Why the image is not displayed?

Question

I have searched the topic in the Google, but it wasn't very clear about what I should do. my question is:

1. What's wrong with my code? How to revise it.

2. The answer in many websites always use URL, how can I know the URL of my image?

Here is the code:

import java.awt.BorderLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class TwoPanel {
    public static void main(String[] args) {
        JFrame frame = new JFrame ("Key test");
        MyDrawPanel1 dp1 = new MyDrawPanel1();
        //MyDrawPanel2 dp2 = new MyDrawPanel2();
        //MyDrawPanel3 dp3 = new MyDrawPanel3();
        //MyDrawPanel4 dp4 = new MyDrawPanel4();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible (true);
        JPanel p = new JPanel ();
        p.setLayout(new BorderLayout());
        p.add (dp1);
        //p.add (dp2);
        //p.add(dp3);
        //p.add(dp4);
        frame.getContentPane().add(p);
        frame.pack();
    }
}

then the drawpanel:

public class MyDrawPanel1 extends JPanel {
    /**
    *
    */
    private static final long serialVersionUID = 1L;
    Image image = new ImageIcon("Koala.jpg").getImage();/*something new: if i use the 

    path to the disk name(D:/Java/workspace/firstJava/src/com/founder/panel/Koala.jpg'),     

    it can work.*/

    public void paintComponent (Graphics g) {
        super.paintComponent(g);
        //Graphics2D g2 = (Graphics2D) g;
        g.drawImage(image, 3, 4, null);
    }
    public Dimension getPreferredSize() {
        if (image != null) {
            return new Dimension(image.getWidth(null), image.getHeight(null));
        }
        return super.getPreferredSize(); // default
    }
}

And this panel can work, although i cann't understand why other doesn't work

package com.founder.panel;

import java.awt.Graphics;

import javax.swing.JPanel;
import java.awt.*;
import java.awt.image.*;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;

import javax.imageio.ImageIO;

public class MyDrawPanel4 extends JPanel{

/**
 * 
 */
private static final long serialVersionUID = 1L;

private static final String URL_PATH = "file:///D:/Java/workspace/firstJava/src/com/founder/panel/" +
        "Koala.jpg";

BufferedImage image = null;

public MyDrawPanel4() {
    // TODO Auto-generated constructor stub

      setBackground(Color.white);
      try {
         image = ImageIO.read(new URL(URL_PATH));
      } catch (MalformedURLException e) {
         e.printStackTrace();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }

   @Override
   public Dimension getPreferredSize() {
      if (image != null) {
         return new Dimension(image.getWidth(), image.getHeight());
      }
      return super.getPreferredSize(); // default
   }

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

Answer 1

My guess is that the image is not loaded, use the path of the file as an argument to ImageIO.read(new File("Koala.jpg")). Using ImageIO:

import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.*;

public class TwoPanel {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                showMainWindow();
            }
        });
    }

    public static void showMainWindow() {
        JFrame frame = new JFrame ("Key test");
        MyDrawPanel1 dp1 = new MyDrawPanel1();
        //MyDrawPanel2 dp2 = new MyDrawPanel2();
        //MyDrawPanel3 dp3 = new MyDrawPanel3();
        //MyDrawPanel4 dp4 = new MyDrawPanel4();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel p = new JPanel ();
        p.setLayout(new BorderLayout());
        p.add (dp1);
        //p.add (dp2);
        //p.add(dp3);
        //p.add(dp4);
        frame.getContentPane().add(p);
        frame.setLocationRelativeTo(null); // center frame
        frame.setMinimumSize(new Dimension(300,400)); // make the frame at least 300x400
        frame.pack();
        frame.setVisible(true);
    }
}

The MyDrawPanel1:

 class MyDrawPanel1 extends JPanel {

    private static final long serialVersionUID = 1L;
    //Image image = new ImageIcon("Koala.jpg").getImage();
     BufferedImage image;

     MyDrawPanel1() {
         super();
         try {
             this.image = ImageIO.read(getClass().getResource("com/founder/pane/Koala.jpg"))
         } catch (IOException e) {
             System.err.println("Koala.jpg does not exist!");
             e.printStackTrace();
         }
     }
     @Override
     public void paintComponent (Graphics g) {
        super.paintComponent(g);
        g.drawImage(image, 3, 4, null);
    }
}