- * One of the risks of this pattern is that bugs resulting from setting a singleton up in a + *
Singleton pattern ensures that the class can have only one existing instance per Java + * classloader instance and provides global access to it.
+ * + *One of the risks of this pattern is that bugs resulting from setting a singleton up in a * distributed environment can be tricky to debug, since it will work fine if you debug with a * single classloader. Additionally, these problems can crop up a while after the implementation of * a singleton, since they may start out synchronous and only become async with time, so you it may - * not be clear why you are seeing certain changes in behaviour. - *
- * There are many ways to implement the Singleton. The first one is the eagerly initialized instance - * in {@link IvoryTower}. Eager initialization implies that the implementation is thread safe. If - * you can afford giving up control of the instantiation moment, then this implementation will suit - * you fine. - *
- * The other option to implement eagerly initialized Singleton is enum based Singleton. The example - * is found in {@link EnumIvoryTower}. At first glance the code looks short and simple. However, you - * should be aware of the downsides including committing to implementation strategy, extending the - * enum class, serializability and restrictions to coding. These are extensively discussed in Stack - * Overflow: + * not be clear why you are seeing certain changes in behaviour.
+ * + *There are many ways to implement the Singleton. The first one is the eagerly initialized + * instance in {@link IvoryTower}. Eager initialization implies that the implementation is thread + * safe. If you can afford giving up control of the instantiation moment, then this implementation + * will suit you fine.
+ * + *The other option to implement eagerly initialized Singleton is enum based Singleton. The + * example is found in {@link EnumIvoryTower}. At first glance the code looks short and simple. + * However, you should be aware of the downsides including committing to implementation strategy, + * extending the enum class, serializability and restrictions to coding. These are extensively + * discussed in Stack Overflow: * http://programmers.stackexchange.com/questions/179386/what-are-the-downsides-of-implementing - * -a-singleton-with-javas-enum - *
- * {@link ThreadSafeLazyLoadedIvoryTower} is a Singleton implementation that is initialized on + * -a-singleton-with-javas-enum
+ * + *{@link ThreadSafeLazyLoadedIvoryTower} is a Singleton implementation that is initialized on * demand. The downside is that it is very slow to access since the whole access method is - * synchronized. - *
- * Another Singleton implementation that is initialized on demand is found in + * synchronized.
+ * + *Another Singleton implementation that is initialized on demand is found in * {@link ThreadSafeDoubleCheckLocking}. It is somewhat faster than * {@link ThreadSafeLazyLoadedIvoryTower} since it doesn't synchronize the whole access method but - * only the method internals on specific conditions. - *
- * Yet another way to implement thread safe lazily initialized Singleton can be found in + * only the method internals on specific conditions.
+ * + *Yet another way to implement thread safe lazily initialized Singleton can be found in * {@link InitializingOnDemandHolderIdiom}. However, this implementation requires at least Java 8 - * API level to work. + * API level to work.
*/ public class App { diff --git a/singleton/src/main/java/com/iluwatar/singleton/EnumIvoryTower.java b/singleton/src/main/java/com/iluwatar/singleton/EnumIvoryTower.java index 2028b7d76..753201dba 100644 --- a/singleton/src/main/java/com/iluwatar/singleton/EnumIvoryTower.java +++ b/singleton/src/main/java/com/iluwatar/singleton/EnumIvoryTower.java @@ -24,10 +24,10 @@ package com.iluwatar.singleton; /** - * Enum based singleton implementation. Effective Java 2nd Edition (Joshua Bloch) p. 18 + *Enum based singleton implementation. Effective Java 2nd Edition (Joshua Bloch) p. 18
* - * This implementation is thread safe, however adding any other method and its thread safety - * is developers responsibility. + *This implementation is thread safe, however adding any other method and its thread safety + * is developers responsibility.
