With concrete or abstract classes, inner classes are the only way to produce the effect of multiple implementation inheritance. : Nested Classes « Class Definition « Java Tutorial

1. Java Tutorial
2. Class Definition
3. Nested Classes

class D {
}

abstract class E {
}

class Z extends D {
  E makeE() {
    return new E() {
    };
  }
}

public class MainClass {
  static void takesD(D d) {
  }

  static void takesE(E e) {
  }

  public static void main(String[] args) {
    Z z = new Z();
    takesD(z);
    takesE(z.makeE());
  }
}