如何不扩展Swing组件？

Question

I've seen a number of posters argue that Swing components should not be extended, and that functionality should instead be added via composition. So say I want to reusably create a JPanel with absolute positioning (no layout manager) in which I can reposition components with the mouse:

public class MoveableComponentPanel 
{
    private final JPanel m_panel;

    public MoveableComponentPanel()
    {
        m_panel = new JPanel(null);
    }

    public void add(Component c)
    {
        m_panel.add(c);
        c.addMouseListener(m_mover);
        c.setSize(c.getPreferredSize());
    }   

    private final MouseListener m_mover = new MouseListener()
    {
        private int m_startX, m_startY;

        @Override
        public void mousePressed(MouseEvent e)
        {
            if (e.getButton() == MouseEvent.BUTTON1)
            {
                m_startX = e.getX();
                m_startY = e.getY();
            }
        }

        @Override
        public void mouseReleased(MouseEvent e)
        {
            if (e.getButton() == MouseEvent.BUTTON1)
            {
                Component c = e.getComponent();
                Point p = c.getLocation();
                p.x += e.getX() - m_startX;
                p.y += e.getY() - m_startY;         
                c.setLocation(p);
                c.repaint();
            }
        }   

        @Override
        public void mouseClicked(MouseEvent e) {}

        @Override
        public void mouseEntered(MouseEvent e) {}

        @Override
        public void mouseExited(MouseEvent e) {}
    };
}

Looks nice, but there's no way to add the panel to a container. Further headaches if I want to allow a calling class to interact with the panel, eg change size, background color, register event listeners, etc. Either I add a getPanel() accessor, which breaks encapsulation, or write pass-through methods for the JPanel methods/properties I want to expose (potentially many). Is there some other strategy I'm missing here? What is the accepted best practice?

Answer 1

in which I can reposition components with the mouse:

Then you just add the MouseListener/MouseMotionListener to the components that you want to drag. There is no need to extend any component to add that functionality.

Check out the Component Mover class for examples of ways to do basic dragging of a component as well as a more complex dragging solution.