*/ public enum EnumIvoryTower { diff --git a/singleton/src/main/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiom.java b/singleton/src/main/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiom.java index dfd4951f7..faf53a63e 100644 --- a/singleton/src/main/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiom.java +++ b/singleton/src/main/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiom.java @@ -24,16 +24,16 @@ package com.iluwatar.singleton; /** - * The Initialize-on-demand-holder idiom is a secure way of creating a lazy initialized singleton - * object in Java. - *- * The technique is as lazy as possible and works in all known versions of Java. It takes advantage - * of language guarantees about class initialization, and will therefore work correctly in all - * Java-compliant compilers and virtual machines. - *
- * The inner class is referenced no earlier (and therefore loaded no earlier by the class loader) than - * the moment that getInstance() is called. Thus, this solution is thread-safe without requiring special - * language constructs (i.e. volatile or synchronized). + *
The Initialize-on-demand-holder idiom is a secure way of creating a lazy initialized singleton + * object in Java.
+ * + *The technique is as lazy as possible and works in all known versions of Java. It takes + * advantage of language guarantees about class initialization, and will therefore work correctly + * in all Java-compliant compilers and virtual machines.
+ * + *The inner class is referenced no earlier (and therefore loaded no earlier by the class loader) + * than the moment that getInstance() is called. Thus, this solution is thread-safe without + * requiring special language constructs (i.e. volatile or synchronized).
* */ public final class InitializingOnDemandHolderIdiom { @@ -44,6 +44,8 @@ public final class InitializingOnDemandHolderIdiom { private InitializingOnDemandHolderIdiom() {} /** + * Sigleton instance. + * * @return Singleton instance */ public static InitializingOnDemandHolderIdiom getInstance() { diff --git a/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java b/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java index a1ae935a3..f5d47380f 100644 --- a/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java +++ b/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java @@ -24,11 +24,11 @@ package com.iluwatar.singleton; /** - * Double check locking - *- * http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html - *
- * Broken under Java 1.4. + *
Double check locking.
+ * + *http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
+ * + *Broken under Java 1.4.
* * @author mortezaadi@gmail.com */ @@ -60,17 +60,21 @@ public final class ThreadSafeDoubleCheckLocking { // Joshua Bloch "Effective Java, Second Edition", p. 283-284 var result = instance; - // Check if singleton instance is initialized. If it is initialized then we can return the instance. + // Check if singleton instance is initialized. + // If it is initialized then we can return the instance. if (result == null) { - // It is not initialized but we cannot be sure because some other thread might have initialized it - // in the meanwhile. So to make sure we need to lock on an object to get mutual exclusion. + // It is not initialized but we cannot be sure because some other thread might have + // initialized it in the meanwhile. + // So to make sure we need to lock on an object to get mutual exclusion. synchronized (ThreadSafeDoubleCheckLocking.class) { - // Again assign the instance to local variable to check if it was initialized by some other thread - // while current thread was blocked to enter the locked zone. If it was initialized then we can - // return the previously created instance just like the previous null check. + // Again assign the instance to local variable to check if it was initialized by some + // other thread while current thread was blocked to enter the locked zone. + // If it was initialized then we can return the previously created instance + // just like the previous null check. result = instance; if (result == null) { - // The instance is still not initialized so we can safely (no other thread can enter this zone) + // The instance is still not initialized so we can safely + // (no other thread can enter this zone) // create an instance and make it our singleton instance. instance = result = new ThreadSafeDoubleCheckLocking(); } diff --git a/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java b/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java index 82580acf6..dd6d05fea 100644 --- a/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java +++ b/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java @@ -24,11 +24,11 @@ package com.iluwatar.singleton; /** - * Thread-safe Singleton class. The instance is lazily initialized and thus needs synchronization - * mechanism. + *Thread-safe Singleton class. The instance is lazily initialized and thus needs synchronization + * mechanism.
