get element of Collection knowing the index?

Question

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best way to get value from Collection by index

Say I have a Collection. And I need to get the element at index 2.

How would I do this if there's no get method and iterator doesn't track indexes?

Collection is just an interface, which collection are you using? — clavio, Dec 03 '12 at 21:04
Unless you have an ordered List or a sorted Set, getting an element at index 2 could be meaningless. — Peter Lawrey, Dec 03 '12 at 21:05
Perhpas it would be better to use a list if you want to keep an order. — Pablo, Dec 03 '12 at 21:05

Tomasz Nurkiewicz · Accepted Answer · 2012-12-03T21:10:24.870

First and foremost try to exploit the actual implementation. If it's a List you can downcast and use better API:

if(collection instanceof List) {
  ((List<Foo>)collection).get(1);
}

But the "pure" solution is to create an Iterator and call next() two times. That's the only general interface you have:

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:

Iterators.get(collection.iterator(), 1);

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:

ArrayList<Foo> copy = new ArrayList<Foo>(collection);
copy.get(1); //second

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