java-swing在组件中显示信息

package com.http;
import java.awt.*;

import javax.swing.*;

public class TestSwing2
{
//创建了一个能够绘制的组件
class HelloWorldComponent extends JComponent
{
    public static final int MESSAGE_X = 75;
    public static final int MESSAGE_Y = 100;

    private static final int DEFAULT_WIDTH = 300;
    private static final int DEFAULT_HEIGHT = 200;

    public void paintComponent(Graphics g)
    {
        g.drawString("hello world",MESSAGE_X,MESSAGE_Y);
    }

    public Dimension getPreferredSize()
    {
        return new Dimension(DEFAULT_WIDTH,DEFAULT_HEIGHT);
    }
}

//添加内容到frame
class HelloWorldFrame extends JFrame
{
    public HelloWorldFrame()
    {
        add(new HelloWorldComponent());
        pack();
    }
}

public static void main(String[] argvs)
{
    EventQueue.invokeLater(new Runnable()
    {
        public void run()
        {

　　　　　　　//在实例化内部类时，需要先实例化外部类
            JFrame frame = new TestSwing2().new HelloWorldFrame();
            frame.setTitle("hello world");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setVisible(true);
        }
    });

}

}