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How to get the first element of the List or Set? [duplicate]

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How do I get the first element of a list?

The get() method of the ArrayList class accepts an integer representing the index value and, returns the element of the current ArrayList object at the specified index. Therefore, if you pass 0 to this method you can get the first element of the current ArrayList and, if you pass list.

How do you get the first value in a set in Python?

We can access the first item in the set by using iter() function, we have to apply next() to it to get the first element.

How do you check if an element appears twice in a list?

Check if list contains duplicates using list.If count > 1 then it means this element has duplicate entries.


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()

Collection c;

Iterator iter = c.iterator();

Object first = iter.next();

(This is the closest you'll get to having the "first" element of a Set. You should realize that it has absolutely no meaning for most implementations of Set. This may have meaning for LinkedHashSet and TreeSet, but not for HashSet.)


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

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

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


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


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)