Java 在JLabel中绘制线并在JFrame中添加为JPanel不起作用

我对Java非常陌生，所以很抱歉有任何愚蠢的问题

我的问题是：

我只想画一条线-如果我将

JLabel

Myline（）

直接作为

label

添加到

JFrame MyFrame

，那么它将显示在生成的

JFrame

-对话框窗口中，但是如果我添加

Myline（）

到

JPanel

面板

和

面板

到

MyFrame

则没有行

为什么?

---

MyFrame

没有首选尺寸，直到

pack（）

查询封闭组件的首选尺寸。给MyLine指定一个首选尺寸，以查看效果。另见

package examples;


import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.Toolkit;
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 {

    private class MyLine extends JLabel {

        int width;

        public MyLine(int width) {
            this.width=width;
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.BLACK);
            g.drawLine(0, 0, getWidth(), 0); 
            g.dispose();


        }

    }

    public MyFrame() {

        JLabel label=new MyLine(getWidth());
        JPanel panel=new JPanel(new GridBagLayout());

        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();

        setVisible(true);


    }



}

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class Bsp {

    public static void main(String[] args) {
        JFrame frame = new MyFrame();
    }
}

class MyFrame extends JFrame {

    private class MyLine extends JLabel {

        int width;

        public MyLine(int width) {
            this.width = width;
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.BLACK);
            g.drawLine(0, 0, getWidth(), getHeight());
            g.dispose();
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(width, 16);
        }

    }

    public MyFrame() {

        JLabel label = new MyLine(32);
        System.out.println(getWidth());
        JPanel panel = new JPanel(new GridBagLayout());

        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();
        setVisible(true);

    }
}