long keydowmtime=0;//按键按下时的时间long eventtime=0;//事件发生的时间    @Override    public boolean onKeyDown(int keyCode, KeyEvent event) {        // TODO Auto-generated method stub        eventtime=event.getEventTime();        if(keydowmtime==0){            keydowmtime=event.getDownTime();        }else {            if((eventtime-keydowmtime)<1000){//即两次点击时间小于1s时退出程序，否则，复位                MainActivity.this.finish();            }else{                keydowmtime=0;            }        }        if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {            Toast.makeText(MainActivity.this,"再按一次退出", Toast.LENGTH_SHORT).show();        }        return super.onKeyDown(keyCode, event);    }}