* - * Note: if created by reflection then a singleton will not be created but multiple options in the - * same classloader + *Note: if created by reflection then a singleton will not be created but multiple options + * in the same classloader
*/ public final class ThreadSafeLazyLoadedIvoryTower { diff --git a/singleton/src/test/java/com/iluwatar/singleton/AppTest.java b/singleton/src/test/java/com/iluwatar/singleton/AppTest.java index 66974e472..75043eae0 100644 --- a/singleton/src/test/java/com/iluwatar/singleton/AppTest.java +++ b/singleton/src/test/java/com/iluwatar/singleton/AppTest.java @@ -26,9 +26,7 @@ package com.iluwatar.singleton; import org.junit.jupiter.api.Test; /** - * - * Application test - * + * Application test. */ public class AppTest { diff --git a/singleton/src/test/java/com/iluwatar/singleton/EnumIvoryTowerTest.java b/singleton/src/test/java/com/iluwatar/singleton/EnumIvoryTowerTest.java index 49dfae6b0..6a19ca752 100644 --- a/singleton/src/test/java/com/iluwatar/singleton/EnumIvoryTowerTest.java +++ b/singleton/src/test/java/com/iluwatar/singleton/EnumIvoryTowerTest.java @@ -24,14 +24,14 @@ package com.iluwatar.singleton; /** - * Date: 12/29/15 - 19:20 PM + * Date: 12/29/15 - 19:20 PM. * * @author Jeroen Meulemeester */ public class EnumIvoryTowerTest extends SingletonTestThis class provides several test case that test singleton construction.
* - * The first proves that multiple calls to the singleton getInstance object are the same when called - * in the SAME thread. The second proves that multiple calls to the singleton getInstance object are - * the same when called in the DIFFERENT thread. + *The first proves that multiple calls to the singleton getInstance object are the same when + * called in the SAME thread. The second proves that multiple calls to the singleton getInstance + * object are the same when called in the DIFFERENT thread.
+ * + *Date: 12/29/15 - 19:25 PM
* - * Date: 12/29/15 - 19:25 PM * @param- * Before Java 8 the Strategies needed to be separate classes forcing the developer + *
The Strategy pattern (also known as the policy pattern) is a software design pattern that + * enables an algorithm's behavior to be selected at runtime.
+ * + *Before Java 8 the Strategies needed to be separate classes forcing the developer * to write lots of boilerplate code. With modern Java it is easy to pass behavior - * with method references and lambdas making the code shorter and more readable. - *
- * In this example ({@link DragonSlayingStrategy}) encapsulates an algorithm. The containing object - * ({@link DragonSlayer}) can alter its behavior by changing its strategy. + * with method references and lambdas making the code shorter and more readable.
+ * + *In this example ({@link DragonSlayingStrategy}) encapsulates an algorithm. The containing + * object ({@link DragonSlayer}) can alter its behavior by changing its strategy.
* */ public class App { @@ -44,7 +44,7 @@ public class App { private static final Logger LOGGER = LoggerFactory.getLogger(App.class); /** - * Program entry point + * Program entry point. * * @param args command line args */ diff --git a/strategy/src/main/java/com/iluwatar/strategy/DragonSlayer.java b/strategy/src/main/java/com/iluwatar/strategy/DragonSlayer.java index 0455edaca..f6b91d967 100644 --- a/strategy/src/main/java/com/iluwatar/strategy/DragonSlayer.java +++ b/strategy/src/main/java/com/iluwatar/strategy/DragonSlayer.java @@ -24,9 +24,7 @@ package com.iluwatar.strategy; /** - * * DragonSlayer