Question

I have initialized MotePanel, Command Panel and LEDPanel before setting their Layout, then how so am I getting this exception.

Please help.

Exception in thread "main" java.awt.AWTError: BoxLayout can't be shared
    at javax.swing.BoxLayout.checkContainer(BoxLayout.java:462)
    at javax.swing.BoxLayout.invalidateLayout(BoxLayout.java:246)
    at javax.swing.BoxLayout.addLayoutComponent(BoxLayout.java:279)
    at java.awt.Container.addImpl(Container.java:1107)
    at java.awt.Container.add(Container.java:974)
    at javax.swing.JFrame.addImpl(JFrame.java:556)
    at java.awt.Container.add(Container.java:377)
    at Window.<init>(Window.java:54)

---

public class Window extends JFrame{
    private JPanel MotePanel;
    private JPanel LEDPanel;
    private JPanel CommandPanel;
    private JCheckBox motes[];
    private JRadioButton Leds[];

    public Window(){
        this.setLayout(new BoxLayout(this,BoxLayout.X_AXIS));
        this.setTitle("Sensor Networks Lab");
        this.setSize(300, 200);
        this.setLocationRelativeTo(null);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);

        MotePanel = new JPanel();
        LEDPanel = new JPanel();
        CommandPanel = new JPanel();
        motes = new JCheckBox[10];
        Leds = new JRadioButton[3];


        MotePanel.setLayout(new BoxLayout(MotePanel, BoxLayout.Y_AXIS));
        CommandPanel.setLayout(new BoxLayout(CommandPanel, BoxLayout.Y_AXIS));
        LEDPanel.setLayout(new BoxLayout(LEDPanel, BoxLayout.Y_AXIS));

        System.out.println("creating MotePanel");
        for(int i=0; i<10; i++){
            motes[i] = new JCheckBox("Mote "+i);
            MotePanel.add(motes[i]);
        }


        System.out.println("creating LEDPanel");
        for(int i=0; i<3; i++)
            Leds[i] = new JRadioButton();
        Leds[0].setText("RED");
        LEDPanel.add(Leds[0]);
        Leds[1].setText("GREEN");
        LEDPanel.add(Leds[1]);
        Leds[2].setText("BLUE");
        LEDPanel.add(Leds[2]);

        this.add(MotePanel);
        this.add(LEDPanel);
        this.add(CommandPanel);
    }

Answer 1

When calling setLayout on a JFrame, you’re actually adding the layout to the JFrame’s contentPane not the JFrame itself since this method is more of a convenience method that transmits the method call to the contentPane. The BoxLayout constructor must reflect this since you can’t add the BoxLayout to one container and then pass in as a parameter a different container. So change this:

this.setLayout(new BoxLayout(this,BoxLayout.X_AXIS));

to this:

setLayout(new BoxLayout(getContentPane(), BoxLayout.X_AXIS));

Also: no need for all the this. business since this. is implied, and also there is no actual need here to extend JFrame.

edit: this code below is all that is needed to demonstrate your error and its solution:

import javax.swing.*;

public class BoxLayoutFoo extends JFrame {
   public BoxLayoutFoo() {

      // swap the comments below
      setLayout(new BoxLayout(this, BoxLayout.LINE_AXIS)); // comment out this line
      //setLayout(new BoxLayout(getContentPane(), BoxLayout.LINE_AXIS)); // uncomment this line

      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      pack();
      setVisible(true);
   }

   public static void main(String[] args) {
      new BoxLayoutFoo();
   }
}