How to extend a class that is returning this in their methods

Question

The question is easy but I'm not sure it's possible to do it...

if we have a class like

class A {
    private int foo;

    public A(int bar) {
       this.foo = bar;
    }

    public A setFoo(int bar) {
       this.foo = bar;
       return this;
    }

    public int getFoo() {
       return this.foo;
    }

    public void doSomething() {
       this.foo++;
    }
}

we can see that it's just simply a class with a private member and a setter/getter. The interesting thing is, to allow method chaining, the setter is returning this.

So we can do things like this:

A a = new A(0);
a.setFoo(1).doSomething();

The problem here is when I try to extend that class add some functionality implementing an interface like this

class B extends A implements I {
    public B(int bar) {
        this.super(bar);
    }

    public void methodI() {
        // whatever
    }
}

It seems ok, until I start using it like this

B b = new B(1);
b.setFoo(2).methodI();

Because setFoo is actually returning an instance of A, not an instance of B, and in A methodI doesn't exist...

Any workaround? Thanks.

By the way, I just wrote a basic code simply to understand, but if you want to know more, I'm just trying to extend the some of the basic classes of libgdx (like Math.Vector2, Math.Vector3) to implement Poolable.

Answer 1

Class B can override the method setFoo and change the return type to B because B is a more specific version of A. An overridden method can have a more specific return type. e.g.

class B extends A implements I {
    public B(int bar) {
        this.super(bar);
    }

    public void methodI() {
        // whatever
    }

    @Override
    public B setFoo(int bar) {
       this.foo = bar;
       return this;
    }
}

Answer 2

You should use generics here:

public class A<T extends A<?>> {

    public T self() {
        return (T) this;
    }

    private int foo;

    public A(int bar) {
       this.foo = bar;
    }

    public T setFoo(int bar) {
       this.foo = bar;
       return self();
    }

    public int getFoo() {
       return this.foo;
    }

    public void doSomething() {
       this.foo++;
    }

}

public class B<T extends B<?>> extends A<T> implements I {

    public B(int bar) {
        this.super(bar);
    }

    public void methodI() {
        // whatever
    }
}

Now you will be able to use such chained calls:

B b = new B(1);
b.setFoo(2).methodI();