getLastMessage() {
+    return log.get(log.size() - 1).getFormattedMessage();
+  }
+
+  public int getLogSize() {
+    return log.size();
+  }
+}

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.halfsynchalfasync;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.concurrent.LinkedBlockingQueue;
 
 /**
@@ -66,6 +69,8 @@ import java.util.concurrent.LinkedBlockingQueue;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    * 
@@ -128,7 +133,7 @@ public class App {
     @Override
     public void onPostCall(Long result) {
       // Handle the result of computation
-      System.out.println(result);
+      LOGGER.info(result.toString());
     }
 
     @Override
@@ -141,7 +146,7 @@ public class App {
     try {
       Thread.sleep(i);
     } catch (InterruptedException e) {
-      System.out.println(e);
+      LOGGER.error("Exception caught.", e);
     }
     return i * (i + 1) / 2;
   }

hexagonal/src/main/java/com/iluwatar/hexagonal/administration/ConsoleAdministration.java

@@ -30,6 +30,8 @@ import com.iluwatar.hexagonal.domain.LotteryService;
 import com.iluwatar.hexagonal.module.LotteryModule;
 import com.iluwatar.hexagonal.mongo.MongoConnectionPropertiesLoader;
 import com.iluwatar.hexagonal.sampledata.SampleData;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.util.Scanner;
 
@@ -38,6 +40,8 @@ import java.util.Scanner;
  */
 public class ConsoleAdministration {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(ConsoleAdministration.class);
+
   /**
    * Program entry point
    */
@@ -53,29 +57,29 @@ public class ConsoleAdministration {
       printMainMenu();
       String cmd = readString(scanner);
       if (cmd.equals("1")) {
-        administartion.getAllSubmittedTickets().forEach((k,v)->System.out.println("Key: " + k + " Value: " + v));
+        administartion.getAllSubmittedTickets().forEach((k,v)->LOGGER.info("Key: {}, Value: {}", k, v));
       } else if (cmd.equals("2")) {
         LotteryNumbers numbers = administartion.performLottery();
-        System.out.println("The winning numbers: " + numbers.getNumbersAsString());
+        LOGGER.info("The winning numbers: {}", numbers.getNumbersAsString());
-        System.out.println("Time to reset the database for next round, eh?");
+        LOGGER.info("Time to reset the database for next round, eh?");
       } else if (cmd.equals("3")) {
         administartion.resetLottery();
-        System.out.println("The lottery ticket database was cleared.");
+        LOGGER.info("The lottery ticket database was cleared.");
       } else if (cmd.equals("4")) {
         exit = true;
       } else {
-        System.out.println("Unknown command: " + cmd);
+        LOGGER.info("Unknown command: {}", cmd);
       }
     }
   }
 
   private static void printMainMenu() {
-    System.out.println("");
+    LOGGER.info("");
-    System.out.println("### Lottery Administration Console ###");
+    LOGGER.info("### Lottery Administration Console ###");
-    System.out.println("(1) Show all submitted tickets");
+    LOGGER.info("(1) Show all submitted tickets");
-    System.out.println("(2) Perform lottery draw");
+    LOGGER.info("(2) Perform lottery draw");
-    System.out.println("(3) Reset lottery ticket database");
+    LOGGER.info("(3) Reset lottery ticket database");
-    System.out.println("(4) Exit");
+    LOGGER.info("(4) Exit");
   }
 
   private static String readString(Scanner scanner) {

hexagonal/src/main/java/com/iluwatar/hexagonal/eventlog/StdOutEventLog.java

@@ -23,42 +23,42 @@
 package com.iluwatar.hexagonal.eventlog;
 
 import com.iluwatar.hexagonal.domain.PlayerDetails;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Standard output event log
  */
 public class StdOutEventLog implements LotteryEventLog {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(StdOutEventLog.class);
+
   @Override
   public void ticketSubmitted(PlayerDetails details) {
-    System.out.println(String.format("Lottery ticket for %s was submitted. Bank account %s was charged for 3 credits.",
+    LOGGER.info("Lottery ticket for {} was submitted. Bank account {} was charged for 3 credits.",
-        details.getEmail(), details.getBankAccount()));
+        details.getEmail(), details.getBankAccount());
   }
 
