Why is paintComponent executed 2 times?

问题

I have following code:

        canvas=new MembershipFunctionComponent(functions);
        canvas.setPreferredSize(new Dimension((int)this.getWidth(), (int)this.getHeight()));
        canvas.addMouseListener(canvas);
        pane.add(canvas);

MembsershipFunctionComponent extends JComponent. Why is paintComponent method executed 2 times?

回答1:

paintComponent can get called at pretty much any time by the Swing framework. Examples of when this might happen:

• Any time a component is resized
• Any time part of the component is revealed (for example in a scrollable window)
• Any time the repaint() method is called on your component (or possibly a parent or sub-component)
• Any time there is a layout change

None of this should worry you - you should just write your code so that it doesn't care how many times paintComponent is called.

来源：https://stackoverflow.com/questions/9355395/why-is-paintcomponent-executed-2-times