@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (event.getAction() == KeyEvent.ACTION_DOWN
&& event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
if(fm.getBackStackEntryCount() == 0){
ExitBy2Click();
}else{
fm.popBackStack();
}

}
return true;
}
/**
* 雙擊退出程式
*/
public void ExitBy2Click(){
if (isExit == false) {
isExit = true;
Toast.makeText(MainActivity.this, "再按一次退出程式",
Toast.LENGTH_SHORT).show();
new Timer().schedule(new TimerTask() {


@Override
public void run() {
isExit = false;
}
}, 2000);
} else {
finish();
System.exit(0);
}
}