How do you draw shapes on a JPanel, which is withi

问题:

I'm currently trying to draw shapes on a JPanel, which is within another JPanel, within a JFrame.

I've searched Google and Youtube and found out how to draw shapes within a JFrame that has one panel, but have found nothing which can help me with what I'm doing. (maybe I'm not seeing something).

Code I've seen so far:

public class GameScreen 
{
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g.setColor(Color.red);
        g.drawRect(100, 10, 30, 40);
    }

    public static void main(String[] args)
    {
       GameScreen gs = new GameScreen();
       JFrame f = new JFrame();
       f.setTitle("");
       f.setSize(400,400);
       f.setVisible(true);
       f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       f.add(gs);
}

This is all good for when I'm dealing with just one panel, but I wanna display shapes on a panel which is within the 1'st panel I've created.

回答1:

This is all good for when I'm dealing with just one panel,

The code you posted works fine because a frame uses a BorderLayout. So, when you add your panel to the frame is will increase in size and your custom painting will be seen.

I wanna display shapes on a panel which is within the 1'st panel I've created.

When you create a panel it uses a FlowLayout by default. When you add a custom painting panel to that panel it will be displayed at its preferred size, which will be (0, 0) since you didn't specify the preferred size.

On the panels that do custom painting you also need to implement the getPreferredSize() method so the layout manager knows how to arrange the panels. For example:

public Dimension getPreferredSize()
{
    return new Dimension(200, 200);
}

回答2:

Add a JPanel to the JFrame in the same way as you're doing now, but do it with your own subclass of JPanel.

class MyPanel extends JPanel {
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.red);
        g.drawRect(100, 10, 30, 40);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(400,400); // As suggested by camickr
    }
}

You can add this to a JPanel which sits within the JFrame

public static void main(String[] args)
{
   MyPanel mp = new MyPanel();
   JPanel jp = new JPanel();
   jp.add(mp);

   JFrame f = new JFrame();
   f.setTitle("");
   f.setSize(400,400);
   f.setVisible(true);
   f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   f.add(jp);
}

This can work for components within components, if you add them as children components. The key is to extend the component and override the methods you wish to change.