新手低分求助~关于绘图
题目：编写一个程序，以允许用户可以从一个JComboBox中选择一个形状，然后用paint方法在随机的位置用随机的尺寸绘制该形状20次。在第一次调用paint方法时，应将JComboBox中的第一个列表项作为默认形状来显示。
下面是我写的代码。问题有3个：
1、第一次调用paint方法绘制Line时点确认按钮没反应，必须是第二次选择Line后才可以绘制；
2、绘制一次后，第二次按确认按钮无法将前一次的绘制结果清除。
3.JLabel标签没有显示，当注释掉paint()方法后才会出现。
我是初学者，望各位强人不吝赐教，拜谢！



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

public class Heyu3 extends JFrame{
private JComboBox shapeBox;
private JButton yesButton;
private JLabel shapeLabel, label;
private String shapeNames[] = { "Line", "Rectangle", "Oval"};
private String shape;


public Heyu3(){
super("绘制自选图形");

Container container = getContentPane();
container.setLayout( new FlowLayout() );

shapeLabel = new JLabel("请选择您要绘制的图形：");

shapeBox = new JComboBox( shapeNames );
shapeBox.setMaximumRowCount( 3 );
shapeBox.addItemListener(

new ItemListener(){

public void itemStateChanged( ItemEvent event ){
shape = shapeNames[ shapeBox.getSelectedIndex() ];

}
}

);

yesButton = new JButton("确定");
yesButton.addActionListener( new YesButtonHandler() );

container.add( shapeLabel );
container.add( shapeBox );
container.add( yesButton );

setSize( 500, 500 );
setVisible( true );

}

private class YesButtonHandler implements ActionListener{

public void actionPerformed( ActionEvent event ){

if( event.getSource() == yesButton)

repaint();

}
}

public void paint( Graphics g ){
for(int count=0;count<20;count++){

if(shape.equals("Line"))
g.drawLine((int)(Math.random()*100)+50, (int)(Math.random()*100)+150,(int)(Math.random()*100)*(int)(Math.random()*10)+20, (int)(Math.random()*10)*(int)(Math.random()*10)+20);

else if(shape.equals("Rectangle"))
g.drawRect((int)(Math.random()*100)+50, (int)(Math.random()*100)+150,(int)(Math.random()*100)*(int)(Math.random()*10)+20, (int)(Math.random()*10)*(int)(Math.random()*10)+20);

else
g.drawOval((int)(Math.random()*100)+50, (int)(Math.random()*100)+150,(int)(Math.random()*100)*(int)(Math.random()*10)+20, (int)(Math.random()*10)*(int)(Math.random()*10)+20);

}

}
public static void main( String args[]){
Heyu3 application = new Heyu3();
application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
}

}

[解决办法]

Java code

import java.awt.*;import java.awt.event.*;import javax.swing.*;public class Heyu3 extends JFrame {    private JComboBox shapeBox;    private JButton yesButton;    private JLabel shapeLabel, label;    private String shapeNames[] = {"Line","Rectangle","Oval"};    private String shape="";//赋初值，以免报空指针异常    public Heyu3() {        super("绘制自选图形");        Container container = getContentPane();        container.setLayout(new FlowLayout());        shapeLabel = new JLabel("请选择您要绘制的图形：");        shapeBox = new JComboBox(shapeNames);        shapeBox.setMaximumRowCount(3);//        shapeBox.addItemListener(这是为那般??我感觉没用,除非你想根据这个触发.//        new ItemListener(){//            public void itemStateChanged(ItemEvent event) {//                shape = shapeNames[shapeBox.getSelectedIndex()];//            }//        });        yesButton = new JButton("确定");        yesButton.addActionListener(new YesButtonHandler());        container.add(shapeLabel);        container.add(shapeBox);        container.add(yesButton);        setSize(500, 500);        setVisible(true);    }    private class YesButtonHandler implements ActionListener {        public void actionPerformed(ActionEvent event) {            if (event.getSource() == yesButton)                shape = (String) shapeBox.getSelectedItem();//赋值整到这里来                repaint();        }    }    public void paint(Graphics g){        super.paint(g);        for (int count = 0; count < 20; count++) {            if (shape.equals("Line"))                g.drawLine((int) (Math.random() * 100) + 50, (int) (Math                        .random() * 100) + 150, (int) (Math.random() * 100)                        * (int) (Math.random() * 10) + 20,                        (int) (Math.random() * 10) * (int) (Math.random() * 10)                                + 20);            else if (shape.equals("Rectangle"))                g.drawRect((int) (Math.random() * 100) + 50, (int) (Math                        .random() * 100) + 150, (int) (Math.random() * 100)                        * (int) (Math.random() * 10) + 20,                        (int) (Math.random() * 10) * (int) (Math.random() * 10)                                + 20);            else if (shape.equals("Oval"))//是这里。。。，别整个else                g.drawOval((int) (Math.random() * 100) + 50, (int) (Math                        .random() * 100) + 150, (int) (Math.random() * 100)                        * (int) (Math.random() * 10) + 20,                        (int) (Math.random() * 10) * (int) (Math.random() * 10)                                + 20);        }    }    public static void main(String args[]) {        Heyu3 application = new Heyu3();                application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    }}