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.

Further reading:

How to Find an Element in a List with Java

Have a look at some quick ways to find an element in a list in Java

Shuffling Collections In Java

Learn how to shuffle various collections in Java.

Check If Two Lists are Equal in Java

A short article focused on the common problem of testing if two List instances contain the same elements in exactly the same order.

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.

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