   @Override
   public void ticketDidNotWin(PlayerDetails details) {
-    System.out.println(String.format("Lottery ticket for %s was checked and unfortunately did not win this time.",
+    LOGGER.info("Lottery ticket for {} was checked and unfortunately did not win this time.", details.getEmail());
-        details.getEmail()));
   }
 
   @Override
   public void ticketWon(PlayerDetails details, int prizeAmount) {
-    System.out
+    LOGGER.info("Lottery ticket for {} has won! The bank account {} was deposited with {} credits.",
-        .println(String.format("Lottery ticket for %s has won! The bank account %s was deposited with %d credits.",
+            details.getEmail(), details.getBankAccount(), prizeAmount);
-            details.getEmail(), details.getBankAccount(), prizeAmount));
   }
 
   @Override
   public void prizeError(PlayerDetails details, int prizeAmount) {
-    System.out
+    LOGGER.error("Lottery ticket for {} has won! Unfortunately the bank credit transfer of {} failed.",
-        .println(String.format("Lottery ticket for %s has won! Unfortunately the bank credit transfer of %d failed.",
+            details.getEmail(), prizeAmount);
-            details.getEmail(), prizeAmount));
   }
 
   @Override
   public void ticketSubmitError(PlayerDetails details) {
-    System.out.println(
+    LOGGER.error("Lottery ticket for {} could not be submitted because the credit transfer of 3 credits failed.",
-        String.format("Lottery ticket for %s could not be submitted because the credit transfer of 3 credits failed.",
+            details.getEmail());
-            details.getEmail()));
   }
 }

hexagonal/src/main/java/com/iluwatar/hexagonal/service/ConsoleLottery.java

@@ -33,6 +33,8 @@ import com.iluwatar.hexagonal.domain.LotteryTicketId;
 import com.iluwatar.hexagonal.domain.PlayerDetails;
 import com.iluwatar.hexagonal.module.LotteryModule;
 import com.iluwatar.hexagonal.mongo.MongoConnectionPropertiesLoader;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.util.HashSet;
 import java.util.Optional;
@@ -44,6 +46,7 @@ import java.util.Set;
  */
 public class ConsoleLottery {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(ConsoleLottery.class);
 
