How to get the first element of the List or Set?

Question

I'd like to know if I can get the first element of a list or set. Which method to use?

Solution 1

See the javadoc

of List

list.get(0);

or Set

set.iterator().next();

and check the size before using the above methods by invoking isEmpty()

!list_or_set.isEmpty()

Solution 2

In Java >=8 you could also use the Streaming API:

Optional<String> first = set.stream().findFirst();

(Useful if the Set/List may be empty.)

Solution 3

Let's assume that you have a List<String> strings that you want the first item from.

There are several ways to do that:

Java (pre-8):

String firstElement = null;
if (!strings.isEmpty() && strings.size() > 0) {
    firstElement = strings.get(0);
}

Java 8:

Optional<String> firstElement = strings.stream().findFirst();

Guava

String firstElement = Iterables.getFirst(strings, null);

Apache commons (4+)

String firstElement = (String) IteratorUtils.get(strings, 0);

Apache commons (before 4)

String firstElement = (String) CollectionUtils.get(strings, 0);

Followed by or encapsulated within the appropriate checks or try-catch blocks.

Kotlin:

In Kotlin both Arrays and most of the Collections (eg: List) have a first method call. So your code would look something like this

for a List:

val stringsList: List<String?> = listOf("a", "b", null)
val first: String? = stringsList.first()

for an Array:

val stringArray: Array<String?> = arrayOf("a", "b", null)
val first: String? = stringArray.first()

Followed by or encapsulated within the appropriate checks or try-catch blocks.

Kotlin also includes safer ways to do that for kotlin.collections, for example firstOrNull or getOrElse, or getOrDefault when using JRE8

Solution 4

I'm surprised that nobody suggested guava solution yet:

com.google.common.collect.Iterables.get(collection, 0)
// or
com.google.common.collect.Iterables.get(collection, 0, defaultValue)
// or
com.google.common.collect.Iterables.getFirst(collection, defaultValue)

or if you expect single element:

com.google.common.collect.Iterables.getOnlyElement(collection, defaultValue)
// or
com.google.common.collect.Iterables.getOnlyElement(collection)

Solution 5

java8 and further

Set<String> set = new TreeSet<>();
    set.add("2");
    set.add("1");
    set.add("3");
    String first = set.stream().findFirst().get();

This will help you retrieve the first element of the list or set. Given that the set or list is not empty (get() on empty optional will throw java.util.NoSuchElementException)

orElse() can be used as: (this is just a work around - not recommended)

String first = set.stream().findFirst().orElse("");
set.removeIf(String::isEmpty);

Below is the appropriate approach :

Optional<String> firstString = set.stream().findFirst();
if(firstString.isPresent()){
    String first = firstString.get();
}

Similarly first element of the list can be retrieved.

Hope this helps.

View more solutions

Comments

[Steve Kuo]

Use isEmpty() instead of size()>0. It better shows your intent and is possibly more efficient (e.g. LinkList size() is O(n)).

[efritz]

@SteveKuo LinkedList has a size variable so size() is O(1). Additionally, isEmpty() is implemented as size() == 0.

[Guido Medina]

Always use List.iterator().next() or Set.iterator().next() to avoid O(N) for Linked data structures, you never know what implementation you will be receiving, and of course verify by using Set.isEmpty() or List.isEmpty() for the same reason, for both cases will always be O(1) instead of a potential O(N) if, again, an implementation of Linked data structure is passed liked LinkedList or LinkedHashSet

[dpedro]

set.toArray()[0] will have to pass through whole set contents to create an array to get only the first element, which is not very effective.

[Diablo]

This looks great but without get() I couldn't be able to call resultant element specific methods.set.stream().findFirst().get() will allow you to call any methods on the resultant object. eg:set.stream().findFirst().get().getMessage()

[Arturas M]

This should be the accepted answer, although not too verbose, but gives the most accurate answer and works perfectly!

[Magnilex]

@Diablo Be careful though, because get() will throw an exception if the optional is empty. There are plenty of other methods on Optional that might be more suitable.

[ZetaPR]

this will cause a very low performance

[ciekawy]

good to have an access to the first method yet here the question concerns list/set regardless the order.

[molholm]

I know, the question had been answered when i posted this, it is just meant as related information. My thought was that it would be likely that someone looking for an answer to this in some cases would want to work with sorted objects. I edited my answer.

[troosan]

if you want it to return null, just do String first = set.stream().findFirst().orElse(null);

[Paul]

For a set this method returns an Object and not the type of element in the Set.

[Ruslan Stelmachenko]

@troosan This is still isn't null-safe because the first element of the set can be null, in which case findFirst() will throw NPE. The null-safe way is to use something like: set.stream().map(Optional::ofNullable).findFirst().orElseGet‌​(Optional::empty).or‌​Else(null). First, we wrap first element of the stream into Optional, then, after findFirst() (which wraps our Optional into another Optional) we unwrap it back or return Optional.empty() if Set was empty. Then, finally, we return the first element of Set or null.

[troosan]

@djxak indeed you are right, your proposal is of course safer if you are not sure what the set contains (and what Implementation of the Set is used)

[CrazyGreenHand]

"If the stream has no encounter order, then any element may be returned"(docs.oracle.com/javase/8/docs/api/java/util/stream‌​/Stream.html)

[CrazyGreenHand]

"If the stream has no encounter order, then any element may be returned" (docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.h‌​tml)

[Mikołaj Podbielski]

@RuslanStelmachenko Can you elaborate on throwing NPE?

[Mikołaj Podbielski]

System.out.println(new HashSet<>().stream().findFirst().orElse(null)); just prints null

[Ruslan Stelmachenko]

@MikołajPodbielski As I wrote, it will throw NPE only if the first element of the set is null. Your example uses empty set. It's fine. But try to do hashSet.add(null) before you create a stream from it. See findFirst() docs on the topic when it throws NPE.