详谈Java中的事件监听机制

package com.cbs;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;

/**
 * Draw类，用于界面的初始化
 * 
 * @author CBS
 * 
 */
public class Draw {

  public static void main(String[] args) {
    Draw t = new Draw();
    t.showUI();

  }

  // 界面初始化方法
  public void showUI() {
    JFrame jf = new JFrame();
    jf.setTitle("画图");
    jf.setSize(700, 700);
    jf.setDefaultCloseOperation(3);
    jf.setLocationRelativeTo(null);
    FlowLayout layout = new FlowLayout(FlowLayout.LEFT);
    jf.setLayout(layout);
    JButton drawLine = new JButton("画直线");
    jf.add(drawLine);
    JButton drawOval = new JButton("画椭圆");
    jf.add(drawOval);
    JButton drawArc = new JButton("画曲线");
    jf.add(drawArc);
    JButton drawPolygon = new JButton("三角形");
    jf.add(drawPolygon);
    JButton jb1 = new JButton();
    jb1.setBackground(Color.RED);
    jf.add(jb1);
    jb1.setPreferredSize(new Dimension(30, 30));
    JButton jb2 = new JButton();
    jb2.setBackground(Color.GREEN);
    jf.add(jb2);
    jb2.setPreferredSize(new Dimension(30, 30));

    jf.setVisible(true);

    Graphics g = jf.getGraphics();// 获取当前的画笔
    DrawListener dl = new DrawListener(g);// 实例化DrawListener类的对象
    jf.addMouseListener(dl);// 为窗体添加鼠标事件监听方法
    jf.addMouseMotionListener(dl);// 为窗体添加鼠标移动事件监听方法

    // 为按钮添加动作监听
    drawLine.addActionListener(dl);
    drawOval.addActionListener(dl);
    jb1.addActionListener(dl);
    jb2.addActionListener(dl);
    drawArc.addActionListener(dl);
    drawPolygon.addActionListener(dl);

  }

}

package com.cbs;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;

import javax.swing.ImageIcon;
import javax.swing.JButton;

/**
 * 事件处理类
 * 
 * @author CBS
 * 
 */

public class DrawListener implements MouseListener, MouseMotionListener,
    ActionListener {

  private int x1, y1, x2, y2;// 记录两次鼠标的点击坐标
  private Graphics g;// 从界面获取画布对象
  private String str;// 记录当前按钮的信息，区分不同的按钮
  private Color color;// 记录画笔的颜色信息
  private int f = 1;// 控制变量，用于更新坐标

  public DrawListener(Graphics g) {
    this.g = g;
  }

  //鼠标按下时的处理方法
  public void mousePressed(MouseEvent e) {
    // 记录第一次点击的位置；由对象e得到
    if (f == 1) {
      x1 = e.getX();
      y1 = e.getY();
    }
  }

  //鼠标点击时的处理方法
  public void mouseClicked(MouseEvent e) {
    if ("三角形".equals(str)) {
      System.out.println("sanjaioxing");
      int x, y;
      x = e.getX();
      y = e.getY();
      g.setColor(color);
      g.drawLine(x, y, x1, y1);
      g.drawLine(x2, y2, x, y);
      f = 1;

    }

  }

  // 鼠标释放时的处理方法
  public void mouseReleased(MouseEvent e) {
    // 记录鼠标释放时的坐标
    if (f == 1) {
      x2 = e.getX();
      y2 = e.getY();
    }
    // 两个坐标的得到了，可以用于直线的绘制，调用画布对象g方法，在界面上面画出直线
    if ("画直线".equals(str)) {
      g.setColor(color);
      g.drawLine(x1, y1, x2, y2);
    }
    if ("画椭圆".equals(str)) {
      g.setColor(color);
      g.drawOval(x1, y1, x2, y2);
    }
    if ("三角形".equals(str) && f == 1) {
      g.setColor(color);
      g.drawLine(x1, y1, x2, y2);
      f++;
    }
  }

  // 鼠标进入时的处理方法
  public void mouseEntered(MouseEvent e) {

  }

  // 鼠标退出时的处理方法
  public void mouseExited(MouseEvent e) {

  }

  public void actionPerformed(ActionEvent e) {
    if ("".equals(e.getActionCommand())) {
      JButton jb = (JButton) e.getSource();
      color = jb.getBackground();
    } else {
      str = e.getActionCommand();
    }
  }

  // 鼠标拖动时的处理方法
  public void mouseDragged(MouseEvent e) {
    if ("画曲线".equals(str)) {
      int x, y;
      x = e.getX();
      y = e.getY();
      g.setColor(color);
      g.drawLine(x1, y1, x, y);
      x1 = x;
      y1 = y;
    }

  }

  // 鼠标释放时的移动方法
  public void mouseMoved(MouseEvent e) {
  
  }

}