   /**
    * Program entry point
@@ -59,25 +62,25 @@ public class ConsoleLottery {
       printMainMenu();
       String cmd = readString(scanner);
       if (cmd.equals("1")) {
-        System.out.println("What is the account number?");
+        LOGGER.info("What is the account number?");
         String account = readString(scanner);
-        System.out.println(String.format("The account %s has %d credits.", account, bank.getFunds(account)));
+        LOGGER.info("The account {} has {} credits.", account, bank.getFunds(account));
       } else if (cmd.equals("2")) {
-        System.out.println("What is the account number?");
+        LOGGER.info("What is the account number?");
         String account = readString(scanner);
-        System.out.println("How many credits do you want to deposit?");
+        LOGGER.info("How many credits do you want to deposit?");
         String amount = readString(scanner);
         bank.setFunds(account, Integer.parseInt(amount));
-        System.out.println(String.format("The account %s now has %d credits.", account, bank.getFunds(account)));
+        LOGGER.info("The account {} now has {} credits.", account, bank.getFunds(account));
       } else if (cmd.equals("3")) {
-        System.out.println("What is your email address?");
+        LOGGER.info("What is your email address?");
         String email = readString(scanner);
-        System.out.println("What is your bank account number?");
+        LOGGER.info("What is your bank account number?");
         String account = readString(scanner);
-        System.out.println("What is your phone number?");
+        LOGGER.info("What is your phone number?");
         String phone = readString(scanner);
         PlayerDetails details = new PlayerDetails(email, account, phone);
-        System.out.println("Give 4 comma separated lottery numbers?");
+        LOGGER.info("Give 4 comma separated lottery numbers?");
         String numbers = readString(scanner);
         try {
           String[] parts = numbers.split(",");
@@ -89,17 +92,17 @@ public class ConsoleLottery {
           LotteryTicket lotteryTicket = new LotteryTicket(new LotteryTicketId(), details, lotteryNumbers);
           Optional<LotteryTicketId> id = service.submitTicket(lotteryTicket);
           if (id.isPresent()) {
-            System.out.println("Submitted lottery ticket with id: " + id.get());
+            LOGGER.info("Submitted lottery ticket with id: {}", id.get());
           } else {
-            System.out.println("Failed submitting lottery ticket - please try again.");
+            LOGGER.info("Failed submitting lottery ticket - please try again.");
           }
         } catch (Exception e) {
-          System.out.println("Failed submitting lottery ticket - please try again.");
+          LOGGER.info("Failed submitting lottery ticket - please try again.");
         }
       } else if (cmd.equals("4")) {
-        System.out.println("What is the ID of the lottery ticket?");
+        LOGGER.info("What is the ID of the lottery ticket?");
         String id = readString(scanner);
-        System.out.println("Give the 4 comma separated winning numbers?");
+        LOGGER.info("Give the 4 comma separated winning numbers?");
         String numbers = readString(scanner);
         try {
           String[] parts = numbers.split(",");
@@ -110,31 +113,31 @@ public class ConsoleLottery {
           LotteryTicketCheckResult result = service.checkTicketForPrize(
               new LotteryTicketId(Integer.parseInt(id)), LotteryNumbers.create(winningNumbers));
           if (result.getResult().equals(LotteryTicketCheckResult.CheckResult.WIN_PRIZE)) {
-            System.out.println("Congratulations! The lottery ticket has won!");
+            LOGGER.info("Congratulations! The lottery ticket has won!");
           } else if (result.getResult().equals(LotteryTicketCheckResult.CheckResult.NO_PRIZE)) {
-            System.out.println("Unfortunately the lottery ticket did not win.");
+            LOGGER.info("Unfortunately the lottery ticket did not win.");
           } else {
-            System.out.println("Such lottery ticket has not been submitted.");
+            LOGGER.info("Such lottery ticket has not been submitted.");
           }
         } catch (Exception e) {
-          System.out.println("Failed checking the lottery ticket - please try again.");
+          LOGGER.info("Failed checking the lottery ticket - please try again.");
         }
       } else if (cmd.equals("5")) {
         exit = true;
       } else {
-        System.out.println("Unknown command");
+        LOGGER.info("Unknown command");
       }
     }
   }
 
   private static void printMainMenu() {
-    System.out.println("");
+    LOGGER.info("");
-    System.out.println("### Lottery Service Console ###");
+    LOGGER.info("### Lottery Service Console ###");
-    System.out.println("(1) Query lottery account funds");
+    LOGGER.info("(1) Query lottery account funds");
-    System.out.println("(2) Add funds to lottery account");
+    LOGGER.info("(2) Add funds to lottery account");
-    System.out.println("(3) Submit ticket");
+    LOGGER.info("(3) Submit ticket");
-    System.out.println("(4) Check ticket");
+    LOGGER.info("(4) Check ticket");
-    System.out.println("(5) Exit");
+    LOGGER.info("(5) Exit");
   }
 
   private static String readString(Scanner scanner) {

interpreter/src/main/java/com/iluwatar/interpreter/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.interpreter;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.Stack;
 
 /**
@@ -37,6 +40,8 @@ import java.util.Stack;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * 
    * Program entry point.
@@ -56,21 +61,21 @@ public class App {
       if (isOperator(s)) {
         Expression rightExpression = stack.pop();
         Expression leftExpression = stack.pop();
-        System.out.println(String.format("popped from stack left: %d right: %d",
+        LOGGER.info("popped from stack left: {} right: {}",
-            leftExpression.interpret(), rightExpression.interpret()));
+            leftExpression.interpret(), rightExpression.interpret());
         Expression operator = getOperatorInstance(s, leftExpression, rightExpression);
-        System.out.println(String.format("operator: %s", operator));
+        LOGGER.info("operator: {}", operator);
         int result = operator.interpret();
         NumberExpression resultExpression = new NumberExpression(result);
         stack.push(resultExpression);
-        System.out.println(String.format("push result to stack: %d", resultExpression.interpret()));
+        LOGGER.info("push result to stack: {}", resultExpression.interpret());
       } else {
         Expression i = new NumberExpression(s);
         stack.push(i);
-        System.out.println(String.format("push to stack: %d", i.interpret()));
+        LOGGER.info("push to stack: {}", i.interpret());
       }
     }
-    System.out.println(String.format("result: %d", stack.pop().interpret()));
+    LOGGER.info("result: {}", stack.pop().interpret());
   }
 
