java - JPanel 不显示绘图

Question

我在 JPanel 上显示绘图时遇到问题。我创建了三个相互链接的类，如下所示。我想知道为什么这段代码不显示我的绘图。

c.add(pDraw);
pDraw.add(draw);

1)主要

public class mainPage {
    public static void main(String[]args){
      JFrame appFrame = new Frame();
      appFrame.setVisible(true); 
      appFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

2) 框架

 public class Frame extends JFrame implements ActionListener{

    private drawingBoard draw;  

    public Frame (){
         draw = new drawingBoard(); //generate pattern
         GridBagLayout m = new GridBagLayout();
         Container c = (Container)getContentPane();
         c.setLayout (m);
         GridBagConstraints con;
         .......

         JPanel pDraw = new JPanel();       
         pDraw.setPreferredSize(new Dimension(500,500));
             .....  
         c.add(pDraw);
         pDraw.add(draw); // Call other class for drawing

         .....
         setResizable(false); 
         pack();
         setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  
         setVisible(true);
    }
}

3) JPANEL

public class drawingBoard extends JPanel {
      .....
      public void paint(Graphics g) {
      ......
      }
   }

Answer 1

答案在这里，我忘了设置绘图板 JPanel 的大小:-)

 public class drawingBoard extends JPanel {
        public drawingBoard(){
        setPreferredSize(new Dimension (500,500));

        }
    }