当我调整DialogWindow的大小时，PaintComponent（）和getpreferredsize（）如何工作？

Question

下面是一个有用的Java代码，我不明白。 我已经超出了程序调用getPreferredsize（）和paintComponent（）方法的时间。我发现每次调整DialogWindow时，它都会调用getPreferredsize-method（）和两次paintcomponent-method（）。

为什么？

package examples;


import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.Toolkit;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;
import java.awt.image.BufferedImage;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.border.EtchedBorder;
import javax.swing.border.TitledBorder;


public class Bsp {

    public static void main(String[] args) {

        JFrame frame = new MyFrame();

    }

}

class MyFrame extends JFrame {

    JPanel panel=new JPanel(new GridBagLayout());

    private class MyLine extends JLabel {



        @Override
        protected void paintComponent(Graphics g) {

            super.paintComponent(g);

             Graphics2D g2d = (Graphics2D)g;

            g2d.setColor(Color.BLACK);
            g2d.setStroke(new BasicStroke(5));
            System.out.println("PaintComponent: "+getWidth()+" "+getHeight());
            g2d.drawLine(0, panel.getHeight()/2, panel.getWidth(), panel.getHeight()/2);
            g2d.dispose();



        }

        @Override
        public Dimension getPreferredSize() {
            System.out.println("PreferredSize: "+panel.getWidth()+" "+panel.getHeight());
            return new Dimension(panel.getWidth(),panel.getHeight());
        }



    }

    public MyFrame() {

        JLabel label=new MyLine();


        setLocationRelativeTo(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new GridLayout(3, 1));



        panel.add(label);



        add(new JButton("A"));
        add(new JButton("B"));
        add(panel);


        pack();
        centeringWindow();
        setVisible(true);



    }


    public void borderingJPanel(JComponent panel, String jPanelname,
            String fontStyle) {

        Font font = new Font(fontStyle, Font.BOLD + Font.ITALIC, 12);

        if (jPanelname != null) {
            panel.setBorder(BorderFactory.createTitledBorder(BorderFactory
                    .createEtchedBorder(EtchedBorder.LOWERED, Color.GRAY,
                            Color.WHITE), jPanelname,
                    TitledBorder.DEFAULT_JUSTIFICATION,
                    TitledBorder.DEFAULT_POSITION, font));
        } else if (jPanelname == null || fontStyle == null) {
            panel.setBorder(BorderFactory.createTitledBorder(BorderFactory
                    .createEtchedBorder(EtchedBorder.LOWERED, Color.GRAY,
                            Color.WHITE)));
        }

        panel.setOpaque(false);

    }

    public void centeringWindow() {
        Dimension dimension = Toolkit.getDefaultToolkit().getScreenSize();
        int x;
        int y;



        x = (int) (dimension.getWidth() - getWidth()) / 2;
        y = (int) (dimension.getHeight() - getHeight()) / 2;

        setLocation(x, y);
    }


    public BufferedImage getBufferedImage(){

        BufferedImage buffImage=null;

        try {
            buffImage=ImageIO.read(MyFrame.class.getClassLoader().getResourceAsStream("examples/rightAnswerSign.png"));
        } catch (IOException e) {
            e.printStackTrace();
        }

        return buffImage;

    }

}