Converting Between a List and a Set in Java

1. Overview

In this short article, we'll take a look at the conversion between a List and a Set – starting with Plain Java, using Guava and the Apache Commons Collections library, and finally, with Java 10.

This article is part of the “Java – Back to Basic” series here on Baeldung.

2. Convert List to Set

2.1. With Plain Java

Let's start with converting from a List to a Set using Java:

public void givenUsingCoreJava_whenListConvertedToSet_thenCorrect() {
    List<Integer> sourceList = Arrays.asList(0, 1, 2, 3, 4, 5);
    Set<Integer> targetSet = new HashSet<>(sourceList);
}

As you can see, the conversion process is type-safe and straightforward – since the constructors of each collection do accept another collection as a source.

2.2. With Guava

Let's do the same conversion using Guava:

public void givenUsingGuava_whenListConvertedToSet_thenCorrect() {
    List<Integer> sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
    Set<Integer> targetSet = Sets.newHashSet(sourceList);
}

2.3. With Apache Commons Collections

Next, let's use the Commons Collections API to convert between a List and a Set:

public void givenUsingCommonsCollections_whenListConvertedToSet_thenCorrect() {
    List<Integer> sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
    Set<Integer> targetSet = new HashSet<>(6);
    CollectionUtils.addAll(targetSet, sourceList);
}

2.4. With Java 10

One additional option is to use the Set.copyOf static factory method introduced in Java 10:

public void givenUsingJava10_whenListConvertedToSet_thenCorrect() {
    List sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
    Set targetSet = Set.copyOf(sourceList);
}

Note, that a Set created this way is unmodifiable.

3. Convert Set to List

3.1. With Plain Java

Let's now do the reverse conversion – from a Set to a List – using Java:

public void givenUsingCoreJava_whenSetConvertedToList_thenCorrect() {
   Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
   List<Integer> targetList = new ArrayList<>(sourceSet);
}

3.2. With Guava

And the Guava solution:

public void givenUsingGuava_whenSetConvertedToList_thenCorrect() {
    Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
    List<Integer> targetList = Lists.newArrayList(sourceSet);
}

Very similar to the java approach, only with a little less duplicated code.

3.3. With Apache Commons Collections

Let's now see the Commons Collections solution to convert between a Set and a List:

public void givenUsingCommonsCollections_whenSetConvertedToList_thenCorrect() {
    Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
    List<Integer> targetList = new ArrayList<>(6);
    CollectionUtils.addAll(targetList, sourceSet);
}

3.4. With Java 10

Finally, we can use the List.copyOf that's been introduced in Java 10:

public void givenUsingJava10_whenSetConvertedToList_thenCorrect() {
    Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
    List<Integer> targetList = List.copyOf(sourceSet);
}

We need to keep in mind that a resulting List is unmodifiable.

4. Conclusion

The implementation of all these examples and code snippets can be found over on GitHub – this is a Maven-based project, so it should be easy to import and run as it is.