diff --git a/caching/src/main/java/com/iluwatar/caching/LruCache.java b/caching/src/main/java/com/iluwatar/caching/LruCache.java
index 2fc37ce44..fb9127bdc 100644
--- a/caching/src/main/java/com/iluwatar/caching/LruCache.java
+++ b/caching/src/main/java/com/iluwatar/caching/LruCache.java
@@ -167,7 +167,7 @@ public class LruCache {
    * Returns cache data in list form.
    */
   public List<UserAccount> getCacheDataInListForm() {
-    ArrayList<UserAccount> listOfCacheData = new ArrayList<>();
+    List<UserAccount> listOfCacheData = new ArrayList<>();
     Node temp = head;
     while (temp != null) {
       listOfCacheData.add(temp.userAccount);
diff --git a/dao/src/main/java/com/iluwatar/dao/CustomerSchemaSql.java b/dao/src/main/java/com/iluwatar/dao/CustomerSchemaSql.java
index 6b8acffd2..e81e415fe 100644
--- a/dao/src/main/java/com/iluwatar/dao/CustomerSchemaSql.java
+++ b/dao/src/main/java/com/iluwatar/dao/CustomerSchemaSql.java
@@ -22,10 +22,13 @@
  */
 package com.iluwatar.dao;
 
-public interface CustomerSchemaSql {
+public final class CustomerSchemaSql {
 
-  String CREATE_SCHEMA_SQL = "CREATE TABLE CUSTOMERS (ID NUMBER, FNAME VARCHAR(100), " 
+  private CustomerSchemaSql() {}
+
+  public static final String CREATE_SCHEMA_SQL = "CREATE TABLE CUSTOMERS (ID NUMBER, FNAME VARCHAR(100), " 
       + "LNAME VARCHAR(100))";
   
-  String DELETE_SCHEMA_SQL = "DROP TABLE CUSTOMERS";
+  public static final String DELETE_SCHEMA_SQL = "DROP TABLE CUSTOMERS";
+  
 }
diff --git a/factory-kit/src/main/java/com/iluwatar/factorykit/WeaponFactory.java b/factory-kit/src/main/java/com/iluwatar/factorykit/WeaponFactory.java
index 4e8d1f77d..8ee9c6c39 100644
--- a/factory-kit/src/main/java/com/iluwatar/factorykit/WeaponFactory.java
+++ b/factory-kit/src/main/java/com/iluwatar/factorykit/WeaponFactory.java
@@ -23,6 +23,7 @@
 package com.iluwatar.factorykit;
 
 import java.util.HashMap;
+import java.util.Map;
 import java.util.function.Consumer;
 import java.util.function.Supplier;
 
@@ -48,7 +49,7 @@ public interface WeaponFactory {
    * @return factory with specified {@link Builder}s
    */
   static WeaponFactory factory(Consumer<Builder> consumer) {
-    HashMap<WeaponType, Supplier<Weapon>> map = new HashMap<>();
+    Map<WeaponType, Supplier<Weapon>> map = new HashMap<>();
     consumer.accept(map::put);
     return name -> map.get(name).get();
   }
diff --git a/hexagonal/src/main/java/com/iluwatar/hexagonal/banking/MongoBank.java b/hexagonal/src/main/java/com/iluwatar/hexagonal/banking/MongoBank.java
index fd0c1086f..7ccc829ed 100644
--- a/hexagonal/src/main/java/com/iluwatar/hexagonal/banking/MongoBank.java
+++ b/hexagonal/src/main/java/com/iluwatar/hexagonal/banking/MongoBank.java
@@ -29,6 +29,7 @@ import com.mongodb.client.model.UpdateOptions;
 import org.bson.Document;
 
 import java.util.ArrayList;
+import java.util.List;
 
