Anonymous Classes

public class OuterWithAnonymous {
  public void aMethod() {
    Object obj = new Object() {     // Anonymous class overriding
                                    // Object's toString() method
      public String toString() {
        return "A custom toString() " + super.toString();
      }
    };

    System.out.println(obj);
  }
}

anonymous class

A kind of local inner class that combines the class definition with the object instantiation.

Anonymous classes are class definitions that override or implement behavior, and are instantiated immediately. * The class definition does not have class Name, so it is anonymous.

Anonymous classes are used in expressions such as method calls or assignments.

Declaring and Using Anonymous Classes

Declare and use an anonymous class by providing a code block after a constructor call, but before the semicolon.

In general, we declare and instantiate an anonymous instance using the code below.

new Name( [arguments] ) {
  //class definition
}

• If Name is a class name, then the anonymous class extends that class and can provide constructor arguments.

• If Name is an interface name, then the anonymous class implements that interface.

Anonymous Class Rules

Methods of local (including anonymous) classes can only use variables:

• Declared in its enclosing scope {} (usually the method body).

• Declared as parameters to the method.

• AND declared as final or effectively final (not changed after initialization) in the local class's enclosing scope - the enclosing method or block.

Anonymous classes can access members of the enclosing class.

// Anonymous Inner Class
public class Outer3 {
  private int outerField;

  public void aMethod() {
    final int localVar = 0;

    Object in = new Object() { // Assignment statement starts here...
      private int innerField;

      public String toString() {
        return "o: " + outerField + " i: " + innerField
            + " l: " + localVar;
      }
    }; // <-- Note the semicolon to end the assignment statement.

    System.out.println(in);
  }

  public static void main(String[] args) {
    Outer3 out = new Outer3();
    out.aMethod();
  }
}

Practice Exercise

Anonymous classes are compiled into bytecode files named EnclosingClass$n, where n is a counter for anonymous classes in EnclosingClass. We don't use $ in identifiers to avoid name collisions.

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