Swing's GroupLayout and setContentPane - losing Components?

Question

After experimentation, it appears to me that Swing's GroupLayout tends to lose Components intended for re-use across my GUI.

But I haven't seen anything in the documentation that makes this single-use rule clear. This makes me wonder if I have made a mistake, or if I am a poor reader.

For example, I make a JPanel with a GroupLayout of a JButton("Foo"). Then I make another JPanel with a GroupLayout of the same JButton renamed "Bar".

If I switch back to the first JPanel from the second JPanel with JFrame.setContentPane, I lose the JButton in the first JPanel.

Can anyone explain why it is losing Components and further, can anyone provide a way to overcome the tendency to lose Components?

Here is a complete SSCCE demonstrating the problem:

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

public class GroupLayoutTest {

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            deployGroupLayoutTest();
        }
    });
}

static JPanel firstPanel;
static JButton jbtnActionLog;
static JFrame systemFrame;

public static void deployGroupLayoutTest() {
    systemFrame = new JFrame("Group Layout Test");
    systemFrame.setSize(300, 300);

    firstPanel = new JPanel();

    JMenuBar jmbSystem = new JMenuBar();

    JMenu jmuAction = new JMenu("Action");

    JMenuItem jmiActionLog = new JMenuItem("Login");
    jmuAction.add(jmiActionLog);

    jmbSystem.add(jmuAction);

    jbtnActionLog = new JButton("Login");
    jbtnActionLog.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            setContentPaneToSecondPanel();
        }
    });

    systemFrame.setJMenuBar(jmbSystem);

    GroupLayout gl = new GroupLayout(firstPanel);
    firstPanel.setLayout(gl);
    gl.setAutoCreateContainerGaps(true);
    gl.setAutoCreateGaps(true);

    GroupLayout.ParallelGroup hGroup = gl.createParallelGroup(GroupLayout.Alignment.CENTER);
    hGroup
            .addComponent(jbtnActionLog);

    gl.setHorizontalGroup(hGroup);

    GroupLayout.SequentialGroup vGroup = gl.createSequentialGroup();
    vGroup
            .addComponent(jbtnActionLog);
    gl.setVerticalGroup(vGroup);

    systemFrame.getContentPane().add(firstPanel);
    systemFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    systemFrame.setLocationByPlatform(true);
    systemFrame.setVisible(true);

}

public static void setContentPaneToSecondPanel() {
    jbtnActionLog.setText("Logout");
    ActionListener[] listenerList = jbtnActionLog.getActionListeners();
    jbtnActionLog.removeActionListener(listenerList[0]);
    jbtnActionLog.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            systemFrame.setContentPane(firstPanel);
            systemFrame.revalidate();
        }
    });

    JPanel secondPanel = new JPanel();

    GroupLayout gl = new GroupLayout(secondPanel);
    secondPanel.setLayout(gl);
    gl.setAutoCreateContainerGaps(true);
    gl.setAutoCreateGaps(true);

    GroupLayout.ParallelGroup hGroup = gl.createParallelGroup(GroupLayout.Alignment.CENTER);
    hGroup
            .addComponent(jbtnActionLog);

    gl.setHorizontalGroup(hGroup);

    GroupLayout.SequentialGroup vGroup = gl.createSequentialGroup();
    vGroup
            .addComponent(jbtnActionLog);
    gl.setVerticalGroup(vGroup);

    systemFrame.setContentPane(secondPanel);
    systemFrame.revalidate();
}

}

score 2 · Accepted Answer · edited May 23 '17 at 12:12

I did not go through all of your code, but it is simply not possible to add a single Swing component to multiple parents. Each component can only be present in one location in the Swing hierarchy. So the code

JPanel firstPanel = ...;
JPanel secondPanel = ...;
JButton button = ...;
firstPanel.add( button );
secondPanel.add( button );

will result in button only contained in one of the panels, and not in both. This has nothing to do with the GroupLayout.

A relevant SO question contains the link to the Swing tutorial explaining this:

Each GUI component can be contained only once. If a component is already in a container and you try to add it to another container, the component will be removed from the first container and then added to the second

Aha! Thank you for explaining this issue. I guess the answer to the problem is to generate new Components but with the same Action or ActionListener ... +1 for a clear and instructive answer. — Arvanem, Apr 20 '12 at 19:28

Swing's GroupLayout and setContentPane - losing Components?

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