MIDP 2.0 开发J2ME游戏起步之一

MIDP2.0(Mobile Internet Device Profile)技术进行游戏开发中用到的最重要的包是:javax.microedition.lcdui.game，本文通过对样例游戏Tumbleweed的代码分析，将展示MIDP2.0技术的几个主要部分。游戏的主要情景是一个牛仔跳着闪避风滚草，这是一个简单的游戏，但你可以从中学到将来写复杂游戏必须具备的大部分基础知识。

　　从MIDlet类开始

　　象通常一样，应用从MIDlet类开始。本例中，我的MIDlet子类是Jump。Jump类几乎是完全继承的MIDIet子类，唯一的不同是用到了另一个独立类GameThread，用来动态设置当前窗口允许的有效按钮，比如当游戏处于非暂停状态时，玩家才可以使用暂停这个有效按钮，而激活按钮有效则是在游戏处于暂停状态时候，与此类似游戏停止后，开始按钮才是有效按钮。

　　Listing 1是游戏的MIDlet子类——Jump.java源码。

　　Listing 1. Jump.java

package net.frog_parrot.jump; import javax.microedition.midlet.*; import javax.microedition.lcdui.*; /** * This is the main class of the Tumbleweed game. * * @author Carol Hamer */ public class Jump extends MIDlet implements CommandListener { 　//--------------------------------------------------------- 　// Commands 　/** 　* The command to end the game. 　*/ 　private Command myExitCommand = new Command("Exit", Command.EXIT, 99); 　/** 　* The command to start moving when the game is paused. 　*/ 　private Command myGoCommand = new Command("Go", Command.SCREEN, 1); 　/** 　* The command to pause the game. 　*/ 　private Command myPauseCommand = new Command("Pause", Command.SCREEN, 1); 　/** 　* The command to start a new game. 　*/ 　private Command myNewCommand = new Command("Play Again", Command.SCREEN, 1); 　//--------------------------------------------------------- 　// Game object fields 　/** 　* The canvas that all of the game will be drawn on. 　*/ 　private JumpCanvas myCanvas; 　/** 　* The thread that advances the cowboy. 　*/ 　private GameThread myGameThread; 　//----------------------------------------------------- 　// Initialization and game state changes 　/** 　* Initialize the canvas and the commands. 　*/ 　public Jump() { 　　try { 　　　myCanvas = new JumpCanvas(this); 　　　myCanvas.addCommand(myExitCommand); 　　　myCanvas.addCommand(myPauseCommand); 　　　myCanvas.setCommandListener(this); 　　} catch(Exception e) 　} 　/** 　* Switch the command to the play again command. 　*/ 　void setNewCommand () 　/** 　* Switch the command to the go command. 　*/ 　private void setGoCommand() 　/** 　* Switch the command to the pause command. 　*/ 　private void setPauseCommand () 　//---------------------------------------------------------------- 　// Implementation of MIDlet. 　// These methods may be called by the application management 　// software at any time, so you always check fields for null 　// before calling methods on them. 　/** 　　* Start the application. 　*/ 　public void startApp() throws MIDletStateChangeException { 　　if(myCanvas != null) { 　　　if(myGameThread == null) { 　　　　myGameThread = new GameThread(myCanvas); 　　　　myCanvas.start(); 　　　　myGameThread.start(); 　　　} else 　　} 　} 　/** 　　* Stop and throw out the garbage. 　*/ 　public void destroyApp(boolean unconditional) 　　throws MIDletStateChangeException { 　　　if(myGameThread != null) 　　　myGameThread = null; 　　　myCanvas = null; 　　　System.gc(); 　　} 　/** 　* Request the thread to pause. 　*/ 　public void pauseApp() { 　　if(myCanvas != null) 　　If(myGameThread != null) 　} 　//---------------------------------------------------------------- 　// Implementation of CommandListener 　/* 　* Respond to a command issued on the Canvas. 　* (either reset or exit). 　*/ 　public void commandAction(Command c, Displayable s) { 　　if(c == myGoCommand) else if(c == myPauseCommand) else if(c == myNewCommand) else if((c == myExitCommand) || (c == Alert.DISMISS_COMMAND)) { 　　　try catch (MIDletStateChangeException ex) 　} } //------------------------------------------------------- // Error methods /** * Converts an exception to a message and displays * the message. */ 　void errorMsg(Exception e) { 　　if(e.getMessage() == null) else { 　　　errorMsg(e.getClass().getName() + ":" + e.getMessage()); 　　} 　} 　/** 　* Displays an error message alert if something goes wrong. 　*/ 　void errorMsg(String msg) { 　　Alert errorAlert = new Alert("error", msg, null, AlertType.ERROR); 　　errorAlert.setCommandListener(this); 　　errorAlert.setTimeout(Alert.FOREVER); 　　Display.getDisplay(this).setCurrent(errorAlert); 　} }