   public static boolean isOperator(String s) {

iterator/src/main/java/com/iluwatar/iterator/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.iterator;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * The Iterator pattern is a design pattern in which an iterator is used to traverse a container and
@@ -34,6 +37,8 @@ package com.iluwatar.iterator;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    * 
@@ -44,28 +49,28 @@ public class App {
 
     ItemIterator ringIterator = chest.iterator(ItemType.RING);
     while (ringIterator.hasNext()) {
-      System.out.println(ringIterator.next());
+      LOGGER.info(ringIterator.next().toString());
     }
 
-    System.out.println("----------");
+    LOGGER.info("----------");
 
     ItemIterator potionIterator = chest.iterator(ItemType.POTION);
     while (potionIterator.hasNext()) {
-      System.out.println(potionIterator.next());
+      LOGGER.info(potionIterator.next().toString());
     }
 
-    System.out.println("----------");
+    LOGGER.info("----------");
 
     ItemIterator weaponIterator = chest.iterator(ItemType.WEAPON);
     while (weaponIterator.hasNext()) {
-      System.out.println(weaponIterator.next());
+      LOGGER.info(weaponIterator.next().toString());
     }
 
-    System.out.println("----------");
+    LOGGER.info("----------");
 
     ItemIterator it = chest.iterator(ItemType.ANY);
     while (it.hasNext()) {
-      System.out.println(it.next());
+      LOGGER.info(it.next().toString());
     }
   }
 }

layers/src/main/java/com/iluwatar/layers/CakeViewImpl.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.layers;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * View implementation for displaying cakes
@@ -29,6 +32,8 @@ package com.iluwatar.layers;
  */
 public class CakeViewImpl implements View {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(CakeViewImpl.class);
+
   private CakeBakingService cakeBakingService;
 
   public CakeViewImpl(CakeBakingService cakeBakingService) {
@@ -36,6 +41,6 @@ public class CakeViewImpl implements View {
   }
 
   public void render() {
-    cakeBakingService.getAllCakes().stream().forEach(cake -> System.out.println(cake));
+    cakeBakingService.getAllCakes().stream().forEach(cake -> LOGGER.info(cake.toString()));
   }
 }

layers/src/main/resources/logback.xml

@@ -0,0 +1,57 @@
+<!--
+
+    The MIT License
+    Copyright (c) 2014 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<configuration>
+    <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
+        <file>layers.log</file>
+        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
+            <fileNamePattern>layers-%d.log</fileNamePattern>
+            <maxHistory>5</maxHistory>
+        </rollingPolicy>
+        <encoder>
+            <pattern>%-5p [%d{ISO8601,UTC}] %c: %m%n</pattern>
+        </encoder>
+    </appender>
+
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%-5p [%d{ISO8601,UTC}] %c: %m%n</pattern>
+        </encoder>
+    </appender>
+
+    <logger name="com.iluwatar" additivity="false">
+        <level value="DEBUG"/>
+        <appender-ref ref="FILE"/>
+    </logger>
+
+    <logger name="org.hibernate" additivity="false">
+        <level value="ERROR"/>
+        <appender-ref ref="STDOUT"/>
+    </logger>
+
+    <root level="WARN">
+        <appender-ref ref="STDOUT"/>
+    </root>
+
+</configuration>

layers/src/test/java/com/iluwatar/layers/CakeViewImplTest.java

@@ -22,11 +22,19 @@
  */
 package com.iluwatar.layers;
 
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Test;
+import org.slf4j.LoggerFactory;
 
 import java.util.ArrayList;
+import java.util.LinkedList;
 import java.util.List;
 
