JPanel with preferred size

Test.java

import java.awt.*;
import javax.swing.*;

public class Test extends JPanel {
	public Test() {
		setPreferredSize(new Dimension(400, 400));
	}
	
	public static void main(String[] args) {
		JFrame frame = new JFrame();
		frame.add(new Test());
		frame.pack();
		frame.setVisible(true);
	}
}