uses different strategies to slay the dragon. - * */ public class DragonSlayer { diff --git a/strategy/src/main/java/com/iluwatar/strategy/DragonSlayingStrategy.java b/strategy/src/main/java/com/iluwatar/strategy/DragonSlayingStrategy.java index eb89523ad..537b521f4 100644 --- a/strategy/src/main/java/com/iluwatar/strategy/DragonSlayingStrategy.java +++ b/strategy/src/main/java/com/iluwatar/strategy/DragonSlayingStrategy.java @@ -24,9 +24,7 @@ package com.iluwatar.strategy; /** - * * Strategy interface. - * */ @FunctionalInterface public interface DragonSlayingStrategy { diff --git a/strategy/src/main/java/com/iluwatar/strategy/MeleeStrategy.java b/strategy/src/main/java/com/iluwatar/strategy/MeleeStrategy.java index 8cb2f24c1..12f467b07 100644 --- a/strategy/src/main/java/com/iluwatar/strategy/MeleeStrategy.java +++ b/strategy/src/main/java/com/iluwatar/strategy/MeleeStrategy.java @@ -27,9 +27,7 @@ import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** - * * Melee strategy. - * */ public class MeleeStrategy implements DragonSlayingStrategy { diff --git a/strategy/src/main/java/com/iluwatar/strategy/ProjectileStrategy.java b/strategy/src/main/java/com/iluwatar/strategy/ProjectileStrategy.java index 4b6031ddf..769b0d7d9 100644 --- a/strategy/src/main/java/com/iluwatar/strategy/ProjectileStrategy.java +++ b/strategy/src/main/java/com/iluwatar/strategy/ProjectileStrategy.java @@ -27,9 +27,7 @@ import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** - * * Projectile strategy. - * */ public class ProjectileStrategy implements DragonSlayingStrategy { diff --git a/strategy/src/main/java/com/iluwatar/strategy/SpellStrategy.java b/strategy/src/main/java/com/iluwatar/strategy/SpellStrategy.java index ffe85c7a2..dfa47f72b 100644 --- a/strategy/src/main/java/com/iluwatar/strategy/SpellStrategy.java +++ b/strategy/src/main/java/com/iluwatar/strategy/SpellStrategy.java @@ -27,9 +27,7 @@ import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** - * * Spell strategy. - * */ public class SpellStrategy implements DragonSlayingStrategy { diff --git a/strategy/src/test/java/com/iluwatar/strategy/AppTest.java b/strategy/src/test/java/com/iluwatar/strategy/AppTest.java index 598085ce4..713eecf43 100644 --- a/strategy/src/test/java/com/iluwatar/strategy/AppTest.java +++ b/strategy/src/test/java/com/iluwatar/strategy/AppTest.java @@ -26,9 +26,7 @@ package com.iluwatar.strategy; import org.junit.jupiter.api.Test; /** - * - * Application test - * + * Application test. */ public class AppTest { diff --git a/strategy/src/test/java/com/iluwatar/strategy/DragonSlayerTest.java b/strategy/src/test/java/com/iluwatar/strategy/DragonSlayerTest.java index 52dfb3ff1..c95612683 100644 --- a/strategy/src/test/java/com/iluwatar/strategy/DragonSlayerTest.java +++ b/strategy/src/test/java/com/iluwatar/strategy/DragonSlayerTest.java @@ -23,21 +23,21 @@ package com.iluwatar.strategy; -import org.junit.jupiter.api.Test; - import static org.mockito.Mockito.mock; import static org.mockito.Mockito.verify; import static org.mockito.Mockito.verifyNoMoreInteractions; +import org.junit.jupiter.api.Test; + /** - * Date: 12/29/15 - 10:50 PM + * Date: 12/29/15 - 10:50 PM. * * @author Jeroen Meulemeester */ public class DragonSlayerTest { /** - * Verify if the dragon slayer uses the strategy during battle + * Verify if the dragon slayer uses the strategy during battle. */ @Test public void testGoToBattle() { @@ -50,7 +50,7 @@ public class