为什么我不能用红，黄，蓝画矩形啊。。求大神指教。。

Question

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;
public class RecJFrame extends JFrame implements ActionListener,ComponentListener {
private JRadioButton colorsbutton[]; //单选按钮组
private RecNCanvas canvas; //自定义画布组件
public RecJFrame()
{
super("图画—颜色"); //框架边布局
Dimension dim=getToolkit().getScreenSize(); //获得屏幕分辨率
this.setBounds(dim.width/4,dim.height/4,dim.width/2,dim.height/2); //窗口居中
this.setDefaultCloseOperation(EXIT_ON_CLOSE);
this.addComponentListener(this); //注册组件事件监听器

    JToolBar toolbar=new JToolBar();                   //创建工具栏，默认水平方向
    this.getContentPane().add(toolbar,"North");        //工具栏添加到框架内容窗格北部
    JButton button_color = new JButton("选择颜色");
    toolbar.add(button_color);
    button_color.addActionListener(this);
    String colors[]={"红","黄","蓝"};
    ButtonGroup bgroup = new ButtonGroup();            //按钮组
    colorsbutton = new JRadioButton[colors.length];    //单选按钮数组
    for (int i=0; i<colorsbutton.length; i++)
    {
        colorsbutton[i]=new JRadioButton(colors[i]);   //单选按钮
        colorsbutton[i].addActionListener(this);
        bgroup.add(colorsbutton[i]);                    //单选按钮添加到按钮组
        toolbar.add(colorsbutton[i]);                   //单选按钮添加到工具栏
    }
    canvas = new RecNCanvas();                //创建自定义画布组件
    this.getContentPane().add(canvas,"Center");
    this.setVisible(true); //创建自定义画布组件this.getBackground()Color.yellow
}
    public void actionPerformed(ActionEvent e)             //按钮动作事件处理方法
    {
       if(e.getActionCommand().equals("红"))
        {
            canvas.setColor(Color.red);
            }
       if(e.getActionCommand().equals("蓝"))
            {
                canvas.setColor(Color.blue);
                } 
       if(e.getActionCommand().equals("黄"))
            {
                canvas.setColor(Color.yellow);
                }
       if (e.getActionCommand().equals("选择颜色"))
        {
            Color c=JColorChooser.showDialog(this,"选择颜色",Color.blue); //弹出JColorChooser颜色选择对话框，返回选中颜色
            canvas.setColor(c);
        }
    }
    public void componentHidden(ComponentEvent arg0) {}
    public void componentMoved(ComponentEvent arg0) {}
    public void componentResized(ComponentEvent arg0) {}
    public void componentShown(ComponentEvent arg0) {}
    public static void main(String arg[])
    {
        new RecJFrame();
    }

}
class RecNCanvas extends Canvas
{
private Color color;
public void setColor(Color color)
{
this.color=color;
}
public void paint(Graphics g)
{
int x0=this.getWidth()/4;
int y0=this.getHeight()/4;
g.setColor(this.color);
g.drawRect(x0,y0,250,100);

}
}

Answer 1

没有repaint
再设置颜色后repaint

 public void actionPerformed(ActionEvent e)             //按钮动作事件处理方法
    {
       if(e.getActionCommand().equals("红"))
        {
            canvas.setColor(Color.red);
            }
       if(e.getActionCommand().equals("蓝"))
            {
                canvas.setColor(Color.blue);
                } 
       if(e.getActionCommand().equals("黄"))
            {
                canvas.setColor(Color.yellow);
                }
       if (e.getActionCommand().equals("选择颜色"))
        {
            Color c=JColorChooser.showDialog(this,"选择颜色",Color.blue); //弹出JColorChooser颜色选择对话框，返回选中颜色
            canvas.setColor(c);
        }
                canvas.repaint();//需要在这里加上repaint，
    }