Can an overriding method have a different return type than the overridden method?

Before Java 5.0, when you override a method, both parameters and return type must match exactly. In Java 5.0, it introduces a new facility called covariant return type. You can override a method with the same signature but returns a subclass of the object returned. In another words, a method in a subclass can return an object whose type is a subclass of the type returned by the method with the same signature in the superclass.

For example, the following code compiles and the narrower type B is a legal return type for the getObject method in the subclass, Sub.

class A {
}
class B extends A {
}

class Super {
  public A getObject() {
    System.out.println("Super::getObject");
    return new A();
  }
}

class Sub extends Super {
  public B getObject() {
    System.out.println("Sub::getObject");
    return new B();
  }

  public static void main(String[] args) {
    Super s = new Sub();
    s.getObject();
  }
}

The output of the above code is:

Sub::getObject

But, the following code will not compile because String is not a legal return type for the getObject method in the subclass, Sub. String does not extends from either A or B.

class A {
}
class B extends A {
}

class Super {
  public A getObject() {
    System.out.println("Super::getObject");
    return new A();
  }
}

class Sub extends Super {
  public B getObject() {
    System.out.println("Sub::getObject");
    return new B();
  }

  public String getObject() {
    return "getObject()";
  }

  public static void main(String[] args) {
    Super s = new Sub();
    s.getObject();
  }
}

Printer-friendly version | Mail this to a friend