Android 兩次返回按鍵退出程序

	public boolean onKeyDown(int keyCode, KeyEvent event) {
		// 兩次返回退出程序
		if (keyCode == KeyEvent.KEYCODE_BACK) {
			if ((System.currentTimeMillis() - mExitTime) > 2000) {
				Toast.makeText(this, R.string.toast_twice_press_back_to_exit,
						Toast.LENGTH_SHORT).show();
				mExitTime = System.currentTimeMillis();
			} else {
				finish();
			}
			return true;
		}
		return super.onKeyDown(keyCode, event);
	}