为什么要打印两次？

Question

I've noticed that when i use paintComponent in java if I use System.out.println(); things will print out 2, 3, and sometimes 4 times. I know that when you use extends JPanel it will automatically be called, but why more then once.

Here is some code to try yourself.

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

public class stack extends JPanel{
public stack(){
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
    frame.add(this);
    frame.setLocationRelativeTo(null);
    frame.setSize(200, 200);
    }
public static void main(String args[]){
    stack s = new stack();
    }
public void paintComponent(Graphics g){
    super.paintComponents(g);
    g.drawString("Thank You!", 100, 100);
    System.out.println("Why?");
}
}

Answer 1

System.out.println gets called once - but your method could be called repeatedly. Everytime paintComponent is called it prints Why? .

This method is being called multiple times, ie, for frame resizes or such and in turn makes it appear that sysout is being executed more than once.