How to Convert a List to Set in Java

How can we convert a list to a set in Java?

Suppose we have a list list, we want to convert this list into a set.

List<T> list = new ArrayList<>();

Note that converting a list to a set will remove duplicates from the collection.

1. Using Set constructor

The simplest way to convert a list to a set is using the Set constructor.

Set<T> hashSet = new HashSet<T>(list);
Set<T> treeSet = new TreeSet<T>(list);

The HashSet returns an unsorted set while the TreeSet returns a sorted one. However, if list is null, this will return a NullPointerException.

2. Using Set.addAll()

We can also initialize the set and add all the elements later on.

Set<T> set = new HashSet<>();
set.addAll(list);

3. Using Java 8 Streams

With Java 8 streams, we can accumulate the input elements into a new, mutable set.

Set<T> set = list.stream().collect(Collectors.toSet());
Set<T> set = list.stream().collect(Collectors.toCollection(HashSet::new));

This is especially useful if we need to perform some preprocessing on the elements.

If we’re working with a list that may be null, we can use Optional.ofNullable and assign the set to null if the list is null.

Set<T> set = Optional.ofNullable(list).map(HashSet::new).orElse(null);

4. Using Guava

Guava provides a succinct way to convert a list into a set as well.

Set<T> set = Sets.newHashSet(list);

5. Using Apache Commons Collection

Apache Commons allows us to add elements of a list to some target set.

Set<T> set = new HashSet<>();
CollectionUtils.addAll(set, list);

6. Using Java 10’s Set.copyOf()

In Java 10, we can use copyOf() to create an unmodifiable set from an existing Collection or Map.

Set<T> set = Set.copyOf(list);
Set<T> set = ImmutableSet.copyOf(list);

Any null elements in the list will throw a NullPointerException.

Any mutator method called on the set will throw an UnsupportedOperationException.

Note that copyOf() is idempotent. Copying a previously copied collection does not create another copy.