java-immutable-list
Immutable ArrayList in Java
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.
Further reading:
Collect a Java Stream to an Immutable Collection
Learn how to collect Java Streams to immutable Collections.
Introduction to Immutables
A quick and practical intro to the Immutables library – used to generate immutable objects via the use of annotations.
Java – Get Random Item/Element From a List
A quick and practical guide to picking a random item/items from a List in Java.
2. With the JDK
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<String>(Arrays.asList("one", "two", "three"));
List<String> unmodifiableList = Collections.unmodifiableList(list);
unmodifiableList.add("four");
}3. With Guava
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<String>(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 returned the strong-typed ImmutableList instead of simply List:
@Test(expected = UnsupportedOperationException.class)
public void givenUsingGuavaBuilder_whenUnmodifiableListIsCreated_thenNoLongerModifiable() {
List<String> list = new ArrayList<String>(Arrays.asList("one", "two", "three"));
ImmutableList<String> unmodifiableList = ImmutableList.<String>builder().addAll(list).build();
unmodifiableList.add("four");
}4. With the Apache Collections Commons
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<String>(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 implementation of all these examples and code snippets can be found over on Github – this is a Maven-based project, so it should be easy to import and run as it is.
