Question

im wondering how a jcomponent gets painted on the screen, is it painted inside paintComponent() from Graphics? Or is it painted separately. Im asking this because its weird how a jbutton changes color on mousehover even though repaint() is never called.

Thanks for your time.

Answer 1

Components are painted with their paint method. repaint is just a useful method that will call paint at some point in the near future on the Event Dispatch Thread.

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When the mouse enters a JButton, the following method is called (for JButtons with a default UI):

public void mouseEntered(MouseEvent e) {
    AbstractButton b = (AbstractButton) e.getSource();
    ButtonModel model = b.getModel();
    if (b.isRolloverEnabled() && !SwingUtilities.isLeftMouseButton(e)) {
        model.setRollover(true);
    }
    if (model.isPressed())
            model.setArmed(true);
}

ButtonModel.setRollover will fire a ChangeEvent, which is handled by AbstractButton in the following way:

public void stateChanged(ChangeEvent e) {
    Object source = e.getSource();

    updateMnemonicProperties();
    if (isEnabled() != model.isEnabled()) {
        setEnabled(model.isEnabled());
    }
    fireStateChanged();
    repaint();
}

So repaint is called when the mouse enters a JButton.