Question

I would like my BallUserInterfaceFactory to return an instance of a user interface that has the proper generic type. I am stuck in the example below getting the error:

Bound mismatch: The generic method getBaseballUserInterface(BASEBALL)
of type BallUserInterfaceFactory is not applicable for the arguments
(BALL). The inferred type BALL is not a valid substitute for the
bounded parameter

public class BallUserInterfaceFactory {
    public static <BALL extends Ball> BallUserInterface<BALL> getUserInterface(BALL ball) {

        if(ball instanceof Baseball){
            return getBaseballUserInterface(ball);
        }
        //Other ball types go here

        //Unable to create a UI for ball
        return null;
    }

    private static <BASEBALL extends Baseball> BaseballUserInterface<BASEBALL> getBaseballUserInterface(BASEBALL ball){
        return new BaseballUserInterface<BASEBALL>(ball);
    }
}

I understand that it cannot guarantee that BALL is a Baseball, and so there is a parameter type mismatch on the getBaseballUserInterface method call.

If I cast the ball parameter in the getBaseballUserInterface method call, then I get the error:

Type mismatch: cannot convert from BaseballUserInterface<Baseball>
to BallUserInterface<BALL>

Because it can’t guarantee that what I am returning is the same type of BALL.

My question is, what is the strategy for dealing with this situation?

(For completeness, here are the other classes required in the example)

public class Ball {

}

public class Baseball extends Ball {

}

public class BallUserInterface <BALL extends Ball> {

    private BALL ball;

    public BallUserInterface(BALL ball){
        this.ball = ball;
    }
}

public class BaseballUserInterface<BASEBALL extends Baseball> extends BallUserInterface<BASEBALL>{

    public BaseballUserInterface(BASEBALL ball) {
        super(ball);
    }

}

Answer 1

This is a VERY GOOD question.

You could cast brutely

    return (BallUserInterface<BALL>)getBaseballUserInterface((Baseball)ball);

The answer is theoretically flawed, since we force BASEBALL=Baseball.

It works due to erasure. Actually it depends on erasure.

I hope there is a better answer that is reification safe.