Flattening Nested Collections in Java

Last updated: April 4, 2025

Written by: Zeger Hendrikse

Reviewed by: Zeger Hendrikse

Java List

>= Java 8

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1. Overview

In this quick article, we’ll explore how to flatten a nested collection in Java.

2. Example of a Nested Collection

Suppose we have a list of lists of type String.

List<List<String>> nestedList = asList(
  asList("one:one"), 
  asList("two:one", "two:two", "two:three"), 
  asList("three:one", "three:two", "three:three", "three:four"));

3. Flattening the List With forEach

In order to flatten this nested collection into a list of strings, we can use forEach together with a Java 8 method reference:

public <T> List<T> flattenListOfListsImperatively(
    List<List<T>> nestedList) {
    List<T> ls = new ArrayList<>();
    nestedList.forEach(ls::addAll);
    return ls;
}

And here you can see the method in action:

@Test
public void givenNestedList_thenFlattenImperatively() {
    List<String> ls = flattenListOfListsImperatively(nestedList);
    
    assertNotNull(ls);
    assertTrue(ls.size() == 8);
    assertThat(ls, IsIterableContainingInOrder.contains(
      "one:one",
      "two:one", "two:two", "two:three", "three:one",
      "three:two", "three:three", "three:four"));
}

4. Flattening the List With flatMap

We can also flatten the nested list by utilizing the flatMap method from the Stream API.

This allows us to flatten the nested Stream structure and eventually collect all elements to a particular collection:

public <T> List<T> flattenListOfListsStream(List<List<T>> list) {
    return list.stream()
      .flatMap(Collection::stream)
      .collect(Collectors.toList());    
}

And here’s the logic in action:

@Test
public void givenNestedList_thenFlattenFunctionally() {
    List<String> ls = flattenListOfListsStream(nestedList);
    
    assertNotNull(ls);
    assertTrue(ls.size() == 8);
}

5. Conclusion

A simple forEach or flatMap methods in Java 8, in combination with method references, can be used for flattening nested collections.

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.