HashSet toArray() Method in Java

Last updated: January 8, 2024

Written by: Mohamed Helmy

Reviewed by: Michal Aibin

Java Collections

Java Set

Refactor Java code safely — and automatically — with OpenRewrite.

Refactoring big codebases by hand is slow, risky, and easy to put off. That’s where OpenRewrite comes in. The open-source framework for large-scale, automated code transformations helps teams modernize safely and consistently.

Each month, the creators and maintainers of OpenRewrite at Moderne run live, hands-on training sessions — one for newcomers and one for experienced users. You’ll see how recipes work, how to apply them across projects, and how to modernize code with confidence.

Join the next session, bring your questions, and learn how to automate the kind of work that usually eats your sprint time.

Distributed systems often come with complex challenges such as service-to-service communication, state management, asynchronous messaging, security, and more.

Dapr (Distributed Application Runtime) provides a set of APIs and building blocks to address these challenges, abstracting away infrastructure so we can focus on business logic.

In this tutorial, we'll focus on Dapr's pub/sub API for message brokering. Using its Spring Boot integration, we'll simplify the creation of a loosely coupled, portable, and easily testable pub/sub messaging system:

>> Flexible Pub/Sub Messaging With Spring Boot and Dapr

1. Introduction

HashSet is one of the common data structures that we can utilize in Java Collеctions.

In this tutorial, we’ll dive into the toArray() method of the HashSet class, illustrating how to convert a HashSet to an array.

2. Convеrting HashSеt to Array

Let’s look at a set of examples that illustrate how to apply the toArray() method to convert a HashSet into an array.

2.1. HashSet to an Array of Strings

In the following method, we are seeking to convert a HashSet of strings into an array of strings:

@Test
public void givenStringHashSet_whenConvertedToArray_thenArrayContainsStringElements() {
    HashSet<String> stringSet = new HashSet<>();
    stringSet.add("Apple");
    stringSet.add("Banana");
    stringSet.add("Cherry");

    // Convert the HashSet of Strings to an array of Strings
    String[] stringArray = stringSet.toArray(new String[0]);

    // Test that the array is of the correct length
    assertEquals(3, stringArray.length);

    for (String str : stringArray) {
        assertTrue(stringSet.contains(str));
    }
}

Here, a HashSet named stringSet is initialized with three String elements: (“Apple” “Banana” and “Cherry“). To be specific, the test method ensures that the resulting array has a length of 3, matching the number of elements in the HashSet.

Then, it iterates through the stringArray and checks if each element is contained within the original stringSet, asserting that the array indeed contains the String elements, confirming the successful conversion of the HashSet to a String array.

2.2. HashSet to an Array of Integers

Additionally, we can utilize the toArray() method to convert an Integer HashSet into an array of Integers as follows:

@Test
public void givenIntegerHashSet_whenConvertedToArray_thenArrayContainsIntegerElements() {
    HashSet<Integer> integerSet = new HashSet<>();
    integerSet.add(5);
    integerSet.add(10);
    integerSet.add(15);

    // Convert the HashSet of Integers to an array of Integers
    Integer[] integerArray = integerSet.toArray(new Integer[0]);

    // Test that the array is of the correct length
    assertEquals(3, integerArray.length);

    for (Integer num : integerArray) {
        assertTrue(integerSet.contains(num));
    }

    assertTrue(integerSet.contains(5));
    assertTrue(integerSet.contains(10));
    assertTrue(integerSet.contains(15));
}

Here, we create a HashSet named integerSet with three Integer elements: (5, 10, and 15). The test method is responsible for verifying the conversion of this Integer HashSet into an array of Integers, referred to as integerArray.

Moreover, it confirms that the resulting array has length = 3, corresponding to the number of elements in the original HashSet. Subsequently, the method iterates through integerArray, ensuring each element is contained within the original integerSet.

3. Conclusion

In conclusion, it is easy to convert a HashSet into an array using the toArray() method of the HashSet class. This can also be useful while handling array-based data structures or some other components in our Java apps.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.