Immutable ArrayList in Java

Last updated: April 4, 2025

Written by: Eugen Paraschiv

Java List

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1. Overview

This quick tutorial will show how to make an ArrayList immutable with the core JDK, with Guava and finally with Apache Commons Collections 4.

This article is part of the “Java – Back to Basic” series here on Baeldung.

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2. With the JDK

First, the JDK provides a nice way to get an unmodifiable collection out of an existing one:

Collections.unmodifiableList(list);

The new collection should no longer be modifiable at this point:

@Test(expected = UnsupportedOperationException.class)
public void givenUsingTheJdk_whenUnmodifiableListIsCreated_thenNotModifiable() {
    List<String> list = new ArrayList<>(Arrays.asList("one", "two", "three"));
    List<String> unmodifiableList = Collections.unmodifiableList(list);
    unmodifiableList.add("four");
}

2.1. With Java 9

Since Java 9, we can use a List<E>.of(E… elements) static factory method to create an immutable list:

@Test(expected = UnsupportedOperationException.class)
public final void givenUsingTheJava9_whenUnmodifiableListIsCreated_thenNotModifiable() {
    final List<String> list = new ArrayList<>(Arrays.asList("one", "two", "three"));
    final List<String> unmodifiableList = List.of(list.toArray(new String[]{}));
    unmodifiableList.add("four");
}

Notice how we have to convert the existing list into an array. This is because List.of(elements) accepts vararg parameters.

3. With Guava

Guava provides similar functionality for creating its own version of ImmutableList:

ImmutableList.copyOf(list);

Similarly – the resulting list should not be modifiable:

@Test(expected = UnsupportedOperationException.class)
public void givenUsingGuava_whenUnmodifiableListIsCreated_thenNotModifiable() {
    List<String> list = new ArrayList<>(Arrays.asList("one", "two", "three"));
    List<String> unmodifiableList = ImmutableList.copyOf(list);
    unmodifiableList.add("four");
}

Note that this operation will actually create a copy of the original list, not just a view.

Guava also provides a builder – this will return the strong-typed ImmutableList instead of simply List:

@Test(expected = UnsupportedOperationException.class)
public void givenUsingGuavaBuilder_whenUnmodifiableListIsCreated_thenNoLongerModifiable() {
    List<String> list = new ArrayList<>(Arrays.asList("one", "two", "three"));
    ImmutableList<String> unmodifiableList = ImmutableList.<String>builder().addAll(list).build();
    unmodifiableList.add("four");
}

4. With the Apache Collections Commons

Finally, Commons Collection also provides an API to create an unmodifiable list:

ListUtils.unmodifiableList(list);

And again, modifying the resulting list should result in an UnsupportedOperationException:

@Test(expected = UnsupportedOperationException.class)
public void givenUsingCommonsCollections_whenUnmodifiableListIsCreated_thenNotModifiable() {
    List<String> list = new ArrayList<>(Arrays.asList("one", "two", "three"));
    List<String> unmodifiableList = ListUtils.unmodifiableList(list);
    unmodifiableList.add("four");
}

5. Conclusion

This tutorial illustrates how to easily create an unmodifiable List out of an existing ArrayList using either the core JDK, Google Guava or Apache Commons 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.