Question

This question has been asked in a C++ context but I’m curious about Java. The concerns about virtual methods don’t apply (I think), but if you have this situation:

abstract class Pet
{
    private String name;
    public Pet setName(String name) { this.name = name; return this; }        
}

class Cat extends Pet
{
    public Cat catchMice() { 
        System.out.println("I caught a mouse!"); 
        return this; 
    }
}

class Dog extends Pet
{
    public Dog catchFrisbee() { 
        System.out.println("I caught a frisbee!"); 
        return this; 
    }
}

class Bird extends Pet
{
    public Bird layEgg() {
        ...
        return this;
    }
}


{
    Cat c = new Cat();
    c.setName("Morris").catchMice(); // error! setName returns Pet, not Cat
    Dog d = new Dog();
    d.setName("Snoopy").catchFrisbee(); // error! setName returns Pet, not Dog
    Bird b = new Bird();
    b.setName("Tweety").layEgg(); // error! setName returns Pet, not Bird
}

In this sort of class hierarchy, is there any way to return this in a way that doesn’t (effectively) upcast the the object type?

Answer 1

If you want to avoid unchecked cast warnings from your compiler (and don’t want to @SuppressWarnings(“unchecked”)), then you need to do a little more:

First of all, your definition of Pet must be self-referential, because Pet is always a generic type:

abstract class Pet <T extends Pet<T>>

Secondly, the (T) this cast in setName is also unchecked. To avoid this, use the “getThis” technique in the excellent Generics FAQ by Angelika Langer:

The “getThis” trick provides a way to
recover the exact type of the this
reference.

This results in the code below, which compiles and runs without warnings. If you want to extend your subclasses, then the technique still holds (though you’ll probably need to genericise your intermediate classes).

The resulting code is:

public class TestClass {

  static abstract class Pet <T extends Pet<T>> {
    private String name;

    protected abstract T getThis();

    public T setName(String name) {
      this.name = name;
      return getThis(); }  
  }

  static class Cat extends Pet<Cat> {
    @Override protected Cat getThis() { return this; }

    public Cat catchMice() {
      System.out.println("I caught a mouse!");
      return getThis();
    }
  }

  static class Dog extends Pet<Dog> {
    @Override protected Dog getThis() { return this; }

    public Dog catchFrisbee() {
      System.out.println("I caught a frisbee!");
      return getThis();
    }
  }

  public static void main(String[] args) {
    Cat c = new Cat();
    c.setName("Morris").catchMice();
    Dog d = new Dog();
    d.setName("Snoopy").catchFrisbee();
  }
}