Rule for inheritance — return type of overriding method can be child class of return type declared in overridden method

Question

I was going through inheritance rules. I came across below rule

return type of overriding method can be child class of return type declared in overridden method.

I don't understand the purpose behind it. If any one can explain it.

Answer 1

It allows you to use more specific return type when you're using specific subclass. A common example would be to use more specific return type when overriding clone() method. It's declared like

protected native Object clone() throws CloneNotSupportedException;

So it returns Object . But in your particular class (say, MyObject ) you can redefine clone like this:

public MyObject clone() {
    try {
        return (MyObject)super.clone();
    }
    catch(CloneNotSupportedException ex) {
        throw new InternalError();
    }
}

This way users of your MyObject class can call the clone() method and get the MyObject result without additional casting. It's quite convenient.

Answer 2

Okay, so here's what I think can happen.

Let's say you have the following:

public class Animal {
...
   public Animal anotherAnimal() {
      return new Animal();
   }
}

public class Dog extends Animal {
...
   public Dog anotherAnimal() {
      return new Dog();
   }
}

So what's happening here is we're overriding the superclass method anotherAnimal , and the rule is that in the child class (Dog), we are allowed to return a different object than the superclass' method did, but it has to return an object that's a subclass.

In other words, because Dog is a subclass of Animal we're able to return it in the overridden Dog method.