为什么这个小程序不能工作/显示图像？

Question

I am new to Applets, and am trying to get comfortable with some basics - like how to display a jpg image in one. I've read what I think are the relevant parts of the Java Tutorials on Applets, and I don't understand why this doesn't work, all I get is a blank applet area. "/cards/as.jpg" - that file exists in subdirectory of the directory the applet is running from. Can anyone see what I'm doing wrong? I'm at a loss.

import javax.swing.JApplet;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class TestApplet extends JApplet {
    public void init() {
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                public void run() {
                    createGUI();
                }
            });
        }
        catch (Exception e) {
            System.err.println("createGUI didn't complete successfully");
        }
    }

    private void createGUI() {
        JPanel newContentPane = new JPanel();

        ImageComponent card = new ImageComponent();
        card.setImage(getCodeBase() + "/cards/as.jpg");

        newContentPane.add(card);        

        newContentPane.setOpaque(true);
        setContentPane(newContentPane);
    }
}


import java.awt.*;
import javax.swing.JComponent;
import javax.imageio.*;
import java.net.*;
import java.io.*;

/**
   A component that draws an Image.
*/
public class ImageComponent extends JComponent {  
    private Image image;
    private String url;

    public ImageComponent() {
        image = null;
        url = null;
    }

    public void setImage(String urlCardName) {
        url = urlCardName;
        if (url != null) {
            try {
                image = ImageIO.read(new URL(url));
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
        }
        else {
            image = null;
        }
        this.repaint();
    }

    public void paintComponent(Graphics g) { 
        if (image == null) return;
        // draw the images in the upper-left corner of the component
        g.drawImage(image, 0, 0, null);
    }
}

Answer 1

I think it's your layouts. Normally a contentPane uses BorderLayout by default, but when you use your own JPanel as contentPane, you're using the JPanel default layout which is FlowLayout, and components added to a container using FlowLayout won't expand to fill the container as they would with BorderLayout. Check to see how large your ImageComponent is when rendered -- I'll bet you it's very small, 0 size even.

A solution is to use the contentPane that's already present in the applet with its default BorderLayout, or else if you must use your own JPanel, give it a BorderLayout and add the new ImageComponent BorderLayout.CENTER. For example:

    JPanel newContentPane = new JPanel();

    newContentPane.setLayout(new BorderLayout()); // *** added this

    ImageComponent card = new ImageComponent();
    card.setImage(getCodeBase() + "/cards/as.jpg");

    newContentPane.add(card, BorderLayout.CENTER);  // *** changed

Answer 2

Check to see how large your ImageComponent is when rendered -- I'll bet you it's very small, 0 size even.

This is because you never set a preferred size for you component.

The real question is why are you drawing the image yourself. There is no need to create a custom component. Just use a JLabel with a Icon and this would not be a problem.