Question

I have run into a problem with generics in Java and I can’t find a solution or example online of someone using generics in a similar fashion. I have a set of methods that are in the same request/response structure. For example, a user populates fields in the Request object, passes the object to a helper method, and they are returned a Response object. All of my request objects extend from a common Request super class (and similarly, Response objects from a Response super class). I would like my helper classes to also have a consistent structure so I have used an interface. Here is some code to illustrate…

Request super class:

public class SuperRequest {
    // ...
}

Example Request subclass:

public class SubRequest extends SuperRequest {
    // ...
}

Response super class:

public class SuperResponse {
    // ...
}

Example response subclass:

public class SubResponse extends SuperResponse{
    // ...
}

The interface:

public interface SomeInterface {
    public <T extends SuperRequest, U extends SuperResponse> U someMethod(T request);
}

As you can see from the interface, I want to pass an object that is a child of SuperRequest and I want to return an object that is a child of SuperResponse. Here is my implementation:

public class SomeImplementation implements SomeInterface {

    @Override
    public SubResponse someMethod(SubRequest request) {
        return null;
    }
}

In Eclipse, I get compilation errors because the compiler tells me I have unimplemented methods and the @Override notation “does not override a supertype method”.

Can anyone help me here? Is my syntax incorrect or is my understanding of generics a little off? Any help is greatly appreciated!

Answer 1

Try changing your interface and implementation to:

interface SomeInterface<T extends SuperRequest, U extends SuperResponse> {
    public U someMethod(T request);
}

class SomeImplementation implements SomeInterface<SubRequest, SubResponse> {

    @Override
    public SubResponse someMethod(SubRequest request) {
        return null;
    }
}