如何在J2ME應用裡控制移動設備的音量

下邊的代碼描述了如何在J2ME應用裡控制移動設備的音量，VolumeControl類可以用來控制播放器Player類的音量

import Javax.microedition.midlet.*;
import Javax.microedition.lcdui.*;
import Javax.microedition.lcdui.Ticker;
import Javax.microedition.media.*;

public class VolumeControlDemo extends MIDlet implements CommandListener {
   
    private Display display;
    private Command exit,incr,decr;
    Form frm;
   
    VolumeControl vc;
    int vol;
    Player player;
   
    public VolumeControlDemo() {
        display = Display.getDisplay(this);
       
    }
   
    public void startApp() {
       
        frm=new Form("VolumeControlDemo  Demo");
       
        exit= new Command("Exit",Command.EXIT,1);
        decr= new Command("Decrease",Command.EXIT,1);
        incr= new Command("Increase",Command.EXIT,1);
        frm.addCommand(exit);
        frm.addCommand(decr);
        frm.addCommand(incr);
       
        frm.setCommandListener(this);
       
       
        display.setCurrent(frm);
       
        try {
           
            // 創建播放器Player對象
            player = Manager.createPlayer("/demo.wav");
           
            // 設置循環計數器
            player.setLoopCount(-1);
           
            // 播放
            player.start();
            Control cs[];
           
            // 獲得播放器的所有控制對象
            cs = player.getControls();
           
            for (int i = 0; i < cs.length; i++) {
                if (cs[i] instanceof VolumeControl)
                    // 獲得音量控制類實例
                    vc=(VolumeControl)cs[i];
            }
           
        } catch (Exception e) {}
       
    }
   
    public void pauseApp() {
       
    }
   
    public void destroyApp(boolean un) {
       
    }
   
    public void commandAction(Command cmd,Displayable d) {
        try {
            if(decr) {
                if(vol>0) vol--;
                vc.setLevel(vol);
            } else if() {
                if(vol<99) vol--;
                vc.setLevel(vol);
            }
           
            frm.appent("vol="+vc.getLevel());
        }catch(Exception e){}
       
    }
}