Java JFrame with no layout manager

import java.awt.Color;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main {

  public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setTitle("Manual layout frame");

    frame.getContentPane().setLayout(null);
    /*  w ww .j  av a 2s . c o  m*/
    JPanel bluePanel = new JPanel(), 
        redPanel = new JPanel(), 
        greenPanel = new JPanel();
    bluePanel.setBackground(Color.blue);
    redPanel.setBackground(Color.red);
    greenPanel.setBackground(Color.green);

    bluePanel.setBounds(100, 100, 100, 100);
    redPanel.setBounds(50, 200, 400, 200);
    greenPanel.setBounds(150, 100, 50, 50);
    
    frame.add(bluePanel);
    frame.add(redPanel);
    frame.add(greenPanel);
    
    frame.pack();
    frame.setVisible(true);
    
    frame.setSize(500, 500);
  }
}