Converting Between an Array and a Set in Java

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.

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.

Related posts:

Java Program to Perform Insertion in a 2 Dimension K-D Tree
Form Validation with AngularJS and Spring MVC
Interface trong Java 8 – Default method và Static method
Hướng dẫn sử dụng Java Generics
Sử dụng JDBC API thực thi câu lệnh truy vấn dữ liệu
Enum trong java
Java Program to Find Path Between Two Nodes in a Graph
“Stream has already been operated upon or closed” Exception in Java
Guide to Mustache with Spring Boot
Java Program to Implement Bellman-Ford Algorithm
Java Program to Implement First Fit Decreasing for 1-D Objects and M Bins
Guava Collections Cookbook
Spring Boot - Scheduling
Spring Boot - Sending Email
Java Program to Compute Discrete Fourier Transform Using the Fast Fourier Transform Approach
Java Program to Implement CopyOnWriteArraySet API
Introduction to Project Reactor Bus
Java Program to Print only Odd Numbered Levels of a Tree
Iterating over Enum Values in Java
Java Program to Implement Horner Algorithm
How to Delay Code Execution in Java
OAuth2 Remember Me with Refresh Token
Mệnh đề Switch-case trong java
Wiring in Spring: @Autowired, @Resource and @Inject
Java Program to Implement ConcurrentLinkedQueue API
Guide to CountDownLatch in Java
Java Program to Construct an Expression Tree for an Postfix Expression
Spring Boot - Service Components
Java Program to Check Whether it is Weakly Connected or Strongly Connected for a Directed Graph
Java Program to Implement AttributeList API
Control the Session with Spring Security
Java Program to Implement Bresenham Line Algorithm