Guarantee covariant return type with generics in Java

Question

I have a class called Point, with a method neighbors() that returns an array of Points:

public class Point {
    public Point[] neighbors() { /* implementation not shown */ }
}

I have a subclass of Point, called SpecialPoint that overrides neighbors() to return an array of SpecialPoints instead of Points. I think this is called covariant return types.

public class SpecialPoint extends Point {
    public SpecialPoint[] neighbors() { /* implementation not shown */ }
}

In a separate class, I want to make use of Point and SpecialPoint with generics

public <P extends Point> P doStuff(P point) {
    P[] neighbors = point.neighbors();
    // more stuff here including return
}

This will not compile, because the compiler can only guarantee that P is some subclass of Point, but there is no guarantee that every subclass of Point will override neighbors() to return an array of itself as I happen to have done with SpecialPoint, so Java only knows that P#neighbors() returns Point[], not P[].

How do I guarantee that each subclass overrides neighbors() with a covariant return type so I can use it with generics?

Answer 1

You may use an interface:

public interface Point<P extends Point<P>> {
    P[] neighbors();
}

public class SimplePoint implements Point<SimplePoint> {
    @Override
    public SimplePoint[] neighbors() { /* ... */ }
}

public class SpecialPoint implements Point<SpecialPoint> {
    @Override
    public SpecialPoint[] neighbors() { /* ... */ }
}

Then:

public <P extends Point<P>> P doStuff(P point) {
    P[] neighbors = point.neighbors();
    /* ... */
}

---

If you still need to factorize code between the implementations, then better use an abstract class:

public abstract class Point<P extends Point<P>> {
    public abstract P[] neighbors();
    public void commonMethod() { /* ... */ }
}

public class SimplePoint extends Point<SimplePoint> { /* ... */ }

public class SpecialPoint extends Point<SpecialPoint> { /* ... */ }