Baeldung Pro – NPI EA (cat = Baeldung)
announcement - icon

Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Baeldung)
announcement - icon

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.

Partner – Microsoft – NPI EA (cat= Spring Boot)
announcement - icon

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, 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.

Partner – Orkes – NPI EA (cat=Spring)
announcement - icon

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.

Partner – Orkes – NPI EA (tag=Microservices)
announcement - icon

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.

eBook – Guide Spring Cloud – NPI EA (cat=Spring Cloud)
announcement - icon

Let's get started with a Microservice Architecture with Spring Cloud:

>> Join Pro and download the eBook

eBook – Mockito – NPI EA (tag = Mockito)
announcement - icon

Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.

Get started with mocking and improve your application tests using our Mockito guide:

Download the eBook

eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
announcement - icon

Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Reactive – NPI EA (cat=Reactive)
announcement - icon

Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:

>> Join Pro and download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
announcement - icon

Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Jackson – NPI EA (cat=Jackson)
announcement - icon

Do JSON right with Jackson

Download the E-book

eBook – HTTP Client – NPI EA (cat=Http Client-Side)
announcement - icon

Get the most out of the Apache HTTP Client

Download the E-book

eBook – Maven – NPI EA (cat = Maven)
announcement - icon

Get Started with Apache Maven:

Download the E-book

eBook – Persistence – NPI EA (cat=Persistence)
announcement - icon

Working on getting your persistence layer right with Spring?

Explore the eBook

eBook – RwS – NPI EA (cat=Spring MVC)
announcement - icon

Building a REST API with Spring?

Download the E-book

Course – LS – NPI EA (cat=Jackson)
announcement - icon

Get started with Spring and Spring Boot, through the Learn Spring course:

>> LEARN SPRING
Course – RWSB – NPI EA (cat=REST)
announcement - icon

Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

>> The New “REST With Spring Boot”

Course – LSS – NPI EA (cat=Spring Security)
announcement - icon

Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.

I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.

You can explore the course here:

>> Learn Spring Security

Course – All Access – NPI EA (cat= Spring)
announcement - icon

All Access is finally out, with all of my Spring courses. Learn JUnit is out as well, and Learn Maven is coming fast. And, of course, quite a bit more affordable. Finally.

>> GET THE COURSE
Course – LSD – NPI EA (tag=Spring Data JPA)
announcement - icon

Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.

Get started with Spring Data JPA through the guided reference course:

>> CHECK OUT THE COURSE

Partner – LambdaTest – NPI EA (cat=Testing)
announcement - icon

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

Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
announcement - icon

Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

Course – Spring Sale 2025 – NPI (cat=Baeldung)
announcement - icon

Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

1. Overview

In this article, we’ll be exploring the SetUtils API of Apache Commons Collections library. Simply put, these utilities can be used to execute certain operations on Set data structures in Java.

2. Dependency Installation

In order for us to use the SetUtils library in our project, we need to add the following dependency to our project’s pom.xml file:

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

Make sure to get the latest version of commons-collections4.

Alternatively, if our project is Gradle-based, we should add the dependency to our project’s build.gradle file. Also, we need to add mavenCentral() to the repositories section of the build.gradle file:

compile 'org.apache.commons:commons-collections4:4.5.0-M2'

3. Predicated Set

The predicatedSet() method of the SetUtils library permits defining conditions that should be met by all elements that are to be inserted into a set. It accepts a source Set object and a predicate.

We can use this to easily validate that all elements of a Set satisfy a certain condition, which can be handy when developing a third-party library/API.

If the validation fails for any element, an IllegalArgumentException will be thrown. The snippet below prevents the addition of strings that do not start with ‘L’ into the sourceSet or the returned validatingSet:

Set<String> validatingSet
  = SetUtils.predicatedSet(sourceSet, s -> s.startsWith("L"));

The library also has predicatedSortedSet() and predicatedNavigableSet() for working with SortedSet and NavigableSet respectively.

4. Union, Difference, and Intersection of a Set

The library has methods that can compute union, difference, and the intersection of Set elements.

The difference() method takes two Set objects and returns an immutable SetUtils.SetView object. The returned SetUtils.SetView contains the elements that are in set a but not in set b:

Set<Integer> a = new HashSet<>(Arrays.asList(1, 2, 5));
Set<Integer> b = new HashSet<>(Arrays.asList(1, 2));
SetUtils.SetView<Integer> result = SetUtils.difference(a, b);
 
assertTrue(result.size() == 1 && result.contains(5));

Note that, trying to perform write operations, like add() or addAll(), on the returned SetUtils.SetView will throw an UnsupportedOperationException.

To modify the returned result, we need to call the toSet() method of the returned SetUtils.SetView to obtain a writable Set object:

Set<Integer> mutableSet = result.toSet();

The union method of the SetUtils library does exactly what it sounds like – it returns all the elements of set a and b. The union method also returns a SetUtil.SetView object that is immutable:

Set<Integer> expected = new HashSet<>(Arrays.asList(1, 2, 5));
SetUtils.SetView<Integer> union = SetUtils.union(a, b);
 
assertTrue(SetUtils.isEqualSet(expected, union));

Take note of the isEqualSet() method used in the assert statement. It is a convenient static method of SetUtils library that effectively checks if two sets are equal.

