一个关于paintComponent的小细节问题

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

public class TestPolyline extends JFrame {
    public TestPolyline() {
        add(new PolylinePanel());
    }
    public static void main(String[] args) {
        TestPolyline frame = new TestPolyline();
        frame.setTitle("TestPolyline");
        frame.setSize(300,200);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
    
    static class PolylinePanel extends JPanel {
        private Polygon p = new Polygon();
        
        public PolylinePanel() {
            
        }
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            p.addPoint(getWidth() / 2, getHeight() / 2);
            p.addPoint(50, 50);
            g.drawPolyline(p.xpoints, p.ypoints, p.npoints);
            g.drawString("" + (p.xpoints).length, 50, 50);
        }
    }
}

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

public class TestPolyline extends JFrame {
    public TestPolyline() {
        add(new PolylinePanel());
    }
    public static void main(String[] args) {
        TestPolyline frame = new TestPolyline();
        frame.setTitle("TestPolyline");
        frame.setSize(300,200);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
    
    static class PolylinePanel extends JPanel {
        private Polygon p = new Polygon();
        
        public PolylinePanel() {
            p.addPoint(getWidth() / 2, getHeight() / 2);
            p.addPoint(50, 50);
        }
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawPolyline(p.xpoints, p.ypoints, p.npoints);
            g.drawString("" + (p.xpoints).length, 50, 50);
        }
    }
}