java 获取知道索引的 Collection 元素？
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get element of Collection knowing the index?
提问by Leon Kingston
Possible Duplicate:
best way to get value from Collection by index
可能的重复：
通过索引从集合中获取价值的最佳方式
Say I have a Collection. And I need to get the element at index 2.
说我有一个Collection. 我需要在索引 2 处获取元素。
How would I do this if there's no get method and iterator doesn't track indexes?
如果没有 get 方法并且迭代器不跟踪索引，我该怎么做？
回答by Tomasz Nurkiewicz
First and foremost try to exploit the actual implementation. If it's a Listyou can downcast and use better API:
首先，最重要的是尝试利用实际实现。如果是这样，List您可以向下转型并使用更好的 API：
if(collection instanceof List) {
((List<Foo>)collection).get(1);
}
But the "pure" solution is to create an Iteratorand call next()two times. That's the only general interface you have:
但是“纯”的解决方案是创建Iterator和调用next()两次。这是您拥有的唯一通用界面：
Iterator<Foo> fooIter = collection.iterator();
fooIter.next();
Foo second = fooIter.next();
This can be easily generalized to k-th element. But don't bother, there is already a method for that: Iterators.html#get(Iterator, int)in Guava:
这可以很容易地推广到第 k 个元素。但是别担心，已经有一个方法：Iterators.html#get(Iterator, int)在番石榴中：
Iterators.get(collection.iterator(), 1);
...or with Iterables.html#get(Iterable, int):
...或与Iterables.html#get(Iterable, int)：
Iterables.get(collection, 1);
If you need to do this many, many times, it might be cheaper to create a copy of the collection in ArrayList:
如果您需要多次执行此操作，则在以下位置创建集合的副本可能更便宜ArrayList：
ArrayList<Foo> copy = new ArrayList<Foo>(collection);
copy.get(1); //second
