birdfly.java

实现大鸟飞行

package birdfly;

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

/**
 * <p>Title: 飞行的大鸟</p>
 * <p>Description: 在沙漠中飞行的大鸟</p>
 * <p>Copyright: Copyright (c) 2004</p>
 * <p>Company: 北京师范大学计算机系</p>
 * @author 孙一林
 * @version 1.0
 */

public class BirdFly extends MultiThreadApplet {
  boolean isStandalone = false;
  Image offScreenImg;					//存放备用屏幕的内容
  Graphics offScreenG;					//备用屏幕的绘图环境
  MediaTracker tracker;					//媒体跟踪器
  Image walkerImgs[]=new Image[4];		//存放大鸟走路姿势图像
  Image currentImg;						//当前放映的大鸟动作图像
  int xpos,ypos=0;						//大鸟动作图像显示的位置
  int walk_step=20;						//大鸟图像每次移动的距离
  int delay = 250;						//每帧时延（毫秒）
  Image bgImage;						//存放草原背景图像
  int applet_width,applet_height;
  int birdImg_width;						//大鸟图像宽度

  //Get a parameter value
  public String getParameter(String key, String def) {
    return isStandalone ? System.getProperty(key, def) :
      (getParameter(key) != null ? getParameter(key) : def);
  }

  //Construct the applet
  public BirdFly() {
  }
  //Initialize the applet
  public void init() {
    try {
      jbInit();
    }
    catch(Exception e) {
      e.printStackTrace();
    }
  }
  //Component initialization
  private void jbInit() throws Exception {
    tracker = new MediaTracker(this);
    for (int i=0;i<walkerImgs.length;i++){					//获取大鸟动作图像
      walkerImgs[i]=getImage(getCodeBase(),"images/bird"+i+".gif");
      tracker.addImage(walkerImgs[i], 0);				//列入0组跟踪范围
    }
    bgImage = getImage(getCodeBase(),"images/"+"bg.gif");	//获取草原背景图像
    tracker.addImage(bgImage, 0);						//列入0组跟踪范围
    applet_width = getSize().width;
    applet_height = getSize().height;
    try {
      offScreenImg = createImage (applet_width,applet_height);//创建备用屏幕
      offScreenG = offScreenImg.getGraphics ();//获取备用屏幕的绘图环境
    }
    catch (Exception e) {
      offScreenG = null;		//若出错，就置备用屏幕的绘图上下文环境为null
    }
  }

  public void run(){
    try{
      tracker.waitForID(0);			//等待0组中所有图像的到达
    }
    catch(InterruptedException e){
      return;
    }
    birdImg_width=walkerImgs[0].getWidth(this);
    int i=0;
    while(true){
      for(xpos=-birdImg_width;xpos<=applet_width;xpos+=walk_step){//计算位置
        currentImg=walkerImgs[i];
        repaint();
        i=(i+1)%walkerImgs.length;			//计算下一帧是哪幅图像
        try{
          Thread.sleep(delay);
        }
        catch(InterruptedException e){
        }
      }
    }
  }

  public void paint(Graphics g){
    g.drawImage(bgImage,0,0,this);
    g.drawImage(currentImg,xpos,ypos,this);
  }
  public void update(Graphics g){
    if (offScreenG!=null) {					//如果备用屏幕创建成功
      paint(offScreenG);
      g.drawImage(offScreenImg,0,0,this);	//将备用屏幕内容画到当前屏幕来
    }
    else{
      paint(g);
    }
  }

  //Get Applet information
  public String getAppletInfo() {
    return "飞行的大鸟";
  }
  //Get parameter info
  public String[][] getParameterInfo() {
    return null;
  }
}