Java generics in an interface

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!

driangle · Accepted Answer · 2012-02-20 16:43:00Z

7

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;
    }
}

answered Feb 20, 2012 at 16:43

driangle

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1 Comment

ggrigery Over a year ago

Awesome! I had a feeling I was just a bit off on the syntax/declaration. Thanks for the help!

DNA · Accepted Answer · 2012-02-20 16:43:18Z

1

As it stands, the implementation of SomeInterface must implement the parameterized method, i.e.:

public <T extends SuperRequest, U extends SuperResponse> U someMethod(T request);

It can't choose a particular Subclass to substitute for T and U.

The interface itself must be generic SomeInterface<T extends SuperRequest, U extends SuperResponse> and the subclass can then choose concrete implementations for T and U.

answered Feb 20, 2012 at 16:43

DNA

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Comments

jlemos · Accepted Answer · 2012-02-20 16:44:17Z

1

You have to declare the types as well when you implement the interface. Try this:

public class SomeImplementation implements SomeInterface<SubRequest,SubResponse> {

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

answered Feb 20, 2012 at 16:44

jlemos

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