To get the intersection of a set, i.e. elements that are both present in set a and set b, we’ll use the SetUtils.intersection() method. This method also returns a SetUtil.SetView object:

Set<Integer> expected = new HashSet<>(Arrays.asList(1, 2));
SetUtils.SetView<Integer> intersect = SetUtils.intersection(a, b);
 
assertTrue(SetUtils.isEqualSet(expected, intersect));

5. Transforming Set Elements

Let’s take a look at another exciting method – SetUtils.transformedSet(). This method accepts a Set object and a Transformer interface. Backed by the source set, it uses the transform() method of the Transformer interface to transform every element of a set.

The transforming logic is defined in the transform() method of the Transformer interface, which is applied to every element added to the set. The code snippet below multiplies every element added to the set by 2:

Set<Integer> a = SetUtils.transformedSet(new HashSet<>(), e -> e * 2  );
a.add(2);
 
assertEquals(a.toArray()[0], 4);

The transformedSet() method is pretty handy – they can even be used to cast elements of a set – say from String to Integer. Just make sure that the type of the output is a subtype of the input.

Let’s say we are working with SortedSet or NavigableSet instead of HashSet, we can use the transformedSortedSet() or transformedNavigableSet() respectively.

Note that a new HashSet instance is passed to the transformedSet() method. In situations where an existing, non-empty Set is passed to the method, the pre-existing elements will not be transformed.

If we want to transform pre-existing elements (and those added thereafter), we need to use the transformedSet() method of org.apache.commons.collections4.set.TransformedSet:

Set<Integer> source = new HashSet<>(Arrays.asList(1));
Set<Integer> newSet = TransformedSet.transformedSet(source, e -> e * 2);
 
assertEquals(newSet.toArray()[0], 2);
assertEquals(source.toArray()[0], 2);

Note that elements from the source set are transformed and the result is copied to the returned newSet.

6. Set Disjunction

The SetUtils library provides a static method that can be used to find set disjunctions. The disjunction of set a and set b are all the elements that are unique to set a and set b.

Let’s see how to use the disjunction() method of the SetUtils library:

Set<Integer> a = new HashSet<>(Arrays.asList(1, 2, 5));
Set<Integer> b = new HashSet<>(Arrays.asList(1, 2, 3));
SetUtils.SetView<Integer> result = SetUtils.disjunction(a, b);
 
assertTrue(
  result.toSet().contains(5) && result.toSet().contains(3));

7. Other Methods in SetUtils Library

There are other methods in the SetUtils library that make processing of set data a breeze:

  • We can use the synchronizedSet() or synchronizedSortedSet() to get a thread-safe Set. However, as stated in the docs, we must manually synchronize the returned set’s iterator to avoid non-deterministic behavior
  • We can use the SetUtils.unmodifiableSet() to get a read-only set. Note that an attempt to add elements to the returned Set Object will throw an UnsupportedOperationException
  • There is also the SetUtils.emptySet() method that returns a type-safe, immutable empty set
  • The SetUtils.emptyIfNull() method accepts a nullable Set object. It returns an empty, read-only, Set if the supplied Set is null; otherwise, it returns the supplied Set
  • SetUtils.orderedSet() will return a Set object that maintains the order in which elements are added
  • SetUtils.hashCodeForSet() can generate a hashcode for a set – in such a way that two sets of the same elements will have the same hashcode
  • SetUtils.newIdentityHashSet() will return a HashSet that uses == to match an element instead of the equals() method. Please read about its caveats here

8. Conclusion

In this article, we’ve explored the nitty-gritty of the SetUtils library. The utility class offers static methods that make working with a set data structure easy and exciting. It also boosts productivity.

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.
Next »
Apache Commons Collections OrderedMap
« Previous
Apache Commons Collections Bag
Baeldung Pro – NPI EA (cat = Baeldung)
announcement - icon

Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Spring Boot)
announcement - icon

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.

Partner – Orkes – NPI EA (cat = Spring)
announcement - icon

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.

Partner – Orkes – NPI EA (tag = Microservices)
announcement - icon

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.

eBook – HTTP Client – NPI EA (cat=HTTP Client-Side)
announcement - icon

The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

>> Download the eBook

eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
announcement - icon

Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
announcement - icon

Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Persistence – NPI EA (cat=Persistence)
announcement - icon

Working on getting your persistence layer right with Spring?

Explore the eBook

Course – LS – NPI EA (cat=REST)

announcement - icon

Get started with Spring Boot and with core Spring, through the Learn Spring course:

>> CHECK OUT THE COURSE

Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
announcement - icon

Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

Course – Spring Sale 2025 – NPI (All)
announcement - icon

Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

eBook Jackson – NPI EA – 3 (cat = Jackson)