Implement two interfaces in an anonymous class

Question

I have two interfaces:

interface A {     void foo(); }  interface B {     void bar(); } 

I am able to create anonymous instances of classes implementing either of these interfaces like so:

new A() {     void foo() {} } 

or:

new B() {     void bar() {} } 

I want to create an anonymous class that implements both interfaces. Something like (the fictitious):

new A implements B {     void foo() {}     void bar() {} } 

This obviously gives a compile error: "B cannot be resolved to a type".

The workaround is quite simple:

class Aggregate implements A, B {     void foo() {}     void bar() {} } 

I then use Aggregate where ever I would have used the anonymous class.

I was wondering if it is even legal for an anonymous class to implement two interfaces.

Answer 1

"An anonymous inner class can extend one subclass or implement one interface. Unlike non-anonymous classes (inner or otherwise), an anonymous inner class cannot do both. In other words, it cannot both extend a class and implement an interface, nor can it implement more than one interface. " (http://scjp.wikidot.com/nested-classes)

Answer 2

If you are determined to do this, you could declare a third interface, C:

public interface C extends A, B { } 

In this way, you can declare a single anonymous inner class, which is an implementation of C.

A complete example might look like:

public class MyClass {    public interface A {     void foo();   }    public interface B {     void bar();   }    public interface C extends A, B {     void baz();   }    public void doIt(C c) {     c.foo();     c.bar();     c.baz();   }    public static void main(String[] args) {     MyClass mc = new MyClass();      mc.doIt(new C() {       @Override       public void foo() {         System.out.println("foo()");       }        @Override       public void bar() {         System.out.println("bar()");       }        @Override       public void baz() {         System.out.println("baz()");       }     });   }  } 

The output of this example is:

foo() bar() baz()