用Java徒手绘制，将图形添加到JPanel

Question

In my class Test I create Three panels. In my class Draw I draw by free hand.

I want to add my object d to centerPanel. When I do that nothing draws. But if i add it to the frame (using getContentPane().add) it draws. Does anyone knows where the problem is?

   topPanel = new JPanel(); 
   centerPanel = new JPanel();    
   bottomPanel = new JPanel();  

   Draw d = new Draw();
   getContentPane().add(d, BorderLayout.CENTER);  //This works
   add(topPanel, BorderLayout.PAGE_START);                 
   add(bottomPanel, BorderLayout.PAGE_END);  


   /* I WANT THIS TO WORK INSTEAD                */
   /* centerPanel.add(d);                        */  //How can I write this line of code?
   /* add(topPanel, BorderLayout.PAGE_START);    */             
   /* add(centerPanel, BorderLayout.CENTER);     */     
   /* add(bottomPanel, BorderLayout.PAGE_END);   */

Class Draw:

public class FreeHand extends JComponent, MouseListener, MouseMotionListener {      
    int x;
    int y;
    int posX;
    int posY;

    public FreeHand()
    {
        addMouseListener(this);
        addMouseMotionListener(this);
    }

    @Override
    public void mousePressed(MouseEvent me) {            
        posX = me.getX();
        posY = me.getY();
    }

    @Override
    public void mouseDragged(MouseEvent me) {
        Graphics g = getGraphics();
        g.setColor(Color.RED);
        g.drawLine(posX, posY, me.getX(), me.getY());
        posX = me.getX();
        posY = me.getY();
    }

    @Override
    public void mouseMoved(MouseEvent me) {}

    @Override
    public void mouseClicked(MouseEvent me) {}

    @Override
    public void mouseEntered(MouseEvent me) {}

    @Override
    public void mouseExited(MouseEvent me) {}

    @Override
    public void mouseReleased(MouseEvent me) {}
}           

Answer 1

getContentPane().add(d, BorderLayout.CENTER); //This works

That works because the content pane uses a BorderLayout and the layout manager will give all the available space to the Draw component when it is added to the CENTER.

centerPanel.add(d);                     
add(centerPanel, BorderLayout.CENTER); 

This doesn't work because the BorderLayout will give all the space to the "centerPanel". But the "centerPanel uses a FlowLayout and by default the FlowLayout will respect the preferred size of any component added to it. Your Draw class doesn't have a preferred size so the size is zero.

You can change the layout manager of the centerPanel to also use a BorderLayout or you can override the getPreferredSize() method of your Draw class to return the appropriate preferred size of your panel.

The question is why do you want to create an extra "centerPanel" when you don't need it?