java JPanel How to fixed sizes

Question

I want to have a resizable panel, that always has the top green panel of a fixed depth. i.e. all changes in height should effect the yellow panel only.

My code below is almost OK, except the green panel varies in size a little.

How do I do this?

            Panel.setLayout(new BoxLayout(Panel, BoxLayout.Y_AXIS));
        Panel.setAlignmentX(Component.LEFT_ALIGNMENT);

        JPanel TopPanel = new JPanel();
        TopPanel.setPreferredSize(new Dimension(80,150));
        TopPanel.setVisible(true);
        TopPanel.setBackground(Color.GREEN);
        JPanel MainPanel = new JPanel();
        MainPanel.setPreferredSize(new Dimension(80,750));
        MainPanel.setVisible(true);
        MainPanel.setOpaque(true);
        MainPanel.setBackground(Color.YELLOW);

        Panel.add(TopPanel);
        Panel.add(MainPanel);

Answer 1

Your question didn't restrict the solution to a BoxLayout, so I am going to suggest a different layout manager.

I would attack this with a BorderLayout and put the green panel in the PAGE_START location. Then put the yellow panel in the CENTER location without a preferredSize call.

http://docs.oracle.com/javase/tutorial/uiswing/layout/border.html

Here is an SSCCE example of the solution:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class TestPad extends JFrame {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.getContentPane().setLayout(new BorderLayout());

        JPanel green = new JPanel();
        green.setPreferredSize(new Dimension(80, 150));
        green.setBackground(Color.GREEN);

        JPanel yellow = new JPanel();
        yellow.setBackground(Color.YELLOW);

        frame.getContentPane().add(green, BorderLayout.PAGE_START);
        frame.getContentPane().add(yellow, BorderLayout.CENTER);

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

Answer 2

If you make your Panel use BorderLayout instead of BoxLayout and put TopPanel in BorderLayout.NORTH and MainPanel in BorderLayout.CENTER, then they will both resize horizontally, but only the MainPanel will resize vertically.

See the BorderLayout documentation