How to make JPanel size apply to JFrame?

I'm having an issue with the size of the JPanel not increasing a trhe size of a JFrame.

Here is my code:

package pong;

import java.awt.Dimension;

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

public class Test {

	public static void main(String[] args) {
		JFrame frame = new JFrame();
		
		JPanel p = new JPanel();
		p.setPreferredSize(new Dimension(500, 500));

		frame.add(p);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setResizable(false);
		frame.setVisible(true);
		
		frame.setLocationRelativeTo(null);
	}
}

The frame is displayed with little width or height at all, even though the panel's preferred size is 500 by 500 px.

> The JFrame is displayed with little width or height at all,

The code should be:

frame.pack(); // added
frame.setVisible(true);

The pack() method will invoke the layout manager used by the frame and all the Swing components will be displayed at their preferred size.

Also, as a general rule there should be no need to set the preferred size of the panel manually since you will be adding components to the panel and therefore the preferred size should be based on the components you add to the panel.

Read the section from the Swing tutorial on Using Layout Managers for more information and examples.