自定义绘制的JPanel将不会出现在JFrame中

Question

I've been trying to come up with a program that for a physics project, but I'm having problems. Here's the code:

import java.awt.event.*;
import javax.swing.*;
import java.awt.*;

public class Cannon extends JFrame {
  Cannon() {
  setTitle("Cannonball Experiment");
  setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

  Container c = getContentPane();
  c.setLayout(new FlowLayout());

  c.add(new ImagePanel());
  c.add(new ModifierPanel());

  setSize(400,600);
  setVisible(true);
       }

   class ImagePanel extends JPanel {
  public void paintComponent(Graphics g) {
     super.paintComponent(g);
     g.drawRect(0,100,200,300);
  }
   }

   class ModifierPanel extends Panel {
  JLabel Text = new JLabel("Speed"); 
  JTextField Tf = new JTextField(10);

  ModifierPanel() {
     add(Text);
     add(Tf);
  }
   }
   public static void main(String[] args) {
  Cannon frame = new Cannon();
   }
}

ModifierPanel displays fine but the JPanel , which I added before the Modifier, is not being displayed.

Answer 1

JPanel ImageJPanel doesn't show anything, so its dimension is (0,0). Try to give your JPanel a dimension overriding method getPreferredSize like this:

class ImagePanel extends JPanel {
   @Override
   public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawRect(0,100,200,300);
   }

   @Override
   public Dimension getPreferredSize() {
        return new Dimension(500,300); //for example
   }
}

You're using FlowLayout that honors getPreferredSize so you won't have any problems. Try and let me know...

Answer 2

try buffered image in image panel ..

 public class ImagePanel extends JPanel{

private BufferedImage image;

public ImagePanel() {
   try {                
      image = ImageIO.read(new File("specify image name and path"));
   } catch (IOException ex) {
        //  exception...
   }
}

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.drawImage(image, 0, 0, null);           
}