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 );
  }

}

------解决方案--------------------

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);
    }

}