Java中Set和Array之间的转换

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[0]);
}

Note, that toArray(new T[0]) is the preferred way to use the method over the toArray(new T[size]). As Aleksey Shipilëv proves in his blog post, it seems faster, safer, and cleaner.

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.

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.