DragonSlayerTest { } /** - * Verify if the dragon slayer uses the new strategy during battle after a change of strategy + * Verify if the dragon slayer uses the new strategy during battle after a change of strategy. */ @Test public void testChangeStrategy() { diff --git a/strategy/src/test/java/com/iluwatar/strategy/DragonSlayingStrategyTest.java b/strategy/src/test/java/com/iluwatar/strategy/DragonSlayingStrategyTest.java index 67e4b92cc..15106cdd9 100644 --- a/strategy/src/test/java/com/iluwatar/strategy/DragonSlayingStrategyTest.java +++ b/strategy/src/test/java/com/iluwatar/strategy/DragonSlayingStrategyTest.java @@ -23,45 +23,48 @@ package com.iluwatar.strategy; +import static org.junit.jupiter.api.Assertions.assertEquals; + import ch.qos.logback.classic.Logger; import ch.qos.logback.classic.spi.ILoggingEvent; import ch.qos.logback.core.AppenderBase; + +import java.util.Collection; +import java.util.LinkedList; +import java.util.List; + import org.junit.jupiter.api.AfterEach; import org.junit.jupiter.api.BeforeEach; import org.junit.jupiter.params.ParameterizedTest; import org.junit.jupiter.params.provider.MethodSource; import org.slf4j.LoggerFactory; -import java.util.Collection; -import java.util.LinkedList; -import java.util.List; - -import static org.junit.jupiter.api.Assertions.assertEquals; - /** - * Date: 12/29/15 - 10:58 PM + * Date: 12/29/15 - 10:58 PM. * * @author Jeroen Meulemeester */ public class DragonSlayingStrategyTest { /** + * Assembles test parameters. + * * @return The test parameters for each cycle */ static Collection- * In this example there is a unit hierarchy beginning from {@link Commander}. This hierarchy is + *
Visitor pattern defines mechanism to apply operations on nodes in hierarchy. New operations + * can be added without altering the node interface.
+ * + *In this example there is a unit hierarchy beginning from {@link Commander}. This hierarchy is * traversed by visitors. {@link SoldierVisitor} applies its operation on {@link Soldier}s, - * {@link SergeantVisitor} on {@link Sergeant}s and so on. + * {@link SergeantVisitor} on {@link Sergeant}s and so on.
* */ public class App { /** - * Program entry point + * Program entry point. * * @param args command line args */ diff --git a/visitor/src/main/java/com/iluwatar/visitor/Commander.java b/visitor/src/main/java/com/iluwatar/visitor/Commander.java index 938c42f8d..782d0116c 100644 --- a/visitor/src/main/java/com/iluwatar/visitor/Commander.java +++ b/visitor/src/main/java/com/iluwatar/visitor/Commander.java @@ -24,9 +24,7 @@ package com.iluwatar.visitor; /** - * - * Commander - * + * Commander. */ public class Commander extends Unit { diff --git a/visitor/src/main/java/com/iluwatar/visitor/CommanderVisitor.java b/visitor/src/main/java/com/iluwatar/visitor/CommanderVisitor.java index 19ea5846e..c34c8ed9e 100644 --- a/visitor/src/main/java/com/iluwatar/visitor/CommanderVisitor.java +++ b/visitor/src/main/java/com/iluwatar/visitor/CommanderVisitor.java @@ -27,9 +27,7 @@ import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** - * - * CommanderVisitor - * + * CommanderVisitor. */ public class CommanderVisitor implements UnitVisitor { diff --git a/visitor/src/main/java/com/iluwatar/visitor/Sergeant.java b/visitor/src/main/java/com/iluwatar/visitor/Sergeant.java index 95de25772..bdc96892a 100644 --- a/visitor/src/main/java/com/iluwatar/visitor/Sergeant.java +++ b/visitor/src/main/java/com/iluwatar/visitor/Sergeant.java @@ -24,9 +24,7 @@ package