#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")