 /**
  * Mongo based banking adapter
@@ -110,7 +111,7 @@ public class MongoBank implements WireTransfers {
   @Override
   public int getFunds(String bankAccount) {
     Document search = new Document("_id", bankAccount);
-    ArrayList<Document> results = accountsCollection.find(search).limit(1).into(new ArrayList<Document>());
+    List<Document> results = accountsCollection.find(search).limit(1).into(new ArrayList<Document>());
     if (results.size() > 0) {
       return results.get(0).getInteger("funds");
     } else {
diff --git a/hexagonal/src/main/java/com/iluwatar/hexagonal/database/MongoTicketRepository.java b/hexagonal/src/main/java/com/iluwatar/hexagonal/database/MongoTicketRepository.java
index a68eee59a..ac747d4ea 100644
--- a/hexagonal/src/main/java/com/iluwatar/hexagonal/database/MongoTicketRepository.java
+++ b/hexagonal/src/main/java/com/iluwatar/hexagonal/database/MongoTicketRepository.java
@@ -35,8 +35,10 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
 
 /**
  * Mongo lottery ticket database
@@ -142,7 +144,7 @@ public class MongoTicketRepository implements LotteryTicketRepository {
   @Override
   public Optional<LotteryTicket> findById(LotteryTicketId id) {
     Document find = new Document("ticketId", id.getId());
-    ArrayList<Document> results = ticketsCollection.find(find).limit(1).into(new ArrayList<Document>());
+    List<Document> results = ticketsCollection.find(find).limit(1).into(new ArrayList<Document>());
     if (results.size() > 0) {
       LotteryTicket lotteryTicket = docToTicket(results.get(0));
       return Optional.of(lotteryTicket);
@@ -166,7 +168,7 @@ public class MongoTicketRepository implements LotteryTicketRepository {
   @Override
   public Map<LotteryTicketId, LotteryTicket> findAll() {
     Map<LotteryTicketId, LotteryTicket> map = new HashMap<>();
-    ArrayList<Document> docs = ticketsCollection.find(new Document()).into(new ArrayList<Document>());
+    List<Document> docs = ticketsCollection.find(new Document()).into(new ArrayList<Document>());
     for (Document doc: docs) {
       LotteryTicket lotteryTicket = docToTicket(doc);
       map.put(lotteryTicket.getId(), lotteryTicket);
@@ -183,7 +185,7 @@ public class MongoTicketRepository implements LotteryTicketRepository {
     PlayerDetails playerDetails = new PlayerDetails(doc.getString("email"), doc.getString("bank"),
         doc.getString("phone"));
     int[] numArray = Arrays.asList(doc.getString("numbers").split(",")).stream().mapToInt(Integer::parseInt).toArray();
-    HashSet<Integer> numbers = new HashSet<>();
+    Set<Integer> numbers = new HashSet<>();
     for (int num: numArray) {
       numbers.add(num);
     }
diff --git a/layers/src/main/java/com/iluwatar/layers/CakeBakingServiceImpl.java b/layers/src/main/java/com/iluwatar/layers/CakeBakingServiceImpl.java
index 85a362d90..e8deee73a 100644
--- a/layers/src/main/java/com/iluwatar/layers/CakeBakingServiceImpl.java
+++ b/layers/src/main/java/com/iluwatar/layers/CakeBakingServiceImpl.java
@@ -163,7 +163,7 @@ public class CakeBakingServiceImpl implements CakeBakingService {
       CakeToppingInfo cakeToppingInfo =
           new CakeToppingInfo(cake.getTopping().getId(), cake.getTopping().getName(), cake
               .getTopping().getCalories());
-      ArrayList<CakeLayerInfo> cakeLayerInfos = new ArrayList<>();
+      List<CakeLayerInfo> cakeLayerInfos = new ArrayList<>();
       for (CakeLayer layer : cake.getLayers()) {
         cakeLayerInfos.add(new CakeLayerInfo(layer.getId(), layer.getName(), layer.getCalories()));
       }
diff --git a/module/error.txt b/module/error.txt
new file mode 100644
index 000000000..e69de29bb
diff --git a/module/output.txt b/module/output.txt
new file mode 100644
index 000000000..e69de29bb
diff --git a/object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java b/object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java
index 790641690..510d9dc88 100644
--- a/object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java
+++ b/object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java
@@ -23,6 +23,7 @@
 package com.iluwatar.object.pool;
 
 import java.util.HashSet;
+import java.util.Set;
 
 /**
  * 
@@ -30,8 +31,8 @@ import java.util.HashSet;
  */
 public abstract class ObjectPool<T> {
 
