按键事件笔记

public boolean onKeyDowen(int keyCode,KeyEvent event){    System.out.println("onKeyDown" +keyCode);        if(keyCode==4){//按下返回键 keyCode =25 音量下键，24音量上键            if(System.currentTimeMillis()-currentTime>2000){                Toast.makeText(getApplicationContext(), "再按一次退出系统", Toast.LENGTH_SHORT).show();                currentTime=System.currentTimeMillis();            }else{                exitSystem();            }//          finish();        }        return true;//      return super.onKeyDown(keyCode, event);}