Java: returning subclass in superclass method signature

Question

I\'m working on a problem where there are several implementations of Foo, accompanied by several FooBuilder\'s. While Foo\'s share sev

Answer 1

Having found this excellent answer I am now sharing it around.

public class SuperClass<I extends SuperClass>
{
    @SuppressWarnings( "unchecked" ) // If you're annoyed by Lint.
    public I doStuff( Object withThings )
    {
        // Do stuff with things.
        return (I)this ; // Will always cast to the subclass. Causes the Lint warning.
    }
}

public class ImplementationOne
extends SuperClass<ImplementationOne>
{} // doStuff() will return an instance of ImplementationOne

public class ImplementationTwo
extends SuperClass<ImplementationTwo>
{} // doStuff() will return an instance of ImplementationTwo

Answer 2

This is a good question and a real problem.

The easiest way to deal with it in Java likely involves the use of generics, as mentioned in Jochen's answer.

There's a good discussion of the issue and a reasonable solution in this blog entry on Using Inheritance with Fluent Interfaces, which combines generics with the definition of a getThis() method overriden in builder subclasses to solve the problem of always returning a builder of the correct class.

Answer 3

Generics might be the way to go here.

If you declare setA() something like this (pseudo-code)

<T> T setA(int a)

the compiler should be able to figure out the real types, and if it doesn't you can give hints in the code like

obj.<RealFoo>setA(42)