Error: "NoClassDefFoundError"

Can anyone please explain to me why this html code will keep showing "NoClassDefFoundError" with a description "SimpleApp (wrong name: applet/simpleapp/SimpleApp)"?

It's a very simple code:

package applet.simpleapp;  
  
import java.awt.BorderLayout;  
import javax.swing.Box;  
import javax.swing.JApplet;  
import javax.swing.JLabel;  
  
public class SimpleApp extends JApplet {  
    private double result;  
      
    private JLabel firstNoLabel;  
    private JLabel secondNoLabel;  
    private JLabel resultLabel;  
      
    @Override  
    public void init() {  
        String firstNo = javax.swing.JOptionPane.showInputDialog(null,   
                "Enter the first number:");  
        String secondNo = javax.swing.JOptionPane.showInputDialog(null,   
                "Enter the second number:");  
          
        double firstNoDbl = Double.parseDouble(firstNo);  
        double secondNoDbl = Double.parseDouble(secondNo);  
        result = firstNoDbl * secondNoDbl;  
          
        // GUI stuff  
        firstNoLabel = new JLabel("1st number: " + firstNo);  
        secondNoLabel = new JLabel("2nd number: " +secondNo);  
        resultLabel = new JLabel("Result: " + result);  
          
        Box boxLayout = Box.createVerticalBox();  
        boxLayout.add(firstNoLabel);  
        boxLayout.add(secondNoLabel);  
        boxLayout.add(resultLabel);  
        add(boxLayout, BorderLayout.NORTH);  
    }  
}

And here is the html code:

<html>
<head>
<title></title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<Applet code="SimpleApp" height="500" width="300" >
</applet>
</body>
</html>

I compiled the code with a specified destination since I like it that my source and class files are separated. I put the html code in the same folder as the SimpleApp.class folder.

Any help appreciated. Thanks!

The class is in the package applet.simpleapp. You try to call it as SimpleApp, but because it is in the package applet.simpleapp the classes real name is applet.simpleapp.SimpleApp (not just SimpleApp, the 'real name' is called the fully specified name and is how you should refer to it from the HTML). In addition, the class SimpleApp must be in a directory structure that reflects the package it is in. So you should create a subfolder called applet, and subfolder of that called simpleapp, and put the SimpleApp.class file there:

Thanks for the reply, Steve!

I get it now. So I should put the code="applet.simpleapp.SimpleApp" in the html code instead of just SimpleApp. And I also need to put the html code on the top directory along with applet directory, in which contains simpleapp directory, in which contains SimpleApp.class.

Thanks a lot! It works ok now!