Why must I use getContentPane() instead of "this" when setting a BoxLayout for a JFrame?

Question

Why must I use getContentPane() instead of this keyword as a parameter argument for BoxLayout when setting a JFrame's layout to BoxLayout. To give a JPanel a BoxLayout, I must use this as a parameter.

I think this is because a JFrame has several layers or parts, which are the glass pane, layered pane, content pane, and menu bar. So the this keyword refers to the JFrame, but it does not refer to the content pane which we want to have a layout manager configured. That is why we invoke getContentPane() instead. I read that the content pane of a JFrame is actually a JPanel.

To summarize: BoxLayout's target parameter accepts JPanels but not JFrame, but JFrame's content pane is a JPanel.

class MyFrame1 extends JFrame {
    public MyFrame1() {    
        // This line does not work, why? 
        setLayout(new BoxLayout(this, BoxLayout.X_AXIS)); 
    }
}

class MyFrame2 extends JFrame {
    public MyFrame2() {    
        // This line works
        setLayout(new BoxLayout(getContentPane(), BoxLayout.X_AXIS));
    }
}

class MyPanel extends JPanel {
    public MyPanel() {
        // JPanel uses "this" keyword
        setLayout(new BoxLayout(this, BoxLayout.X_AXIS));
    }
}

Does a JPanel have multiple panes like a JFrame? What is the actual reason that I must use getContentPane()?

When the compiler says that BoxLayout can't be shared, does that mean that BoxLayout can not be shared between the multiple panes that compose a JFrame?

Yes, it happens every time the first parameter (target) takes a bad argument. — poetryrocksalot, Apr 18 '14 at 13:11
I think no error occured because I had the correct code in MyFrame1 (that was an accident and I have changed it to include the incorrect code). — poetryrocksalot, Apr 18 '14 at 13:24
"Exception in thread "main" java.awt.AWTError: BoxLayout can't be shared" — poetryrocksalot, Apr 18 '14 at 22:19
Err, just as I suspected, that's an exception. Nothing to do with the compiler whatsoever. — user207421, Apr 19 '14 at 00:07

score 1 · Answer 1 · answered Apr 18 '14 at 17:52

A JFrame is a Container, but it's not the Container "that needs to be laid out." In contrast, the Container, returned by getContentPane() is one that can use an instance of BoxLayout. Alternatively consider the variation below that uses Box, "A lightweight container that uses a BoxLayout object as its layout manager."

Box b = Box.createVerticalBox();
b.add(new JLabel("Test"));
f.setContentPane(b);

As tested:

import java.awt.EventQueue;
import javax.swing.Box;
import javax.swing.JFrame;
import javax.swing.JLabel;

/** @see http://stackoverflow.com/a/23159430/230513 */
public class Test {

    private void display() {
        JFrame f = new JFrame("Test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Box b = Box.createVerticalBox();
        b.add(new JLabel("Test"));
        f.setContentPane(b);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new Test().display();
            }
        });
    }
}

Why must I use getContentPane() instead of "this" when setting a BoxLayout for a JFrame?

1 Answers1