Generic type parameter which implements generic interace

Question

In a Java application, I'd like to use a generic type parameter which implements an interface which uses a generic parameter itself.

public interface SuperInterface<T> { ... }

public interface MyInterface extends SuperInterface<MyClass> { ... }


public class Worker<T extends SuperInterface<U>> extends SuperWorker<String, Boolean> {

}

However, the class declaration won't work like that. T should be of type MyInterface (or any other interface which implements the SuperInterface) and U should be of type MyClass (or any other class according to the interface).

Answer 1

You have to declare all of the type parameters at the top level. It's annoying, but that's how it is.

public class Worker<U extends MyClass, T extends SuperInterface<U>> { ...

The order of the parameters doesn't matter; you can also do Worker<T extends..., U extends...> . All that matters is that each is declared at the top level of the nested generics.

Here's a complete class:

public class MyClass {
  public interface SuperInterface<T>{}

  public interface MyInterface extends SuperInterface<MyClass> {}

  public class Worker<U extends MyClass, T extends SuperInterface<U>> {}

  public void compileTest() {
    // just to make sure the declaration compiles
    Worker<MyClass, MyInterface> worker = null;
  }
}