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. Introduction

In this article, we’ll learn different approaches to finding duplicates in a List in Java.

Given a list of integers with duplicate elements, we’ll be finding the duplicate elements in it. For example, given the input list [1, 2, 3, 3, 4, 4, 5], the output List will be [3, 4].

2. Finding Duplicates Using Collections

In this section, we’ll discuss two ways of using Collections to extract duplicate elements present in a list.

2.1. Using the contains() Method of Set

Set in Java doesn’t contain duplicates. The contains() method in Set returns true only if the element is already present in it.

We’ll add elements to the Set if contains() returns false. Otherwise, we’ll add the element to the output list. The output list thus contains the duplicate elements:

List<Integer> listDuplicateUsingSet(List<Integer> list) {
    List<Integer> duplicates = new ArrayList<>();
    Set<Integer> set = new HashSet<>();
    for (Integer i : list) {
        if (set.contains(i)) {
            duplicates.add(i);
        } else {
            set.add(i);
        }
    }
    return duplicates;
}

Let’s write a test to check if the list duplicates contains only the duplicate elements:

@Test
void givenList_whenUsingSet_thenReturnDuplicateElements() {
    List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
    List<Integer> duplicates = listDuplicate.listDuplicateUsingSet(list);
    Assert.assertEquals(duplicates.size(), 2);
    Assert.assertEquals(duplicates.contains(3), true);
    Assert.assertEquals(duplicates.contains(4), true);
    Assert.assertEquals(duplicates.contains(1), false);
}

Here we see that the output list only contains two elements 3 and 4.

This approach takes O(n) time for n elements in a list and extra space of size n for the set.

2.2. Using a Map and Storing the Frequency of Elements

We can use a Map to store the frequency of each element and then add them to the output list only when the frequency of the element isn’t 1:

List<Integer> listDuplicateUsingMap(List<Integer> list) {
    List<Integer> duplicates = new ArrayList<>();
    Map<Integer, Integer> frequencyMap = new HashMap<>();
    for (Integer number : list) {
        frequencyMap.put(number, frequencyMap.getOrDefault(number, 0) + 1);
    }
    for (int number : frequencyMap.keySet()) {
        if (frequencyMap.get(number) != 1) {
            duplicates.add(number);
        }
    }
    return duplicates;
}

Let’s write a test to check if the list duplicates contain only duplicate elements:

@Test
void givenList_whenUsingFrequencyMap_thenReturnDuplicateElements() {
    List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
    List<Integer> duplicates = listDuplicate.listDuplicateUsingMap(list);
    Assert.assertEquals(duplicates.size(), 2);
    Assert.assertEquals(duplicates.contains(3), true);
    Assert.assertEquals(duplicates.contains(4), true);
    Assert.assertEquals(duplicates.contains(1), false);
}

Here we see that the output list contains only two elements, 3 and 4.

This approach takes O(n) time for n elements in a list and extra space of size n for the map.

3. Using Streams in Java 8

In this section, we’ll discuss three ways of using Streams to extract duplicate elements present in a list.

3.1. Using filter() and Set.add() Method

Set.add() adds the specified element to this set if it’s not already present. If this set already contains the element, the call leaves the set unchanged and returns false.

Here, we’ll use a Set and convert the list to a stream. The stream is added to the Set, and the duplicate elements are filtered and collected into List:

List<Integer> listDuplicateUsingFilterAndSetAdd(List<Integer> list) {
    Set<Integer> elements = new HashSet<Integer>();
    return list.stream()
      .filter(n -> !elements.add(n))
      .collect(Collectors.toList());
}

Let’s write a test to check if the list duplicates contain only duplicate elements:

@Test
void givenList_whenUsingFilterAndSetAdd_thenReturnDuplicateElements() {
    List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
    List<Integer> duplicates = listDuplicate.listDuplicateUsingFilterAndSetAdd(list);
    Assert.assertEquals(duplicates.size(), 2);
    Assert.assertEquals(duplicates.contains(3), true);
    Assert.assertEquals(duplicates.contains(4), true);
    Assert.assertEquals(duplicates.contains(1), false);
}

Here we see that the output elements contain only two elements, 3 and 4, as expected.

This approach using filter() with Set.add() is the fastest algorithm to find duplicate elements with O(n) time complexity and extra space of size n for the set.

3.2. Using Collections.frequency()

Collections.frequency() returns the number of elements in the specified collection, which is equal to a specified value. Here we’ll convert List to Stream and filter out only the elements that return a value greater than one from Collections.frequency().

