Java Applet未在Eclipse的内置Applet Viewer中更新

Question

I'm just learning about java applets. I drew a line on the applet with drawLine() and when I pressed run, it compiled normally and displayed the applet with Eclipse's built in applet viewer. Here's the code

import java.applet.*;
import java.awt.*;

public class Lab04b {
    public void paint(Graphics g){
    g.drawLine(0, 0, 200, 200);
    }
}

However, when I commented out the drawLine() and recompiled it and ran it, it displayed the applet with a line on it as if it didn't update when the code changed. Here's the commented out version:

 import java.applet.*;
 import java.awt.*;

public class Lab04b {
    public void paint(Graphics g){
    //g.drawLine(0, 0, 200, 200);
    }
}

I have tried reopening Eclipse but it still shows the applet with a line on it just like the first time it ran. Please tell me how I can get Eclipse to update the applet in the built in applet viewer.

Answer 1

Your Lab04b class is not an Applet:

import java.applet.*;
import java.awt.*;

public class Lab04b {
    public void paint(Graphics g){
    g.drawLine(0, 0, 200, 200);
    }
}

Since an applet class must extend either Applet or JApplet, and yours does neither. I suggest:

• Have the class extend JApplet
• But don't draw directly within it.
• Instead draw within the paintComponent method of a JPanel that is displayed within the applet.
• Be sure to give your applet class an init() method where it will hold its initialization code.

For example:

import java.awt.Graphics;
import java.lang.reflect.InvocationTargetException;
import javax.swing.*;

// an applet class must extend either Applet or JApplet
public class AppletTest extends JApplet {

    // it should have an init() method where it holds its initialization code.
    @Override
    public void init() {
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                public void run() {
                    add(new DrawingPanel());
                }
            });
        } catch (InvocationTargetException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}

// avoid drawing directly within the applet itself
// but instead draw within a JPanel that is added to the applet
class DrawingPanel extends JPanel {

    // this is the method to draw in
    @Override
    protected void paintComponent(Graphics g) {
        // don't forget to call the super method to do "housekeeping" drawing
        super.paintComponent(g);
        g.drawLine(0, 0, 200, 200);
    }
}

• Having said this, consider not learning about applets since they're rarely used now-a-days.