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Well, what is the KnopHandler class?

Also, that syntax is incorrect. What are you trying to do here:

zwart.addActionListener(kh)(
new ActionListener(){
public void...

See how you save the diameter to a value named d, which you change using a JSlider?

You can do the same thing with a Color variable:

Color c = new Color(255, 0, 0); //Color.RED
g.setColor(c);...

You're right; there isn't much code required. But we can't just do your homework for you.

You already use a JSlider to change the diameter of the circles. Changing the colors would be done in a...

Use the highlight tags by checking out the forum rules you were linked to upon joining, or by quoting another post that uses them to see how they work.

More recommended reading: A Visual Guide to...

Please use the highlight tags to preserve formatting. Unformatted code is pretty hard to read.

Recommended reading: http://docs.oracle.com/javase/tutorial/uiswing/components/button.html