We’ll collect these elements into Set to avoid repetitions and finally convert Set to List:

List<Integer> listDuplicateUsingCollectionsFrequency(List<Integer> list) {
    List<Integer> duplicates = new ArrayList<>();
    Set<Integer> set = list.stream()
      .filter(i -> Collections.frequency(list, i) > 1)
      .collect(Collectors.toSet());
    duplicates.addAll(set);
    return duplicates;
}

Let’s write a test to check if the duplicates contain only duplicate elements:

@Test
void givenList_whenUsingCollectionsFrequency_thenReturnDuplicateElements() {
    List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
    List<Integer> duplicates = listDuplicate.listDuplicateUsingCollectionsFrequency(list);
    Assert.assertEquals(duplicates.size(), 2);
    Assert.assertEquals(duplicates.contains(3), true);
    Assert.assertEquals(duplicates.contains(4), true);
    Assert.assertEquals(duplicates.contains(1), false);
}

As expected, the output list contains only two elements, 3 and 4.

This approach using Collections.frequency() is the slowest because it compares each element with a list – Collections.frequency(list, i) whose complexity is O(n). So the overall complexity is O(n*n). It also requires an extra space of size n for the set.

3.3. Using Map and Collectors.groupingBy()

Collectors.groupingBy() returns a collector implementing a cascaded “group by” operation on input elements.

It groups elements according to a classification function and then performs a reduction operation on the associated values with a given key using the specified downstream collector. The classification function maps elements to some key type K. The downstream collector operates on input elements and produces a result of type D. The resulting collector produces a Map<K, D>.

Here we’ll use Function.identity() as the classification function and Collectors.counting() as the downstream collector.

Function.identity() returns a function that always returns its input argument. Collectors.counting() returns a collector accepting elements that count the number of input elements. If no elements are present, the result is zero. Thus we’ll get a map of elements and their frequency using Collectors.groupingBy().

Then we convert the EntrySet of this Map into a Stream, filter out only the elements that have a value greater than 1, and collect them in a Set to avoid repetitions. Then the  Set is converted into a List: 

List<Integer> listDuplicateUsingMapAndCollectorsGroupingBy(List<Integer> list) {
    List<Integer> duplicates = new ArrayList<>();
    Set<Integer> set = list.stream()
      .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
      .entrySet()
      .stream()
      .filter(m -> m.getValue() > 1)
      .map(Map.Entry::getKey)
      .collect(Collectors.toSet());
    duplicates.addAll(set);
    return duplicates;
}

Let’s write a test to check if the list duplicates contains only duplicate elements:

@Test
void givenList_whenUsingMapAndCollectorsGroupingBy_thenReturnDuplicateElements() {
    List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
    List<Integer> duplicates = listDuplicate.listDuplicateUsingCollectionsFrequency(list);
    Assert.assertEquals(duplicates.size(), 2);
    Assert.assertEquals(duplicates.contains(3), true);
    Assert.assertEquals(duplicates.contains(4), true);
    Assert.assertEquals(duplicates.contains(1), false);
}

Here we see that the output elements contain only two elements 3 and 4.

Collectors.groupingBy() takes O(n) time. A filter() operation is done on the resulting EntrySet, but the complexity remains O(n) as the map lookup time is O(1). It also requires an extra space of n for the set.

4. Find Duplicate Values in an Array in Java

In Java, finding duplicate values in an array is a common task often approached with various techniques. One straightforward method involves iterating through the array and comparing each element with every other element to identify duplicates. This approach, however, can be inefficient for large arrays due to its time complexity of O(n^2).

Alternatively, Using HashSet or HashMap presents a more efficient option with a better time complexity of O(n). Iterating through the array and leveraging these data structures for duplicate detection significantly improves scalability and performance, especially for larger datasets.

4.1. Using for Loop for Duplicate Detection

Let’s create a method to find duplicate values in an array using a for loop for duplicate detection:

public static Set<Integer> findDuplicateInArray(int[] array) {
    Set<Integer> duplicates = new HashSet<>();
    Set<Integer> seen = new HashSet<>();
    for (int val : array) {
        if (!seen.add(val)) {
            duplicates.add(val);
        }
    }
    return duplicates;
}

The above method takes a generic array T[] as input and returns a Set containing all the duplicate values within the array using a For loop. It iterates through each element within the array, adding it to a HashSet named “seen“. If a value cannot be added to the “seen” set due to its existence, it is subsequently added to the resulting set of duplicates.

4.2. Using Streams and Collectors for Duplicate Detection

Let’s create a method to find duplicate values in an array using Java streams and collectors for efficient duplicate detection:

public static <T> Set<T> findDuplicateInArrayWithStream(T[] array) {
    Set<T> seen = new HashSet<>();
    return Arrays.stream(array)
      .filter(val -> !seen.add(val))
      .collect(Collectors.toSet());
}

The above method takes a generic array T[] as input and returns a Set containing all the duplicate values within the array using streams. It utilizes Java streams to convert the array into a stream of elements, filters the stream to retain only elements that aren’t successfully added to the seen set (thus identifying duplicates), and collects the filtered elements into a set, eliminating duplicates automatically. This implementation offers a more concise and modern approach to finding duplicates in an array.

5. Conclusion

In this article, we learned about different ways of extracting duplicate elements from a List in Java.

We discussed approaches using Set and Map and their corresponding approaches using Stream. The code using Stream is far more declarative and conveys the intent of the code clearly without the need of external iterators.

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.
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)