这个程序是如何运行的？

Question

我开始学习c ++但是现在已经切换到Java了，因为它似乎更容易进入。

有人可以向我解释一下这个paintcomponent函数是如何运行的，而不是在主函数中严格调用???

package javaapplication3;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Line2D;
import javax.swing.JButton;

import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

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

public class JavaApplication3 extends JPanel{

    public void paintComponent(Graphics g) {
    super.paintComponent(g);

    Graphics2D g2d = (Graphics2D) g;

    g2d.setColor(Color.blue);    

      double y, x, z, q;
      int amp = 30;
      int amp2 = 50;

      for(x = 0; x < 1000; x += .01){

        q = amp * Math.sin(.05 * x);
        z = amp2 * Math.sin(q + x);

        g2d.draw(new Line2D.Double(x ,(z + 100) ,x ,(z + 100)));
      }
  }

    public static void main(String[] args) {
    JavaApplication3 points = new JavaApplication3();
    JFrame frame = new JFrame("Sine");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.add(points);
    frame.setSize(1000, 250);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }

}