Seek guidance getPreferredSize () usage

import javax.swing.JPanel;
 import java.awt.Color;
 import java.awt.Dimension;
 import javax.swing.JFrame;
 import java.awt.Graphics;
 public class ShowRect extends JFrame{
 	private static final long serialVersionUID = 1L;
 	public ShowRect(){
 		setTitle("ShowRect");
 		getContentPane().add( new RectJPanel() );
 	}
 	
 	public static void main(String[] args){
 		ShowRect frame = new ShowRect();
 		frame.setSize(450,450);
 		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 		frame.setVisible(true);
 	}
 	
 	class RectJPanel extends JPanel{
 		private static final long serialVersionUID = 1L;
 
 		protected void paintComponent( Graphics g ){
 			super.paintComponent(g);
 			int xx,yy=0;
 			for(int i = 1;i<=4;i++){
 				xx = 0;
 				for( int j=1;j<=4;j++){
 					g.setColor(Color.white);
 					g.fillRect(xx,yy,50,50);
 					xx += 50;
 					g.setColor(Color.BLACK);
 					g.fillRect(xx,yy,50,50);
 					xx += 50;
 				}
 				yy +=50;
 				xx = 0;
 				for( int j=1;j<=4;j++){
 					g.setColor(Color.BLACK);
 					g.fillRect(xx,yy,50,50);					
 					
 					xx += 50;
 					g.setColor(Color.WHITE);
 					g.fillRect(xx,yy,50,50);
 					xx += 50;
 				}
 				yy += 50;
 			}
 			
 		}
 	public Dimension getPreferredSize(){
 			return new Dimension( 200,200 );
 	}
 	}
 }

I want out of this display screen change automatically as the window changes , but with getPreferredSize () does not work, ask how to use this method , how to get out of the picture as the window display changes automatically change ~ < br> ------ Solution ----------------------------------------- ---
swing with setPreferredSize methods generally do not override the getPreferredSize method .

JFrame calling pack
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top
up ...
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main inside plus a
frame.pack ();
In addition to your black and white plaid number , size is limited, a total of eight grid