图像未在 JPanel.paintComponent 中绘制

Question

I am actually want to load image but only applet dialog open and no error occurred but the image is not loading.the code is below here

import java.awt.Graphics;
import java.awt.Image;
import java.awt.Toolkit;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Ball extends JPanel{
  Image image; 


  public Ball(){
    super();       
    image = Toolkit.getDefaultToolkit().getImage("/D:\\lolololo\\tuto\\bin\\sa.jpg");
  }

private Image getResource(String string) {
    return image;
    // TODO Auto-generated method stub

}

  public void paintComponent(Graphics g){


   // Draw our Image object.
   g.drawImage(image,50,10,574,960, this); // at location 50,10
     // 200 wide and high
  }

  public void main(String arg[]){
   JFrame frame = new JFrame("ShowImage");
   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   frame.setSize(800,500);

   Ball panel = new Ball();
   frame.setContentPane(panel);
   frame.setVisible(true);
  }
}

Answer 1

The way you are loading the image is wrong. This will never work when you extract as Runnable jar.

• Create package ("res") inside inside your src

Now load the image this way

image = ImageIO.read(Ball.class.getResource("/res/sa.jpg"));

This will work.

As indicated by Andrew in his comment main class should be

public static void main(String arg[]) {}

Answer 2

+1 to @AndrewThompsons comments.

1) Below is incorrect, you do not honer the paint chain by calling the supers implementation of super.paintComponent(...) :

public void paintComponent(Graphics g) {
   // Draw our Image object.
   g.drawImage(image,50,10,574,960, this); // at location 50,10
   // 200 wide and high

}

As per docs for paintComponent :

Further, if you do not invoker super's implementation you must honor the opaque property, that is if this component is opaque, you must completely fill in the background in a non-opaque color. If you do not honor the opaque property you will likely see visual artifacts.

should be:

public class Ball extends JPanel {

    BufferedImage image;

    public Ball() {
       super();

       try {
           image=ImageIO.read(new File("c:/test.jpg"));//change to your path of file 
       }catch(Exception ex) {//file did not load properly
           ex.printStackTrace();
       }
    }

    @Override
    protected void paintComponent(Graphics g){
         super.paintComponent(g);

         // Draw our Image object.
         g.drawImage(image,0,0,image.getWidth(),image.getHeight(), this); // at location 50,10
        // 200 wide and high
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(image.getWidth(),image.getHeight());//replace with size of image
    }

}

Also notice:

• I overrode getPreferredSize() of JPanel and returned Dimension s I wanted (ie Image size) or the JPanel will only be as big as the components added to it and not the image (so if no components 0,0).

• I also chose BufferedImage vs Image and surrounded the statement with a try catch to check if any errors are thrown.

• I see you also had g.drawImage(image,50,10...) why 50 and 10 and not 0,0?

2) Also this (as @AndrewThompson has said):

image = Toolkit.getDefaultToolkit().getImage("/D:\\\\lolololo\\\\tuto\\\\bin\\\\sa.jpg");

No need for the / thats only if its located in your classes package etc not when its on your HDD at a set location.

3) also as said by @AndrewThompson a main method should be:

public static void main(String[] args){}

Notice the static modifer applied other wise its just another method.

4) Also dont use JFrame#setSize(..) , rather use LayoutManager and/or override getPreferredSize and than simply call pack() on JFrame instance before setting it visible.

5) Also please have a read on Concurrency in Swing . especially the Event-Dispatch-Thread