Java ClassCastException when invoking method

Question

I am making a game engine and I'm currently working on the component-part of the program. I want the user to be able to provide a special render method to the components so therefore I am using reflection and some other stuff. As the title suggests, this throws a ClassCastException and I can't figure out why. Here's the code:

public class LComponent {
    public Vector pos, size;
    private Class renderClass = getClass();
    private Method renderMethod;

    public LComponent(Vector pos, Vector size) {
        try {
            renderMethod = renderClass.getDeclaredMethod("defaultRender",
                    Graphics.class);
            renderMethod.setAccessible(true);
        } catch (NoSuchMethodException | SecurityException e) {
            e.printStackTrace();
        }
    }

    public void render(Graphics g) {
        try {
            renderMethod.invoke(renderClass, g);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void defaultRender(Graphics g) {
        g.drawRect((int) pos.getX(), (int) pos.getY(), (int) size.getX(),
                (int) size.getY());
    }
}

Do you have a debugger? This is easy to fix with the right information. — duffymo
– duffymo, Commented May 8, 2015 at 11:21
What are you trying to achieve with this, that can't be achieved with simple overriding of methods? — RealSkeptic
– RealSkeptic, Commented May 8, 2015 at 11:33
Questions seeking debugging help ("why isn't this code working?") must include the desired behavior, a specific problem or error and the shortest code necessary to reproduce it in the question itself. — Hot Licks
– Hot Licks, Commented May 8, 2015 at 11:54

Peter Lawrey · Accepted Answer · 2015-05-08 11:37:08Z

1

instead of

renderMethod.invoke(renderClass);

you need

renderMethod.invoke(this, g);

Although you could write all this without refection much simpler.

I want the class to be class to be customizable

I suggest you use an interface

interface Renderable {
    void render(Graphics g);
}

public class LComponent implements Renderable {
    // can be anything which implements Renderable
    final Renderable renderable; // initialise in the constructor

    public void render(Graphics g) {
         renderable.redner(g);
    }

}

edited May 8, 2015 at 11:37

answered May 8, 2015 at 11:21

Peter Lawrey

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3 Comments

xenia Over a year ago

But I want the class to be class to be customizable

Peter Lawrey Over a year ago

@Loovjo You have to have an instance to call the method on. If you make the instance implement an interface, you can call this method on any instance via the interface without reflection. Generally speaking, you don't need reflection unless you are calling code you don't have control over.

Peter Lawrey Over a year ago

@Loovjo Yes, that is why I gave an answer.

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