@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    // TODO Auto-generated method stub
    if(keyCode == KeyEvent.KEYCODE_BACK)
     {  
         for2Click();        //调用双击退出函数
    }
    return false;
}

private static Boolean isExit = false;

private void for2Click() {
    Timer tExit = null;
    if (isExit == false) {
        isExit = true; // 准备退出
        Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
        tExit = new Timer();
        tExit.schedule(new TimerTask() {
            @Override
            public void run() {
                isExit = false; // 取消退出
            }
        }, 2000); // 2秒

    } else {
        finish();
        System.exit(0);//finish();
    }
}