private long exitTime = 0;
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (KeyEvent.KEYCODE_BACK == keyCode) {
/*        //點一次返回相當於home鍵        moveTaskToBack(true);*/
// 判斷是否在兩秒之內連續點選返回鍵，是則退出，否則不退出
if (System.currentTimeMillis() - exitTime > 2000) {
Toast.makeText(getApplicationContext
 
(), "再按一次退出程式", Toast.LENGTH_SHORT).show();
// 將系統當前的時間賦值給exitTime
exitTime = System.currentTimeMillis();
} else {
finish();
}
return true;
}
return super.onKeyDown(keyCode, event);
}