Java: Get first item from a collection

Question

If I have a collection, such as Collection strs, how can I get the first item out? I could just call an Iterator, take its first

Answer 1

There is no such a thing as "first" item in a Collection because it is .. well simply a collection.

From the Java doc's Collection.iterator() method:

There are no guarantees concerning the order in which the elements are returned...

So you can't.

If you use another interface such as List, you can do the following:

String first = strs.get(0);

But directly from a Collection this is not possible.

Answer 2

It sounds like your Collection wants to be List-like, so I'd suggest:

List<String> myList = new ArrayList<String>();
...
String first = myList.get(0);

Answer 3

Iterables.get(yourC, indexYouWant)

Because really, if you're using Collections, you should be using Google Collections.

Answer 4

You can do a casting. For example, if exists one method with this definition, and you know that this method is returning a List:

Collection<String> getStrings();

And after invoke it, you need the first element, you can do it like this:

List<String> listString = (List) getStrings();
String firstElement = (listString.isEmpty() ? null : listString.get(0));

Answer 5

It totally depends upon which implementation you have used, whether arraylist linkedlist, or other implementations of set.

if it is set then you can directly get the first element , their can be trick loop over the collection , create a variable of value 1 and get value when flag value is 1 after that break that loop.

if it is list's implementation then it is easy by defining index number.

Answer 6

You could do this:

String strz[] = strs.toArray(String[strs.size()]);
String theFirstOne = strz[0];

The javadoc for Collection gives the following caveat wrt ordering of the elements of the array:

If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.