ClassNotFound Exception when calling Applet

Question

I am trying to set up a simple JApplet using IntelliJ. I have a class called Square and a HTML file which should make it work, but I keep getting a ClassNotFoundException.

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

public class Square extends JApplet {

    int size = 40;

    public void init() {
        JButton butSmall = new JButton("Small");
        JButton butMedium = new JButton("Medium");
        JButton butLarge = new JButton("Large");
        JButton butMessage = new JButton("Say Hi!");

        SquarePanel panel = new SquarePanel(this);
        JPanel butPanel = new JPanel();
        butPanel.add(butSmall);
        butPanel.add(butMedium);
        butPanel.add(butLarge);
        butPanel.add(butMessage);
        add(butPanel, BorderLayout.NORTH);
        add(panel, BorderLayout.CENTER);
    }
}

class SquarePanel extends JPanel {

    Square theApplet;

    SquarePanel(Square app) {
        theApplet = app;
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.green);
        g.fillRect(20, 20, theApplet.size, theApplet.size);
    }
}

and the HTML file

<HTML>
<APPLET CODE="Square.class"> WIDTH=400 HEIGHT=200>
</APPLET>
</HTML>

This is the folder structure. I've tried lots of different combos and names and <> delimiters but I can't get it to open properly.

folder image

score 2 · Answer 1 · edited May 23 '17 at 10:33

The problem is that the applet container, typically a browser, has been told where to find the class Square but not the class SquarePanel. You can do either of two things:

Enclose your classes in a JAR and specify the archive name in your <APPLET\> tag, as shown here.
Nest SquarePanel in Square, as shown below for illustration purposes.

A JAR is the preferred approach, but also consider a hybrid for more flexible testing and deployment. For convenient appletviewer testing, the tag is included in a comment, as shown here.

Command line:

$ appletviewer Square.java

Code, as tested:

// <applet code='Square' width='400' height='200'></applet>
import javax.swing.*;
import java.awt.*;

public class Square extends JApplet {

    int size = 40;

    public void init() {
        JButton butSmall = new JButton("Small");
        JButton butMedium = new JButton("Medium");
        JButton butLarge = new JButton("Large");
        JButton butMessage = new JButton("Say Hi!");

        SquarePanel panel = new SquarePanel(this);
        JPanel butPanel = new JPanel();
        butPanel.add(butSmall);
        butPanel.add(butMedium);
        butPanel.add(butLarge);
        butPanel.add(butMessage);
        add(butPanel, BorderLayout.NORTH);
        add(panel, BorderLayout.CENTER);
    }

    private static class SquarePanel extends JPanel {

        Square theApplet;

        SquarePanel(Square app) {
            theApplet = app;
        }

        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.green);
            g.fillRect(20, 20, theApplet.size, theApplet.size);
        }
    }
}

See also [*Why CS teachers should stop teaching Java applets*](http://programmers.blogoverflow.com/2013/05/why-cs-teachers-should-stop-teaching-java-applets/) and [*Initial Threads*](http://docs.oracle.com/javase/tutorial/uiswing/concurrency/initial.html). — trashgod, Oct 31 '14 at 17:43

ClassNotFound Exception when calling Applet

1 Answers1