按两次返回键退出应用程序

    //按两次返回键退出应用程序    @Override    public boolean onKeyDown(int keyCode, KeyEvent event) {        if(event.getAction()==KeyEvent.ACTION_DOWN&&keyCode==KeyEvent.KEYCODE_BACK){            if(System.currentTimeMillis()-exittime>2000){                Toast.makeText(this,"再按一次退出手机淘宝",0).show();                exittime=System.currentTimeMillis();            }            else{                finish();            }            return true;        }        return super.onKeyDown(keyCode, event);    }