Java 9 - Inner Class Diamond Operator


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Diamond operator was introduced in java 7 to make code more readable but it could not be used with Anonymous inner classes. In java 9, it can be used with annonymous class as well to simplify code and improves readability. Consider the following code prior to Java 9.

Tester.java

Live Demo
public class Tester {
   public static void main(String[] args) {
      Handler<Integer> intHandler = new Handler<Integer>(1) {
         @Override
         public void handle() {
            System.out.println(content);
         }
      };
      intHandler.handle();
      Handler<? extends Number> intHandler1 = new Handler<Number>(2) {
         @Override
         public void handle() {
            System.out.println(content);
         }
      };
      intHandler1.handle();
      Handler<?> handler = new Handler<Object>("test") {
         @Override
         public void handle() {
            System.out.println(content);
         }
      };
      handler.handle();    
   }  
}
abstract class Handler<T> {
   public T content;

   public Handler(T content) {
      this.content = content; 
   }
   
   abstract void handle();
}

Output

1
2
Test

With Java 9, we can use <> operator with anonymous class as well as shown below.

Tester.java

public class Tester {
   public static void main(String[] args) {
      Handler<Integer> intHandler = new Handler<>(1) {
         @Override
         public void handle() {
            System.out.println(content);
         }
      };
      intHandler.handle();
      Handler<? extends Number> intHandler1 = new Handler<>(2) {
         @Override
         public void handle() {
            System.out.println(content);
         }
      };
      intHandler1.handle();
      Handler<?> handler = new Handler<>("test") {
         @Override
         public void handle() {
            System.out.println(content);
         }
      };

      handler.handle();    
   }  
}

abstract class Handler<T> {
   public T content;

   public Handler(T content) {
      this.content = content; 
   }
   
   abstract void handle();
}

Output

1
2
Test


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