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1. Overview

In this short article we’re going to look at converting between an array and a Set – first using plain java, then Guava and the Commons Collections library from Apache.

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

2. Convert Array to a Set

2.1. Using plain Java

Let’s first look at how to turn the array to a Set using plain Java:

@Test
public void givenUsingCoreJavaV1_whenArrayConvertedToSet_thenCorrect() {
    Integer[] sourceArray = { 0, 1, 2, 3, 4, 5 };
    Set<Integer> targetSet = new HashSet<Integer>(Arrays.asList(sourceArray));
}

Alternatively, the Set can be created first and then filled with the array elements:

@Test
public void givenUsingCoreJavaV2_whenArrayConvertedToSet_thenCorrect() {
    Integer[] sourceArray = { 0, 1, 2, 3, 4, 5 };
    Set<Integer> targetSet = new HashSet<Integer>();
    Collections.addAll(targetSet, sourceArray);
}

2.2. Using Google Guava

Next, let’s look at the Guava conversion from array to Set:

@Test
public void givenUsingGuava_whenArrayConvertedToSet_thenCorrect() {
    Integer[] sourceArray = { 0, 1, 2, 3, 4, 5 };
    Set<Integer> targetSet = Sets.newHashSet(sourceArray);
}

2.3. Using Apache Commons Collections

Finally, let’s do the conversion using the Commons Collection library from Apache:

@Test
public void givenUsingCommonsCollections_whenArrayConvertedToSet_thenCorrect() {
    Integer[] sourceArray = { 0, 1, 2, 3, 4, 5 };
    Set<Integer> targetSet = new HashSet<>(6);
    CollectionUtils.addAll(targetSet, sourceArray);
}

3. Convert Set to Array

3.1. Using plain Java

Now let’s look at the reverse – converting an existing Set to an array:

@Test
public void givenUsingCoreJava_whenSetConvertedToArray_thenCorrect() {
    Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
    Integer[] targetArray = sourceSet.toArray(new Integer[sourceSet.size()]);
}

3.2. Using Guava

Next – the Guava solution:

@Test
public void givenUsingGuava_whenSetConvertedToArray_thenCorrect() {
    Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
    int[] targetArray = Ints.toArray(sourceSet);
}

Notice that we’re using the Ints API from Guava, so this solution is specific to the data type that we’re working with.

3.3 Using Commons Collections

And finally – let’s transform the Set into an array using the Apache Commons Collections library:

@Test
public void givenUsingCommonsCollections_whenSetConvertedToArray_thenCorrect() {
    Set<Integer> sourceSet = Sets.newHashSet(0, 1, 2, 3, 4, 5);
    Integer[] targetArray = sourceSet.toArray(new Integer[sourceSet.size()]);
}

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.

I just announced the new Spring Boot 2 material, coming in REST With Spring:

>> CHECK OUT THE LESSONS