Assert Two Lists for Equality Ignoring Order in Java

Last updated: January 8, 2024

Written by: baeldung

Reviewed by: Jonathan Cook

Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.

End-to-end testing is a very useful method to make sure that your application works as intended. This highlights issues in the overall functionality of the software, that the unit and integration test stages may miss.

Playwright is an easy-to-use, but powerful tool that automates end-to-end testing, and supports all modern browsers and platforms.

When coupled with LambdaTest (an AI-powered cloud-based test execution platform) it can be further scaled to run the Playwright scripts in parallel across 3000+ browser and device combinations:

>> Automated End-to-End Testing With Playwright

1. Overview

Sometimes when writing unit tests, we need to make order-agnostic comparisons of lists. In this short tutorial, we’ll take a look at different examples of how we can write such unit tests.

2. Setup

As per the List#equals Java documentation, two lists are equal if they contain the same elements in the same order. Therefore we can’t merely use the equals method as we want to do order agnostic comparison.

Throughout this tutorial, we’ll use these three lists as example inputs for our tests:

List first = Arrays.asList(1, 3, 4, 6, 8);
List second = Arrays.asList(8, 1, 6, 3, 4);
List third = Arrays.asList(1, 3, 3, 6, 6);

There are different ways to make order-agnostic comparisons. Let’s take a look at them one by one.

3. Using JUnit

JUnit is a well-known framework used for unit testing in the Java ecosystem.

We can use the logic below to compare the equality of two lists using the assertTrue and assertFalse methods.

Here we check the size of both lists and check if the first list contains all elements of the second list and vice versa. Although this solution works, it’s not very readable. So now let’s look at some alternatives:

@Test
public void whenTestingForOrderAgnosticEquality_ShouldBeTrue() {
    assertTrue(first.size() == second.size() && first.containsAll(second) && second.containsAll(first));
}

In this first test, the size of both lists is compared before we check if the elements in both lists are the same. As both of these conditions return true, our test will pass.

Let’s now take a look at a failing test:

@Test
public void whenTestingForOrderAgnosticEquality_ShouldBeFalse() {
    assertFalse(first.size() == third.size() && first.containsAll(third) && third.containsAll(first));
}

Contrastingly, in this version of the test, although the size of both lists is the same, all elements don’t match.

4. Using AssertJ

AssertJ is an open-source, community-driven library for writing fluent and rich assertions in Java tests.

To use it in our maven project, let’s add the assertj-core dependency in the pom.xml file:

<dependency>
    <groupId>org.assertj</groupId>
    <artifactId>assertj-core</artifactId>
    <version>3.16.1</version>
</dependency>

Let’s write a test to compare the equality of two list instances of the same element and the same size:

@Test
void whenTestingForOrderAgnosticEqualityBothList_ShouldBeEqual() {
    assertThat(first).hasSameElementsAs(second);
}

In this example, we verify first contains all the elements of the given iterable and nothing else, in any order. The main limitation of this approach is the hasSameElementsAs method ignores duplicates.

Let’s look at this in practice to see what we mean:

@Test
void whenTestingForOrderAgnosticEqualityBothList_ShouldNotBeEqual() {
    List a = Arrays.asList("a", "a", "b", "c");
    List b = Arrays.asList("a", "b", "c");
    assertThat(a).hasSameElementsAs(b);
}

In this test, although we have the same elements, the size of both lists is not equal, but the assertion will still be true, as it ignores the duplicates.

To overcome issues related to hasSameElementsAs(), we can use containsExactlyInAnyOrderElementsOf(), which verifies that both lists contain exactly the same elements and nothing else in any order:

assertThat(a).containsExactlyInAnyOrderElementsOf(b);

The test will indeed fail as expected.

5. Using Hamcrest

If we are already using Hamcrest or want to use it for writing unit tests, here is how we can use the Matchers#containsInAnyOrder method for order-agnostic comparison.

To use Hamcrest in our maven project, let’s add the hamcrest-all dependency in pom.xml file:

<dependency>
    <groupId>org.hamcrest</groupId>
    <artifactId>hamcrest-all</artifactId>
    <version>1.3</version>
</dependency>

Let’s look at the test:

@Test
public void whenTestingForOrderAgnosticEquality_ShouldBeEqual() {
    assertThat(first, Matchers.containsInAnyOrder(second.toArray()));
}

Here the method containsInAnyOrder creates an order agnostic matcher for Iterables, which does matching with examined Iterable elements. This test matches the elements of two lists, ignoring the order of elements in the list.

Thankfully this solution doesn’t suffer from the same problem as explained in the previous section, so we don’t need to compare the sizes explicitly.

6. Using Apache Commons

Another library or framework apart from JUnit, Hamcrest, or AssertJ we can use is Apache CollectionUtils. It provides utility methods for common operations that cover a wide range of use cases and helps us avoid writing boilerplate code.

To use it in our maven project, let’s add the commons-collections4 dependency in pom.xml file:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.5.0-M2</version>
</dependency>

Here is a test using CollectionUtils:

@Test
public void whenTestingForOrderAgnosticEquality_ShouldBeTrueIfEqualOtherwiseFalse() {
    assertTrue(CollectionUtils.isEqualCollection(first, second));
    assertFalse(CollectionUtils.isEqualCollection(first, third));
}

The isEqualCollection method returns true if the given collections contain precisely the same elements with the same cardinalities. Otherwise, it returns false.

7. Conclusion

In this article, we have explored how to check the equality of two List instances where the elements of both lists are ordered differently.

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.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.