Java Multiple Inheritance without Source Code

Question

I have a class called Thing and a class called Robot. Thing has a public void setBlocksExit(). Robot has some methods I also desire.

I have extended Robot but I also want setBlocksExit() from Thing. I would make an interface that has setBlocksExit() and then make a class like:

public class C extends Robot implements BlockExit {}

The problem is I don't have access to the source code for Thing and Robot. I am using an educational package 'becker.jar' and all of the code is compiled so I can't access it to extract interfaces. What are my options?

Answer 1

Your options are the following:

• Extend Thing and have a reference to a Robot which you delegate all Robot methods to.
• Extend Robot and have a reference to a Thing object which you delegate setBlocksExit calls to.
• Create a fresh class and have a reference to a Robot and a reference to a Thing and delegate calls to these two objects.

If you're using an IDE such as Eclipse you can even "extract interfaces" and generate delegate methods automatically.

Option 1:

class C extends Thing {
    final Robot robot;
    public C(Robot robot) {
        this.robot = robot;
    }

    public int robotMethod1() {
        return robot.robotMethod1();
    }

    ...
}

Option 2:

class C extends Robot {
    final Thing thing;
    public C(Thing thing) {
        this.thing = thing;
    }

    public void setBlocksExit(boolean flag) {
        return thing.setBlocksExit(flag);
    }

    ...
}

Option 3:

class C {
    final Thing thing;
    final Robot robot;
    public C(Thing thing, Robot robot) {
        this.thing = thing;
        this.robot = robot;
    }

    public void setBlocksExit(boolean flag) {
        return thing.setBlocksExit(flag);
    }

    public int robotMethod1() {
        return robot.robotMethod1();
    }
    ...
}

If you're using Eclipse you could use this feature:

• Fast implement wrapping (delegate methods) in Eclipse?

I'm sure whatever IDE you're using has a similar feature.