Enforcing return type for an class that implements an interface

Question

How do I enforce that the method getFoo() in the implementing class, returns a list of the type of same implementing class.

public interface Bar{
     ....
          


        

Answer 1

Unfortunately this cannot be enforced by Java's type system.

You can get pretty close, though, by using:

public interface Bar> {
    List getFoo();
}

And then your implementing classes can implement it like so:

public class SomeSpecificBar implements Bar {
    // Compiler will enforce the type here
    @Override
    public List getFoo() {
        // ...
    }
}

But nothing stops another class from doing this:

public class EvilBar implements Bar {
    // The compiler's perfectly OK with this
    @Override
    public List getFoo() {
        // ...
    }
}