Thread: problem with BorderLayout

I'm trying to layout a JPanel with a button and a scrollable textarea so that the button is at the top and the textarea takes up all the available remaining space. From what I read borderlayout should be able to do that but when I run it, all I get is a blank container. When I change to borderlayout to the default flowlayout the button and textarea show up but don't fill out the panel. So I have 2 questions: why is nothing showing up and when it does show up will this do what I want it to do?

package Test;
 
import java.awt.*;
import javax.swing.*;
 
public class TestWindow
{
	private JFrame frame;
	private CustomPanel panel;
 
	public TestWindow()
	{
		frame = new JFrame("Test");
		frame.setPreferredSize(new Dimension(800,600));
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		panel = new CustomPanel();
		//panel.setLayout(new BorderLayout());
		frame.add(panel);
		frame.pack();
		frame.setVisible(true);
	}
 
	public static void main(String[] args)
	{
		SwingUtilities.invokeLater( new Runnable()
		{
			public void run()
			{
				TestWindow test = new TestWindow();		
			}
		});
	}
 
}
 
class CustomPanel extends JPanel
{
	private JTextArea display;
 
	public CustomPanel()
	{
		super();
		JButton button = new JButton("test");
		add(button, BorderLayout.PAGE_START);
		display = new JTextArea();
		display.setText("test message aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");		
		JScrollPane scrollpane = new JScrollPane(display);
		scrollpane.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
		add(scrollpane, BorderLayout.CENTER);		
	}
}

JPanel's layout is not BorderLayout by default, it's FlowLayout. You have to set JPanel's Layout to be BorderLayout:

// in the CustomPanel() constructor:
setLayout( new BorderLayout() );

I know, I forgot to uncomment the line before i copy-pasted the code into my post. (it's right below panel = new CustomPanel(); on line 17)

I'm not getting the results you describe. I have a "test" button across the top and the rest of the panel filled with the JTextArea, BECAUSE the layout is set FIRST and then the components are added to the panel. Do things in the correct place in the correct order. Put the setLayout() statement near the top of the constructor as I suggested.

I see, I had no idea that mattered, but that explains it. It does indeed work now.

Just think about it a bit - I hope you have.

Your original version creates the panel in the TestClass() constructor by calling the CustomPanel() constructor. The CustomPanel() constructor adds all of the components to the panel while it has the default FlowLayout. After the CustomPanel() constructor is done, control returns to the TestClass() constructor which then sets the panel's layout to BorderLayout, but at that point it's too late. The components have already been added to the panel, and changing the layout has no effect - at least not until more components are added to the panel, and then I'm not sure what will happen. I think it won't be pleasant. In fact, that might be why you sometimes saw a blank. Perhaps the pack() statement did something to the panel using the new layout, but I'm not sure.