Java: Fenetre qui se ferme

import java.awt.Color; 
import java.awt.Graphics; 
  
import javax.swing.JFrame; 
import javax.swing.JPanel; 
import java.lang.*; 
  
public class main extends JFrame { 
  
        public main(){ 
                 
                this.setTitle("Ma première fenêtre java"); 
                this.setSize(100, 150); 
                this.setLocationRelativeTo(null);                
                this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
                this.setContentPane(new panneau()); 
                 
                this.setVisible(true); 
        } 
         
}

import java.awt.Color; 
import java.awt.Font; 
import java.awt.GradientPaint; 
import java.awt.Graphics; 
import java.awt.Graphics2D; 
import java.awt.Image; 
import java.io.File; 
import java.io.IOException; 
  
import javax.imageio.ImageIO; 
import javax.swing.JPanel; 
  
public class panneau extends JPanel { 
  
        public void paintComponent(Graphics g){ 
                 
                Graphics2D g2d = (Graphics2D)g;          
                GradientPaint gp = new GradientPaint(0, 0, Color.RED, 30, 30, Color.cyan, true);                 
                g2d.setPaint(gp); 
                g2d.fillRect(0, 0, this.getWidth(), this.getHeight()); 
                 
        }                
}