com.iluwatar.visitor; /** - * - * Sergeant - * + * Sergeant. */ public class Sergeant extends Unit { diff --git a/visitor/src/main/java/com/iluwatar/visitor/SergeantVisitor.java b/visitor/src/main/java/com/iluwatar/visitor/SergeantVisitor.java index fdcdce5ca..59fb405e5 100644 --- a/visitor/src/main/java/com/iluwatar/visitor/SergeantVisitor.java +++ b/visitor/src/main/java/com/iluwatar/visitor/SergeantVisitor.java @@ -27,9 +27,7 @@ import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** - * - * SergeantVisitor - * + * SergeantVisitor. */ public class SergeantVisitor implements UnitVisitor { diff --git a/visitor/src/main/java/com/iluwatar/visitor/Soldier.java b/visitor/src/main/java/com/iluwatar/visitor/Soldier.java index 7ab4a26b9..a2a812416 100644 --- a/visitor/src/main/java/com/iluwatar/visitor/Soldier.java +++ b/visitor/src/main/java/com/iluwatar/visitor/Soldier.java @@ -24,9 +24,7 @@ package com.iluwatar.visitor; /** - * - * Soldier - * + * Soldier. */ public class Soldier extends Unit { diff --git a/visitor/src/main/java/com/iluwatar/visitor/SoldierVisitor.java b/visitor/src/main/java/com/iluwatar/visitor/SoldierVisitor.java index 6585246a0..1f19a6458 100644 --- a/visitor/src/main/java/com/iluwatar/visitor/SoldierVisitor.java +++ b/visitor/src/main/java/com/iluwatar/visitor/SoldierVisitor.java @@ -27,9 +27,7 @@ import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** - * - * SoldierVisitor - * + * SoldierVisitor. */ public class SoldierVisitor implements UnitVisitor { diff --git a/visitor/src/main/java/com/iluwatar/visitor/Unit.java b/visitor/src/main/java/com/iluwatar/visitor/Unit.java index 49ddb8a89..318e84262 100644 --- a/visitor/src/main/java/com/iluwatar/visitor/Unit.java +++ b/visitor/src/main/java/com/iluwatar/visitor/Unit.java @@ -24,9 +24,7 @@ package com.iluwatar.visitor; /** - * * Interface for the nodes in hierarchy. - * */ public abstract class Unit { @@ -37,7 +35,7 @@ public abstract class Unit { } /** - * Accept visitor + * Accept visitor. */ public void accept(UnitVisitor visitor) { for (var child : children) { diff --git a/visitor/src/main/java/com/iluwatar/visitor/UnitVisitor.java b/visitor/src/main/java/com/iluwatar/visitor/UnitVisitor.java index 086075e5e..8300fd1fe 100644 --- a/visitor/src/main/java/com/iluwatar/visitor/UnitVisitor.java +++ b/visitor/src/main/java/com/iluwatar/visitor/UnitVisitor.java @@ -24,9 +24,7 @@ package com.iluwatar.visitor; /** - * * Visitor interface. - * */ public interface UnitVisitor { diff --git a/visitor/src/test/java/com/iluwatar/visitor/AppTest.java b/visitor/src/test/java/com/iluwatar/visitor/AppTest.java index fb2fdd123..45804a9c6 100644 --- a/visitor/src/test/java/com/iluwatar/visitor/AppTest.java +++ b/visitor/src/test/java/com/iluwatar/visitor/AppTest.java @@ -26,9 +26,7 @@ package com.iluwatar.visitor; import org.junit.jupiter.api.Test; /** - * - * Application test - * + * Application test. */ public class AppTest { diff --git a/visitor/src/test/java/com/iluwatar/visitor/CommanderTest.java b/visitor/src/test/java/com/iluwatar/visitor/CommanderTest.java index f3cf823fc..256a0cdbd 100644 --- a/visitor/src/test/java/com/iluwatar/visitor/CommanderTest.java +++ b/visitor/src/test/java/com/iluwatar/visitor/CommanderTest.java @@ -27,14 +27,14 @@ import static org.mockito.Matchers.eq; import static org.mockito.Mockito.verify; /** - * Date: 12/30/15 - 19:45 PM + * Date: 12/30/15 - 19:45 PM. * * @author Jeroen Meulemeester */ public class CommanderTest extends UnitTest