java的repaint()没有重绘，怎么回事呀。

Question

package chapter17;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class showRadioButton extends JFrame{

/**
 * @param args
 */
showRadioButton(){
    creatButtonPanel bp =new creatButtonPanel();
    messagePanel mp = new messagePanel();

    setLayout(new BorderLayout());
    add(bp,BorderLayout.SOUTH);
    add(mp,BorderLayout.CENTER);

    messagePanel  messagepanel = new messagePanel();
    setSize(800,600);
    setLocationRelativeTo(null);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setVisible(true);
}
public static void main(String[] args) {
    // TODO 自动生成的方法存根
    showRadioButton srb = new showRadioButton();


}
//生成String，并操作String
class messagePanel extends JPanel{
    private String s = "Welcome to Java";
    private boolean bl = true;
    private int x ;
    private int y ;

    // 第一次绘制图形
    protected void paintComponent(Graphics g){
        super.paintComponent(g);
        // 设置string的位置   
        if(bl == true){
            FontMetrics fm = g.getFontMetrics();
            int sw = fm.stringWidth(s)/2;
            int sh = fm.getAscent()/2;
            x = getWidth()/2 - sw;
            y = getHeight()/2 - sh;
            //开始绘制sting并且设置布尔值
            g.drawString(s,x,y);
            bl = false;
        }else{
            g.drawString(s,x,y);
        }
    }
    //重绘方法
    protected  void left(){
        x = x - 50;
        repaint();
    }
    public void right() {
        x = x + 50;
        repaint();
    }
}

//绘制button按钮并出册事件

class creatButtonPanel extends JPanel{
    public creatButtonPanel(){
    JButton jbtl = new JButton("left");
    JButton jbtr = new JButton("rigtht");
    jbtl.setMnemonic('L');
    jbtr.setMnemonic('R');
    //showRadioButton srb = new showRadioButton();


    add(jbtl);
    add(jbtr);
    jbtl.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){
            messagePanel mp = new messagePanel();
            mp.left();                    // 重要   外部类调用内部类
        }
    });
    jbtl.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){
            new messagePanel().right();
        }
    });
    //return buttonPanel;
    }
}

}

Answer 1

你对象是内部类的,超出这个作用域之后会直接销毁,那么你每次重绘的都是不一样的对象,而不是同一个,每次都新建对象,你将这个对象设置为全局的看一下