-  private HashSet<T> available = new HashSet<>();
-  private HashSet<T> inUse = new HashSet<>();
+  private Set<T> available = new HashSet<>();
+  private Set<T> inUse = new HashSet<>();
 
   protected abstract T create();
 
diff --git a/semaphore/src/main/java/com/iluwatar/semaphore/FruitBowl.java b/semaphore/src/main/java/com/iluwatar/semaphore/FruitBowl.java
index 4f527f01c..a53672109 100644
--- a/semaphore/src/main/java/com/iluwatar/semaphore/FruitBowl.java
+++ b/semaphore/src/main/java/com/iluwatar/semaphore/FruitBowl.java
@@ -22,6 +22,7 @@
  */
 package com.iluwatar.semaphore;
 
+import java.util.List;
 import java.util.ArrayList;
 
 /**
@@ -29,7 +30,7 @@ import java.util.ArrayList;
  */
 public class FruitBowl {
     
-  private ArrayList<Fruit> fruit = new ArrayList<>();
+  private List<Fruit> fruit = new ArrayList<>();
 
   /**
    * 
diff --git a/servant/src/main/java/com/iluwatar/servant/App.java b/servant/src/main/java/com/iluwatar/servant/App.java
index 33a57e367..0e65e1de2 100644
--- a/servant/src/main/java/com/iluwatar/servant/App.java
+++ b/servant/src/main/java/com/iluwatar/servant/App.java
@@ -26,6 +26,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.util.ArrayList;
+import java.util.List;
 
 
 /**
@@ -58,7 +59,7 @@ public class App {
     King k = new King();
     Queen q = new Queen();
 
-    ArrayList<Royalty> guests = new ArrayList<>();
+    List<Royalty> guests = new ArrayList<>();
     guests.add(k);
     guests.add(q);
 
diff --git a/servant/src/test/java/com/iluwatar/servant/ServantTest.java b/servant/src/test/java/com/iluwatar/servant/ServantTest.java
index e2e61e899..900ccc418 100644
--- a/servant/src/test/java/com/iluwatar/servant/ServantTest.java
+++ b/servant/src/test/java/com/iluwatar/servant/ServantTest.java
@@ -25,6 +25,7 @@ package com.iluwatar.servant;
 import org.junit.Test;
 
 import java.util.ArrayList;
+import java.util.List;
 
 import static org.junit.Assert.*;
 import static org.mockito.Mockito.mock;
@@ -74,12 +75,12 @@ public class ServantTest {
     final Royalty badMoodRoyalty = mock(Royalty.class);
     when(badMoodRoyalty.getMood()).thenReturn(true);
 
-    final ArrayList<Royalty> goodCompany = new ArrayList<>();
+    final List<Royalty> goodCompany = new ArrayList<>();
     goodCompany.add(goodMoodRoyalty);
     goodCompany.add(goodMoodRoyalty);
     goodCompany.add(goodMoodRoyalty);
 
-    final ArrayList<Royalty> badCompany = new ArrayList<>();
+    final List<Royalty> badCompany = new ArrayList<>();
     goodCompany.add(goodMoodRoyalty);
     goodCompany.add(goodMoodRoyalty);
     goodCompany.add(badMoodRoyalty);
diff --git a/singleton/src/main/java/com/iluwatar/singleton/IvoryTower.java b/singleton/src/main/java/com/iluwatar/singleton/IvoryTower.java
index ed7d44705..37917bda8 100644
--- a/singleton/src/main/java/com/iluwatar/singleton/IvoryTower.java
+++ b/singleton/src/main/java/com/iluwatar/singleton/IvoryTower.java
@@ -27,16 +27,16 @@ package com.iluwatar.singleton;
  */
 public final class IvoryTower {
 
-  /**
-   * Static to class instance of the class.
-   */
-  private static final IvoryTower INSTANCE = new IvoryTower();
-
   /**
    * Private constructor so nobody can instantiate the class.
    */
   private IvoryTower() {}
 
+  /**
+   * Static to class instance of the class.
+   */
+  private static final IvoryTower INSTANCE = new IvoryTower();
+
   /**
    * To be called by user to obtain instance of the class.
    *