If I have a collection, such as Collection<String> strs
, how can I get the first item out? I could just call an Iterator
, take its first next()
, then throw the Iterator
away. Is there a less wasteful way to do it?
12 Answers
Looks like that is the best way to do it:
String first = strs.iterator().next();
Great question... At first, it seems like an oversight for the Collection
interface.
Note that "first" won't always return the first thing you put in the collection, and may only make sense for ordered collections. Maybe that is why there isn't a get(item)
call, since the order isn't necessarily preserved.
While it might seem a bit wasteful, it might not be as bad as you think. The Iterator
really just contains indexing information into the collection, not a usually a copy of the entire collection. Invoking this method does instantiate the Iterator
object, but that is really the only overhead (not like copying all the elements).
For example, looking at the type returned by the ArrayList<String>.iterator()
method, we see that it is ArrayList::Itr
. This is an internal class that just accesses the elements of the list directly, rather than copying them.
Just be sure you check the return of iterator()
since it may be empty or null
depending on the implementation.
Iterables.get(yourC, indexYouWant)
Because really, if you're using Collections, you should be using Google Collections.
In java 8:
Optional<String> firstElement = collection.stream().findFirst();
For older versions of java, there is a getFirst method in Guava Iterables:
Iterables.getFirst(iterable, defaultValue)
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.
In Java 8 you have some many operators to use, for instance limit
/**
* Operator that limit the total number of items emitted through the pipeline
* Shall print
* [1]
* @throws InterruptedException
*/
@Test
public void limitStream() throws InterruptedException {
List<Integer> list = Arrays.asList(1, 2, 3, 1, 4, 2, 3)
.stream()
.limit(1)
.collect(toList());
System.out.println(list);
}
Guava provides an onlyElement
Collector
, but only use it if you expect the collection to have exactly one element.
Collection<String> stringCollection = ...;
String string = collection.stream().collect(MoreCollectors.onlyElement())
If you are unsure of how many elements there are, use findFirst
.
Optional<String> optionalString = collection.stream().findFirst();
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));
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.
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.