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{
     ....
     List<? extends Bar> getFoo(); 
}

Right now a class that implements Bar returns objects of any class that implements Bar. I want to make it stricter so that the class that implements Bar returns a List of objects of only its type in getFoo().

Answer 1

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

You can get pretty close, though, by using:

public interface Bar<T extends Bar<T>> {
    List<T> getFoo();
}

And then your implementing classes can implement it like so:

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

But nothing stops another class from doing this:

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