//双击退出1
    /**
     * 菜单、返回键响应
     */
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        // TODO Auto-generated method stub
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            exitBy2Click();      //调用双击退出函数
        }
        return false;
    }

    /**
     * 双击退出函数
     */
    private static Boolean isExit = false;

    private void exitBy2Click() {
        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
                }
            }, 1000); // 如果1秒钟内没有按下返回键，则启动定时器取消掉刚才执行的任务
        } else {
            finish();
            System.exit(0);
        }
    }


    //双击退出2
    private long exitTime = 0;
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){
            if((System.currentTimeMillis()-exitTime) > 2000){
                Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
                exitTime = System.currentTimeMillis();
            } else {
                finish();
                System.exit(0);
            }
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }


//返回键无操作
 @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            return true;  //返回true是告诉系统已经处理了,false则还没处理需要系统再处理
        }
        return super.onKeyDown(keyCode, event);
    }