Could not find or load main class swing.ComboDemo Caused by: java.lang.ClassNotFoundException: swing.ComboDemo on this code

Question

I get error on the code below:

Could not find or load main class swing.ComboDemo Caused by: 
   java.lang.ClassNotFoundException: swing.ComboDemo 

---

package swing;

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

public class ComboDemo extends JFrame implements ItemListener{

    JComboBox<String> box;
    JLabel lbl;
    
ComboDemo(){
    Container c = getContentPane();
    c.setLayout(null);
    box=new JComboBox<String>();
    box.addItem("India");
    box.addItem("America");
    box.addItem("Pakistan");
    box.addItem("Uk");
    box.setBounds(300,300,40,60);
    c.add(box);

    lbl = new JLabel();
    lbl.setBounds(300, 500, 70, 30);
    c.add(lbl);

    box.addItemListener(this);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

public void itemStateChanged(ItemEvent ie) {
    String str = (String)box.getSelectedItem();
    lbl.setText(str);
}

    public static void main(String[] args) {
      ComboDemo demo = new ComboDemo();
      demo.setSize(600,600);
      demo.setVisible(true);
    }
}