Java中的事件監(jiān)聽(tīng)機(jī)制是什么

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類，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("畫(huà)圖");
    jf.setSize(700, 700);
    jf.setDefaultCloseOperation(3);
    jf.setLocationRelativeTo(null);
    FlowLayout layout = new FlowLayout(FlowLayout.LEFT);
    jf.setLayout(layout);
    JButton drawLine = new JButton("畫(huà)直線");
    jf.add(drawLine);
    JButton drawOval = new JButton("畫(huà)橢圓");
    jf.add(drawOval);
    JButton drawArc = new JButton("畫(huà)曲線");
    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();// 獲取當(dāng)前的畫(huà)筆
    DrawListener dl = new DrawListener(g);// 實(shí)例化DrawListener類的對(duì)象
    jf.addMouseListener(dl);// 為窗體添加鼠標(biāo)事件監(jiān)聽(tīng)方法
    jf.addMouseMotionListener(dl);// 為窗體添加鼠標(biāo)移動(dòng)事件監(jiān)聽(tīng)方法

    // 為按鈕添加動(dòng)作監(jiān)聽(tīng)
    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;

/**
 * 事件處理類
 * 
 * @author CBS
 * 
 */

public class DrawListener implements MouseListener, MouseMotionListener,
    ActionListener {

  private int x1, y1, x2, y2;// 記錄兩次鼠標(biāo)的點(diǎn)擊坐標(biāo)
  private Graphics g;// 從界面獲取畫(huà)布對(duì)象
  private String str;// 記錄當(dāng)前按鈕的信息，區(qū)分不同的按鈕
  private Color color;// 記錄畫(huà)筆的顏色信息
  private int f = 1;// 控制變量，用于更新坐標(biāo)

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

  //鼠標(biāo)按下時(shí)的處理方法
  public void mousePressed(MouseEvent e) {
    // 記錄第一次點(diǎn)擊的位置；由對(duì)象e得到
    if (f == 1) {
      x1 = e.getX();
      y1 = e.getY();
    }
  }

  //鼠標(biāo)點(diǎn)擊時(shí)的處理方法
  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;

    }

  }

  // 鼠標(biāo)釋放時(shí)的處理方法
  public void mouseReleased(MouseEvent e) {
    // 記錄鼠標(biāo)釋放時(shí)的坐標(biāo)
    if (f == 1) {
      x2 = e.getX();
      y2 = e.getY();
    }
    // 兩個(gè)坐標(biāo)的得到了，可以用于直線的繪制，調(diào)用畫(huà)布對(duì)象g方法，在界面上面畫(huà)出直線
    if ("畫(huà)直線".equals(str)) {
      g.setColor(color);
      g.drawLine(x1, y1, x2, y2);
    }
    if ("畫(huà)橢圓".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++;
    }
  }

  // 鼠標(biāo)進(jìn)入時(shí)的處理方法
  public void mouseEntered(MouseEvent e) {

  }

  // 鼠標(biāo)退出時(shí)的處理方法
  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();
    }
  }

  // 鼠標(biāo)拖動(dòng)時(shí)的處理方法
  public void mouseDragged(MouseEvent e) {
    if ("畫(huà)曲線".equals(str)) {
      int x, y;
      x = e.getX();
      y = e.getY();
      g.setColor(color);
      g.drawLine(x1, y1, x, y);
      x1 = x;
      y1 = y;
    }

  }

  // 鼠標(biāo)釋放時(shí)的移動(dòng)方法
  public void mouseMoved(MouseEvent e) {
  
  }

}