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{
     ....
     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() {
        // ...
    }
}

Answer 2

This is not possible in Java, but you might wonder what the use-case is to force this in the interface.

• If you program against the interface (which you typically do, why else define the interface) the type wouldn't be known
• If you program against a specific class, this thread already provided options on how you can implement that specific class to return a List<itself>. And since you program against that specific class, the compiler has access to that specific return type and knows about it