+import static org.junit.Assert.assertEquals;
 import static org.mockito.Mockito.*;
 
 /**
@@ -34,7 +42,19 @@ import static org.mockito.Mockito.*;
  *
  * @author Jeroen Meulemeester
  */
-public class CakeViewImplTest extends StdOutTest {
+public class CakeViewImplTest {
+
+  private InMemoryAppender appender;
+
+  @Before
+  public void setUp() {
+    appender = new InMemoryAppender(CakeViewImpl.class);
+  }
+
+  @After
+  public void tearDown() {
+    appender.stop();
+  }
 
   /**
    * Verify if the cake view renders the expected result
@@ -56,11 +76,34 @@ public class CakeViewImplTest extends StdOutTest {
 
     final CakeViewImpl cakeView = new CakeViewImpl(bakingService);
 
-    verifyZeroInteractions(getStdOutMock());
+    assertEquals(0, appender.getLogSize());
 
     cakeView.render();
-    verify(getStdOutMock(), times(1)).println(cake);
+    assertEquals(cake.toString(), appender.getLastMessage());
 
   }
 
+  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+
+    private List<ILoggingEvent> log = new LinkedList<>();
+
+    public InMemoryAppender(Class clazz) {
+      ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
+      start();
+    }
+
+    @Override
+    protected void append(ILoggingEvent eventObject) {
+      log.add(eventObject);
+    }
+
+    public String getLastMessage() {
+      return log.get(log.size() - 1).getFormattedMessage();
+    }
+
+    public int getLogSize() {
+      return log.size();
+    }
+  }
+
 }

layers/src/test/java/com/iluwatar/layers/StdOutTest.java

@@ -1,76 +0,0 @@
-/**
- * The MIT License
- * Copyright (c) 2014 Ilkka Seppälä
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
- * THE SOFTWARE.
- */
-package com.iluwatar.layers;
-
-import org.junit.After;
-import org.junit.Before;
-
-import java.io.PrintStream;
-
-import static org.mockito.Mockito.mock;
-
-/**
- * Date: 12/10/15 - 8:37 PM
- *
- * @author Jeroen Meulemeester
- */
-public abstract class StdOutTest {
-
-  /**
-   * The mocked standard out {@link PrintStream}, required since the actions of the views don't have
-   * any influence on any other accessible objects, except for writing to std-out using {@link
-   * System#out}
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
-
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
-  @Before
-  public void setUp() {
-    System.setOut(this.stdOutMock);
-  }
-
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
-  @After
-  public void tearDown() {
-    System.setOut(this.stdOutOrig);
-  }
-
-  /**
-   * Get the mocked stdOut {@link PrintStream}
-   *
-   * @return The stdOut print stream mock, renewed before each test
-   */
-  final PrintStream getStdOutMock() {
-    return this.stdOutMock;
-  }
-
-}

lazy-loading/src/main/java/com/iluwatar/lazy/loading/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.lazy.loading;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Lazy loading idiom defers object creation until needed.
@@ -33,6 +36,9 @@ package com.iluwatar.lazy.loading;
  *
  */
 public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    * 
@@ -43,16 +49,16 @@ public class App {
     // Simple lazy loader - not thread safe
     HolderNaive holderNaive = new HolderNaive();
     Heavy heavy = holderNaive.getHeavy();
-    System.out.println("heavy=" + heavy);
+    LOGGER.info("heavy={}", heavy);
 
     // Thread safe lazy loader, but with heavy synchronization on each access
     HolderThreadSafe holderThreadSafe = new HolderThreadSafe();
     Heavy another = holderThreadSafe.getHeavy();
-    System.out.println("another=" + another);
+    LOGGER.info("another={}", another);
 
