Calling a JPanel in a JFrame of another Class

Question

I have created a class with one JPanel which will be used for the CardLayout, this works fine if I remove the comment // which has the window size at the bottom of the code. It will run that way and everything functions perfectly. However when I try to call it from another class which has a JFrame in it, it doesn't work.

CardDemo.java:

import java.awt.CardLayout;
import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.event.ListSelectionEvent;
import javax.swing.event.ListSelectionListener;
import java.awt.*;

public class CardDemo extends JPanel implements ListSelectionListener {

CardLayout cl;
JPanel p1, p2, p3, p4, p5;
JList l1;


public CardDemo(){

    p1 = new JPanel();
    p1.setBackground(Color.red);   //the top panel

    String[] list1 = { "One", "Two", "Three"};
    l1 = new JList(list1);
    l1.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
    l1.addListSelectionListener(this);

    p1.add(l1);

    p2 = new JPanel();
    p2.setBackground(Color.blue);  //bottom panel - we never actually see this

    cl = new CardLayout();
    p2.setLayout(cl);


    p3 = new JPanel();
    p3.setBackground(Color.green);
    p4 = new JPanel();
    p4.setBackground(Color.yellow); 
    p5 = new JPanel();
    p5.setBackground(Color.cyan);


    p2.add(p3, "One");
    p2.add(p4, "Two");
    p2.add(p5, "Three");

    //this.setSize(500,400);
   //this.setVisible(true);
   this.setLayout(new GridLayout(2,2));
   this.add(p1);
   this.add(p2);

}

/** The actionPerformed method handles button clicks
 */
public void valueChanged(ListSelectionEvent e) {
    String listLabel = (String) ((JList)e.getSource()).getSelectedValue();    //get the label of the button that was clicked
    cl.show(p2, listLabel);             //use the label to display the relevant panel
}
}   

Test.java

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

public class Test extends JFrame
{

public Test()
{
  JFrame frame = new JFrame("CardLayoutDemo");
  frame.setDefaultCloseOperation(EXIT_ON_CLOSE);

  CardDemo b = new CardDemo();
  b.add(frame);

  frame.pack();
  frame.setVisible(true);
}


}

Everything compiles just fine, but when I run Test.java I get the following error:

java.lang.IllegalArgumentException: adding a window to a container (in java.awt.Container)

What is it that I am doing wrong as I can't seem to pinpoint it.

Answer 1

You need to change

b.add(frame);  

to

frame.add(b);

Answer 2

You're adding your JFrame to your JPanel. It should be the other way around. The frame variable refers to a JFrame which is a top level window. This is the top level container of all your components, and so you'll want to add components to it, not the other way around.