java while (LinkedList.iterator().hasNext()) 不起作用
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java while (LinkedList.iterator().hasNext()) does not work
提问by Yevgraf Andreyevich Zhivago
I have the following while loop, if I put this.boatTripsList.iterator().hasNext() in the while loop condition, it throws error. When I create iterator then put in the while loop condition, it will work then. Why is this? Thanks & Regards. (the second version throws error)
我有以下 while 循环，如果我将 this.boatTripsList.iterator().hasNext() 放在 while 循环条件中，则会引发错误。当我创建迭代器然后放入 while 循环条件时，它就会工作。为什么是这样？感谢和问候。（第二个版本抛出错误）
public Journey(List<BoatTrip> trips) {
this.boatTripsList = new LinkedList<BoatTrip>();
Iterator<BoatTrip> iterator = trips.iterator();
//add the given boat trips to the boattrips list
while (iterator.hasNext()) {
BoatTrip thistrip = iterator.next();
this.boatTripsList.add(thistrip);
}
}
public Journey(List<BoatTrip> trips) {
this.boatTripsList = new LinkedList<BoatTrip>();
//add the given boat trips to the boattrips list
while (trips.iterator().hasNext()) {
BoatTrip thistrip = iterator.next();
this.boatTripsList.add(thistrip);
}
}
回答by fge
This is normal: if your while condition is while(trips.iterator().hasNext()), you create a new iterator each time. If your list is not empty, the condition will therefore always be true...
这是正常的：如果您的 while 条件为while(trips.iterator().hasNext())，则每次都创建一个新的迭代器。如果您的列表不为空，则条件将始终为真...
While in the loop itself, you use the iterator you created before entering the loop... As a result, you'll get a NoSuchElementExceptionwhen this iterator is empty.
在循环本身中，您使用您在进入循环之前创建的迭代器......因此，NoSuchElementException当此迭代器为空时，您将获得 a 。
Use:
利用：
final Iterator<Whatever> = list.iterator();
Whatever whatever;
while (iterator.hasNext()) {
whatever = iterator.next();
// do whatever stuff
}
But for walking lists, a foreach loop is preferred:
但是对于步行列表，首选 foreach 循环：
for (final BoatTrip trip: tripList)
// do whatever is needed
And if you want to add the contents of a list to another, use .addAll():
如果要将列表的内容添加到另一个列表，请使用.addAll()：
// no need for the "this" qualifier, there is no name conflict
boatTripList.addAll(trips);
回答by Quetzalcoatl
You aren't using the iteratoryou requested on the first line of your code there - you're requesting a new one each time, so it will always have a next.
你没有使用iterator你在那里的代码第一行请求的 - 你每次都请求一个新的，所以它总会有下一个。
回答by Regenschein
A call to .iterator() obtains a new iterator. If you do that in the loop, you will always obtain a new iterator rather than iterating over an existing iterator.
调用 .iterator() 获得一个新的迭代器。如果您在循环中这样做，您将始终获得一个新的迭代器，而不是迭代现有的迭代器。
回答by jsfviky
this.boatTripsList.iterator().hasNext() is wrong
this.boatTripsList.iterator().hasNext() 是错误的
this.boatTripsList.hasNext() is correct
this.boatTripsList.hasNext() 是正确的