     // The most efficient lazy loader utilizing Java 8 features
     Java8Holder java8Holder = new Java8Holder();
     Heavy next = java8Holder.getHeavy();
-    System.out.println("next=" + next);
+    LOGGER.info("next={}", next);
   }
 }

lazy-loading/src/main/java/com/iluwatar/lazy/loading/Heavy.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.lazy.loading;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Heavy objects are expensive to create.
@@ -29,16 +32,18 @@ package com.iluwatar.lazy.loading;
  */
 public class Heavy {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Heavy.class);
+
   /**
    * Constructor
    */
   public Heavy() {
-    System.out.println("Creating Heavy ...");
+    LOGGER.info("Creating Heavy ...");
     try {
       Thread.sleep(1000);
     } catch (InterruptedException e) {
-      e.printStackTrace();
+      LOGGER.error("Exception caught.", e);
     }
-    System.out.println("... Heavy created");
+    LOGGER.info("... Heavy created");
   }
 }

lazy-loading/src/main/java/com/iluwatar/lazy/loading/HolderNaive.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.lazy.loading;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Simple implementation of the lazy loading idiom. However, this is not thread safe.
@@ -29,13 +32,15 @@ package com.iluwatar.lazy.loading;
  */
 public class HolderNaive {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(HolderNaive.class);
+
   private Heavy heavy;
 
   /**
    * Constructor
    */
   public HolderNaive() {
-    System.out.println("HolderNaive created");
+    LOGGER.info("HolderNaive created");
   }
 
   /**

lazy-loading/src/main/java/com/iluwatar/lazy/loading/HolderThreadSafe.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.lazy.loading;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Same as HolderNaive but with added synchronization. This implementation is thread safe, but each
@@ -30,13 +33,15 @@ package com.iluwatar.lazy.loading;
  */
 public class HolderThreadSafe {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(HolderThreadSafe.class);
+
   private Heavy heavy;
 
   /**
    * Constructor
    */
   public HolderThreadSafe() {
-    System.out.println("HolderThreadSafe created");
+    LOGGER.info("HolderThreadSafe created");
   }
 
   /**

lazy-loading/src/main/java/com/iluwatar/lazy/loading/Java8Holder.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.lazy.loading;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.function.Supplier;
 
 /**
@@ -32,10 +35,12 @@ import java.util.function.Supplier;
  */
 public class Java8Holder {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(Java8Holder.class);
+
   private Supplier<Heavy> heavy = () -> createAndCacheHeavy();
 
   public Java8Holder() {
-    System.out.println("Java8Holder created");
+    LOGGER.info("Java8Holder created");
   }
 
   public Heavy getHeavy() {

mediator/src/main/java/com/iluwatar/mediator/PartyMemberBase.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.mediator;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * Abstract base class for party members.
@@ -29,23 +32,25 @@ package com.iluwatar.mediator;
  */
 public abstract class PartyMemberBase implements PartyMember {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(PartyMemberBase.class);
+
   protected Party party;
 
   @Override
   public void joinedParty(Party party) {
-    System.out.println(this + " joins the party");
+    LOGGER.info("{} joins the party", this);
     this.party = party;
   }
 
   @Override
   public void partyAction(Action action) {
-    System.out.println(this + " " + action.getDescription());
+    LOGGER.info("{} {}", this, action.getDescription());
   }
 
   @Override
   public void act(Action action) {
     if (party != null) {
-      System.out.println(this + " " + action.toString());
+      LOGGER.info("{} {}", this, action);
       party.act(this, action);
     }
   }

mediator/src/test/java/com/iluwatar/mediator/PartyMemberTest.java

@@ -22,22 +22,25 @@
  */
 package com.iluwatar.mediator;
 
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.Parameterized;
+import org.slf4j.LoggerFactory;
 
-import java.io.PrintStream;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.LinkedList;
+import java.util.List;
 import java.util.function.Supplier;
 
 import static org.junit.Assert.assertEquals;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
-import static org.mockito.Mockito.verifyZeroInteractions;
 
 /**
  * Date: 12/19/15 - 10:13 PM
@@ -57,34 +60,6 @@ public class PartyMemberTest {
     );
   }
 
-  /**
-   * The mocked standard out {@link PrintStream}, required since some actions on a {@link
-   * PartyMember} have any influence on any other accessible objects, except for writing to std-out
-   * using {@link System#out}
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
-
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
-  @Before
-  public void setUp() {
-    System.setOut(this.stdOutMock);
-  }
-
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
-  @After
-  public void tearDown() {
-    System.setOut(this.stdOutOrig);
-  }
-
   /**
    * The factory, used to create a new instance of the tested party member
    */
@@ -99,6 +74,18 @@ public class PartyMemberTest {
     this.memberSupplier = memberSupplier;
   }
 
