#1469 Add lambda expressions implementation for Strategy Pattern (#1631)

* #1469 Add Strategy Pattern lambda implementation

After Java 8.
Also take advantage of enum

* #1469 Update class diagrams

* #1469 Update README.md

Add lambda programmatic example

* #1469 Remove unused imports

Co-authored-by: Subhrodip Mohanta <subhrodipmohanta@gmail.com>

strategy/README.md

@@ -122,6 +122,51 @@ Program output:
     You cast the spell of disintegration and the dragon vaporizes in a pile of dust!    
 ```
 
+What's more, the new feature Lambda Expressions in Java 8 provides another approach for the implementation:
+
+```java
+public class LambdaStrategy {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(LambdaStrategy.class);
+
+  public enum Strategy implements DragonSlayingStrategy {
+    MeleeStrategy(() -> LOGGER.info(
+        "With your Excalibur you severe the dragon's head!")),
+    ProjectileStrategy(() -> LOGGER.info(
+        "You shoot the dragon with the magical crossbow and it falls dead on the ground!")),
+    SpellStrategy(() -> LOGGER.info(
+        "You cast the spell of disintegration and the dragon vaporizes in a pile of dust!"));
+
+    private final DragonSlayingStrategy dragonSlayingStrategy;
+
+    Strategy(DragonSlayingStrategy dragonSlayingStrategy) {
+      this.dragonSlayingStrategy = dragonSlayingStrategy;
+    }
+
+    @Override
+    public void execute() {
+      dragonSlayingStrategy.execute();
+    }
+  }
+}
+```
+
+And here's the dragonslayer in action.
+
+```java
+    LOGGER.info("Green dragon spotted ahead!");
+    dragonSlayer.changeStrategy(LambdaStrategy.Strategy.MeleeStrategy);
+    dragonSlayer.goToBattle();
+    LOGGER.info("Red dragon emerges.");
+    dragonSlayer.changeStrategy(LambdaStrategy.Strategy.ProjectileStrategy);
+    dragonSlayer.goToBattle();
+    LOGGER.info("Black dragon lands before you.");
+    dragonSlayer.changeStrategy(LambdaStrategy.Strategy.SpellStrategy);
+    dragonSlayer.goToBattle();
+```
+
+Program output is the same as above one.
+
 ## Class diagram
 
 ![alt text](./etc/strategy_urm.png "Strategy")

strategy/etc/strategy.urm.puml

@@ -14,6 +14,19 @@ package com.iluwatar.strategy {
   interface DragonSlayingStrategy {
     + execute() {abstract}
   }
+  class LambdaStrategy {
+    - LOGGER : Logger {static}
+    + LambdaStrategy()
+  }
+  enum Strategy {
+    + MeleeStrategy {static}
+    + ProjectileStrategy {static}
+    + SpellStrategy {static}
+    - dragonSlayingStrategy : DragonSlayingStrategy
+    + execute()
+    + valueOf(name : String) : Strategy {static}
+    + values() : Strategy[] {static}
+  }
   class MeleeStrategy {
     - LOGGER : Logger {static}
     + MeleeStrategy()
@@ -30,7 +43,10 @@ package com.iluwatar.strategy {
     + execute()
   }
 }
+Strategy ..+ LambdaStrategy
+Strategy -->  "-dragonSlayingStrategy" DragonSlayingStrategy
 DragonSlayer -->  "-strategy" DragonSlayingStrategy
+Strategy ..|> DragonSlayingStrategy 
 MeleeStrategy ..|> DragonSlayingStrategy 
 ProjectileStrategy ..|> DragonSlayingStrategy 
 SpellStrategy ..|> DragonSlayingStrategy 

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

@@ -60,7 +60,7 @@ public class App {
     dragonSlayer.changeStrategy(new SpellStrategy());
     dragonSlayer.goToBattle();
 
-    // Java 8 Strategy pattern
+    // Java 8 functional implementation Strategy pattern
     LOGGER.info("Green dragon spotted ahead!");
     dragonSlayer = new DragonSlayer(
         () -> LOGGER.info("With your Excalibur you severe the dragon's head!"));
@@ -73,5 +73,16 @@ public class App {
     dragonSlayer.changeStrategy(() -> LOGGER.info(
         "You cast the spell of disintegration and the dragon vaporizes in a pile of dust!"));
     dragonSlayer.goToBattle();
+
+    // Java 8 lambda implementation with enum Strategy pattern
+    LOGGER.info("Green dragon spotted ahead!");
+    dragonSlayer.changeStrategy(LambdaStrategy.Strategy.MeleeStrategy);
+    dragonSlayer.goToBattle();
+    LOGGER.info("Red dragon emerges.");
+    dragonSlayer.changeStrategy(LambdaStrategy.Strategy.ProjectileStrategy);
+    dragonSlayer.goToBattle();
+    LOGGER.info("Black dragon lands before you.");
+    dragonSlayer.changeStrategy(LambdaStrategy.Strategy.SpellStrategy);
+    dragonSlayer.goToBattle();
   }
 }

strategy/src/main/java/com/iluwatar/strategy/LambdaStrategy.java

@@ -0,0 +1,29 @@
+package com.iluwatar.strategy;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class LambdaStrategy {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(LambdaStrategy.class);
+
+  public enum Strategy implements DragonSlayingStrategy {
+    MeleeStrategy(() -> LOGGER.info(
+        "With your Excalibur you severe the dragon's head!")),
+    ProjectileStrategy(() -> LOGGER.info(
+        "You shoot the dragon with the magical crossbow and it falls dead on the ground!")),
+    SpellStrategy(() -> LOGGER.info(
+        "You cast the spell of disintegration and the dragon vaporizes in a pile of dust!"));
+
+    private final DragonSlayingStrategy dragonSlayingStrategy;
+
+    Strategy(DragonSlayingStrategy dragonSlayingStrategy) {
+      this.dragonSlayingStrategy = dragonSlayingStrategy;
+    }
+
+    @Override
+    public void execute() {
+      dragonSlayingStrategy.execute();
+    }
+  }
+}