SwingTest.java

package deltageek.test;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;

public class SwingTest {
    public static void main(String[] args) {
        SwingTest t = new SwingTest();
        t.buildAndShowGui();
    }

    private void buildAndShowGui() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new BorderLayout());

        JPanel panel = new JPanel();
        panel.setPreferredSize(new Dimension(640, 480));

        JPanel sizePanel = new JPanel();
        JLabel lblWidth = new JLabel("Width: ");
        JLabel lblHeight = new JLabel("Height: ");

        sizePanel.add(lblWidth);
        sizePanel.add(lblHeight);

        frame.addComponentListener(new ComponentAdapter() {
            @Override
            public void componentResized(ComponentEvent e) {
                lblWidth.setText("Width: " + panel.getWidth());
                lblHeight.setText("Height: " + panel.getHeight());
            }
        });

        frame.add(panel, BorderLayout.CENTER);
        frame.add(sizePanel, BorderLayout.SOUTH);

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