+  private InMemoryAppender appender;
+
+  @Before
+  public void setUp() {
+    appender = new InMemoryAppender(PartyMemberBase.class);
+  }
+
+  @After
+  public void tearDown() {
+    appender.stop();
+  }
+
   /**
    * Verify if a party action triggers the correct output to the std-Out
    */
@@ -108,10 +95,10 @@ public class PartyMemberTest {
 
     for (final Action action : Action.values()) {
       member.partyAction(action);
-      verify(this.stdOutMock).println(member.toString() + " " + action.getDescription());
+      assertEquals(member.toString() + " " + action.getDescription(), appender.getLastMessage());
     }
 
-    verifyNoMoreInteractions(this.stdOutMock);
+    assertEquals(Action.values().length, appender.getLogSize());
   }
 
   /**
@@ -122,19 +109,19 @@ public class PartyMemberTest {
     final PartyMember member = this.memberSupplier.get();
 
     member.act(Action.GOLD);
-    verifyZeroInteractions(this.stdOutMock);
+    assertEquals(0, appender.getLogSize());
 
     final Party party = mock(Party.class);
     member.joinedParty(party);
-    verify(this.stdOutMock).println(member.toString() + " joins the party");
+    assertEquals(member.toString() + " joins the party", appender.getLastMessage());
 
     for (final Action action : Action.values()) {
       member.act(action);
-      verify(this.stdOutMock).println(member.toString() + " " + action.toString());
+      assertEquals(member.toString() + " " + action.toString(), appender.getLastMessage());
       verify(party).act(member, action);
     }
 
-    verifyNoMoreInteractions(party, this.stdOutMock);
+    assertEquals(Action.values().length + 1, appender.getLogSize());
   }
 
   /**
@@ -147,4 +134,27 @@ public class PartyMemberTest {
     assertEquals(memberClass.getSimpleName(), member.toString());
   }
 
+  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+    private List<ILoggingEvent> log = new LinkedList<>();
+
+    public InMemoryAppender(Class clazz) {
+      ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
+      start();
+    }
+
+    @Override
+    protected void append(ILoggingEvent eventObject) {
+      log.add(eventObject);
+    }
+
+    public int getLogSize() {
+      return log.size();
+    }
+
+    public String getLastMessage() {
+      return log.get(log.size() - 1).getFormattedMessage();
+    }
+  }
+
+
 }

memento/src/main/java/com/iluwatar/memento/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.memento;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.Stack;
 
 /**
@@ -45,6 +48,8 @@ import java.util.Stack;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    */
@@ -52,22 +57,22 @@ public class App {
     Stack<StarMemento> states = new Stack<>();
 
     Star star = new Star(StarType.SUN, 10000000, 500000);
-    System.out.println(star);
+    LOGGER.info(star.toString());
     states.add(star.getMemento());
     star.timePasses();
-    System.out.println(star);
+    LOGGER.info(star.toString());
     states.add(star.getMemento());
     star.timePasses();
-    System.out.println(star);
+    LOGGER.info(star.toString());
     states.add(star.getMemento());
     star.timePasses();
-    System.out.println(star);
+    LOGGER.info(star.toString());
     states.add(star.getMemento());
     star.timePasses();
-    System.out.println(star);
+    LOGGER.info(star.toString());
     while (states.size() > 0) {
       star.setMemento(states.pop());
-      System.out.println(star);
+      LOGGER.info(star.toString());
     }
   }
 }

message-channel/src/main/java/com/iluwatar/message/channel/App.java

@@ -25,6 +25,8 @@ package com.iluwatar.message.channel;
 import org.apache.camel.CamelContext;
 import org.apache.camel.builder.RouteBuilder;
 import org.apache.camel.impl.DefaultCamelContext;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * 
@@ -50,6 +52,8 @@ import org.apache.camel.impl.DefaultCamelContext;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point
    */
@@ -66,7 +70,7 @@ public class App {
     });
 
