best way to get value from Collection by index

Question

What is the best way to get value from java.util.Collection by index?

Thanks.

Answer 1

Convert the collection into an array by using function Object[] toArray(Object[] a)

Answer 2

In general, there is no good way, as Collections are not guaranteed to have fixed indices. Yes, you can iterate through them, which is how toArray (and other functions) work. But the iteration order isn't necessarily fixed, and if you're trying to index into a general Collection, you're probably doing something wrong. It would make more sense to index into a List.

Answer 3

You shouldn't. a Collection avoids talking about indexes specifically because it might not make sense for the specific collection. For example, a List implies some form of ordering, but a Set does not.

    Collection<String> myCollection = new HashSet<String>();
    myCollection.add("Hello");
    myCollection.add("World");

    for (String elem : myCollection) {
        System.out.println("elem = " + elem);
    }

    System.out.println("myCollection.toArray()[0] = " + myCollection.toArray()[0]);

gives me:

    elem = World
    elem = Hello
    myCollection.toArray()[0] = World

Whilst:

    myCollection = new ArrayList<String>();
    myCollection.add("Hello");
    myCollection.add("World");

    for (String elem : myCollection) {
        System.out.println("elem = " + elem);
    }

    System.out.println("myCollection.toArray()[0] = " + myCollection.toArray()[0]);

gives me:

    elem = Hello
    elem = World
    myCollection.toArray()[0] = Hello

Why do you want to do this? Could you not just iterate over the collection?

Answer 4

You must either wrap your collection in a list (new ArrayList(c)) or use c.toArray() since Collections have no notion of "index" or "order".

Answer 5

I agree with Matthew Flaschen's answer and just wanted to show examples of the options for the case you cannot switch to List (because a library returns you a Collection):

List list = new ArrayList(theCollection);
list.get(5);

Or

Object[] list2 = theCollection.toArray();
doSomethingWith(list[2]);

If you know what generics is I can provide samples for that too.

Edit: It's another question what the intent and semantics of the original collection is.

Answer 6

you definitively want a List:

The List interface provides four methods for positional (indexed) access to list elements. Lists (like Java arrays) are zero based.

Also

Note that these operations may execute in time proportional to the index value for some implementations (the LinkedList class, for example). Thus, iterating over the elements in a > list is typically preferable to indexing through it if the caller does not know the implementation.

If you need the index in order to modify your collection you should note that List provides a special ListIterator that allow you to get the index:

    List<String> names = Arrays.asList("Davide", "Francesco", "Angelocola");
    ListIterator<String> i = names.listIterator();

    while (i.hasNext()) {
        System.out.format("[%d] %s\n", i.nextIndex(), i.next());
    }

Answer 7

I agree that this is generally a bad idea. However, Commons Collections had a nice routine for getting the value by index if you really need to:

CollectionUtils.get(collection, index)