Java Swing GUI BorderLayout: Location of components

I'm new to Java and I'm playing around with a simple GUI example:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Rectangle2D;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.WindowConstants;

public class DrawTest {

    class DrawingPanel extends JPanel {

        private Rectangle2D shape;

        public DrawingPanel(Rectangle2D shape) {
            this.shape = shape;
        }

        public void paintComponent(Graphics g) {

            Graphics2D g2D = (Graphics2D) g;            
            super.paintComponent(g2D);  
            g2D.setColor(new Color(31, 21, 1));
            g2D.fill(shape);

        }

    }


    public void draw() {
        JFrame frame = new JFrame();
        Rectangle2D shape = new Rectangle2D.Float();
        final DrawingPanel drawing = new DrawingPanel(shape);

        shape.setRect(0, 0, 400, 400);
        frame.getContentPane().add(BorderLayout.NORTH, new JButton("TestN"));
        frame.getContentPane().add(BorderLayout.SOUTH, new JButton("TestS"));
        frame.getContentPane().add(BorderLayout.EAST, new JButton("TestE"));
        frame.getContentPane().add(BorderLayout.WEST, new JButton("TestW"));
        frame.getContentPane().add(BorderLayout.CENTER, drawing);
        frame.pack();
        frame.setSize(500,500);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setVisible(true);  
    }
}

public class DrawMain {
    public static void main(String[] args) {
        DrawTest test = new DrawTest();
        test.draw();

    }
}

As expected, this code produces a frame with the rectangle at the centre and buttons around it. However, if I change the code like this:

        frame.getContentPane().add(BorderLayout.NORTH, drawing);
        frame.getContentPane().add(BorderLayout.SOUTH, new JButton("TestS"));
        frame.getContentPane().add(BorderLayout.EAST, new JButton("TestE"));
        frame.getContentPane().add(BorderLayout.WEST, new JButton("TestW"));
        frame.getContentPane().add(BorderLayout.CENTER, new JButton("TestC"));

the "TestC" button gets a huge area in the middle while the rectangle doesn't get enough space. This is even true if I remove the other buttons (TestS, TestE, TestW): I get a huge TestC button and a tiny part of the rectangle (even not the scaled rectangle) at the top.

Why doesn't the rectangle get enough space when it's drawn at the top (NORTH) but does get it when it's drawn at the CENTER?

Solution

The DrawingPanel should @Override getPreferredSize() to return an appropriate size.

The layout manager will then take that preferred size as a hint. Some layout managers will expand a component's height or width according to the logic of the layout and constraint. E.G. a BorderLayout will stretch components in the PAGE_START / PAGE_END to the width of the content pane, and LINE_START / LINE_END to the height of the tallest of either of those, or the CENTER. A GridBagLayout OTOH will completely hide / remove a component for which there is not enough space to display it at the preferred size, and that's where 'pack' comes in.

So change frame.setSize(500,500); (which is no better than a guess) to frame.pack();, which will make frame the minimum size it needs to be, in order to display the components it contains.