     context.start();
-    context.getRoutes().stream().forEach(r -> System.out.println(r));
+    context.getRoutes().stream().forEach(r -> LOGGER.info(r.toString()));
     context.stop();
   }
 }

model-view-controller/src/main/java/com/iluwatar/model/view/controller/GiantView.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.model.view.controller;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * 
  * GiantView displays the giant
@@ -29,7 +32,9 @@ package com.iluwatar.model.view.controller;
  */
 public class GiantView {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(GiantView.class);
+
   public void displayGiant(GiantModel giant) {
-    System.out.println(giant);
+    LOGGER.info(giant.toString());
   }
 }

model-view-controller/src/test/java/com/iluwatar/model/view/controller/GiantViewTest.java

@@ -22,15 +22,19 @@
  */
 package com.iluwatar.model.view.controller;
 
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.AppenderBase;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.slf4j.LoggerFactory;
 
-import java.io.PrintStream;
+import java.util.LinkedList;
+import java.util.List;
 
+import static org.junit.Assert.assertEquals;
 import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
 
 /**
  * Date: 12/20/15 - 2:04 PM
@@ -39,32 +43,16 @@ import static org.mockito.Mockito.verifyNoMoreInteractions;
  */
 public class GiantViewTest {
 
-  /**
+  private InMemoryAppender appender;
-   * The mocked standard out {@link PrintStream}, required since the actions of the views don't have
-   * any influence on any other accessible objects, except for writing to std-out using {@link
-   * System#out}
-   */
-  private final PrintStream stdOutMock = mock(PrintStream.class);
 
-  /**
-   * Keep the original std-out so it can be restored after the test
-   */
-  private final PrintStream stdOutOrig = System.out;
-
-  /**
-   * Inject the mocked std-out {@link PrintStream} into the {@link System} class before each test
-   */
   @Before
   public void setUp() {
-    System.setOut(this.stdOutMock);
+    appender = new InMemoryAppender(GiantView.class);
   }
 
-  /**
-   * Removed the mocked std-out {@link PrintStream} again from the {@link System} class
-   */
   @After
   public void tearDown() {
-    System.setOut(this.stdOutOrig);
+    appender.stop();
   }
 
   /**
@@ -78,9 +66,29 @@ public class GiantViewTest {
     final GiantModel model = mock(GiantModel.class);
     view.displayGiant(model);
 
-    verify(this.stdOutMock).println(model);
+    assertEquals(model.toString(), appender.getLastMessage());
-    verifyNoMoreInteractions(model, this.stdOutMock);
+    assertEquals(1, appender.getLogSize());
-
   }
 
+  public class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+    private List<ILoggingEvent> log = new LinkedList<>();
+
+    public InMemoryAppender(Class clazz) {
+      ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
+      start();
+    }
+
+    @Override
+    protected void append(ILoggingEvent eventObject) {
+      log.add(eventObject);
+    }
+
+    public String getLastMessage() {
+      return log.get(log.size() - 1).getMessage();
+    }
+
+    public int getLogSize() {
+      return log.size();
+    }
+  }
 }

monad/src/main/java/com/iluwatar/monad/App.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.monad;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.Objects;
 import java.util.function.Function;
 import java.util.function.Predicate;
@@ -42,6 +45,8 @@ import java.util.function.Predicate;
  */
 public class App {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
   /**
    * Program entry point.
    *
@@ -49,7 +54,7 @@ public class App {
    */
   public static void main(String[] args) {
     User user = new User("user", 24, Sex.FEMALE, "foobar.com");
-    System.out.println(Validator.of(user).validate(User::getName, Objects::nonNull, "name is null")
+    LOGGER.info(Validator.of(user).validate(User::getName, Objects::nonNull, "name is null")
         .validate(User::getName, name -> !name.isEmpty(), "name is empty")
         .validate(User::getEmail, email -> !email.contains("@"), "email doesn't containt '@'")
         .validate(User::getAge, age -> age > 20 && age < 30, "age isn't between...").get().toString());