Is it possible to monkey patch in Java?

I don't want to discuss the merits of this approach, just if it is possible. I believe the answer to be "no". But maybe someone will surprise me!

Imagine you have a core widget class. It has a method calculateHeight(), that returns a height. The height is too big - this result in buttons (say) that are too big. You can extend DefaultWidget to create your own NiceWidget, and implement your own calculateHeight() to return a nicer size.

Now a library class WindowDisplayFactory, instantiates DefaultWidget in a fairly complex method. You would like it to use your NiceWidget. The factory class's method looks something like this:

public IWidget createView(Component parent) {
    DefaultWidget widget = new DefaultWidget(CONSTS.BLUE, CONSTS.SIZE_STUPIDLY);

    // bunch of ifs ...
    SomeOtherWidget bla = new SomeOtherWidget(widget);
    SomeResultWidget result = new SomeResultWidget(parent);
    SomeListener listener = new SomeListener(parent, widget, flags);

    // more widget creation and voodoo here

    return result;
}

That's the deal. The result has the DefaultWidget deep within a hierarchy of other objects. The question - how to get this factory method to use my own NiceWidget? Or at least get my own calculateHeight() in there. Ideally, I'd like to be able to monkey patch the DefaultWidget so that its calculateHeight did the right thing...

public class MyWindowDisplayFactory {
    public IWidget createView(Component parent) {
        DefaultWidget.class.setMethod("calculateHeight", myCalculateHeight);
        return super.createView(parent);
    }
}

Which is what I could do in Python, Ruby, etc. I've invented the name setMethod() though. The other options open to me are:

• Copying and pasting the code of the createView() method into my own class that inherits from the factory class
• Living with w