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1. Introduction

In this quick tutorial, we're going to learn how to clean up the duplicate elements from a List. First, we'll use plain Java, then Guava, and finally, a Java 8 Lambda-based solution.

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

2. Remove Duplicates From a List Using Plain Java

We can easily remove the duplicate elements from a List with the standard Java Collections Framework through a Set:

public void 
  givenListContainsDuplicates_whenRemovingDuplicatesWithPlainJava_thenCorrect() {
    List<Integer> listWithDuplicates = Lists.newArrayList(5, 0, 3, 1, 2, 3, 0, 0);
    List<Integer> listWithoutDuplicates = new ArrayList<>(
      new HashSet<>(listWithDuplicates));

    assertThat(listWithoutDuplicates, hasSize(5));
    assertThat(listWithoutDuplicates, containsInAnyOrder(5, 0, 3, 1, 2));
}

As we can see, the original list remains unchanged.

In the example above, we used HashSet implementation, which is an unordered collection. As a result, the order of the cleaned-up listWithoutDuplicates might be different than the order of the original listWithDuplicates.

If we need to preserve the order, we can use LinkedHashSet instead:

public void 
  givenListContainsDuplicates_whenRemovingDuplicatesPreservingOrderWithPlainJava_thenCorrect() {
    List<Integer> listWithDuplicates = Lists.newArrayList(5, 0, 3, 1, 2, 3, 0, 0);
    List<Integer> listWithoutDuplicates = new ArrayList<>(
      new LinkedHashSet<>(listWithDuplicates));

    assertThat(listWithoutDuplicates, hasSize(5));
    assertThat(listWithoutDuplicates, containsInRelativeOrder(5, 0, 3, 1, 2));
}

Further reading:

Java Collections Interview Questions

A set of practical Collections-related Java interview questions

Java – Combine Multiple Collections

A quick and practical guide to combining multiple collections in Java

How to Find an Element in a List with Java

Have a look at some quick ways to find an element in a list in Java

3. Remove Duplicates From a List Using Guava

We can do the same thing using Guava as well:

public void 
  givenListContainsDuplicates_whenRemovingDuplicatesWithGuava_thenCorrect() {
    List<Integer> listWithDuplicates = Lists.newArrayList(5, 0, 3, 1, 2, 3, 0, 0);
    List<Integer> listWithoutDuplicates 
      = Lists.newArrayList(Sets.newHashSet(listWithDuplicates));

    assertThat(listWithoutDuplicates, hasSize(5));
    assertThat(listWithoutDuplicates, containsInAnyOrder(5, 0, 3, 1, 2));
}

Here also, the original list remains unchanged.

Again, the order of elements in the cleaned-up list might be random.

If we use the LinkedHashSet implementation, we'll preserve the initial order:

public void 
  givenListContainsDuplicates_whenRemovingDuplicatesPreservingOrderWithGuava_thenCorrect() {
    List<Integer> listWithDuplicates = Lists.newArrayList(5, 0, 3, 1, 2, 3, 0, 0);
    List<Integer> listWithoutDuplicates 
      = Lists.newArrayList(Sets.newLinkedHashSet(listWithDuplicates));

    assertThat(listWithoutDuplicates, hasSize(5));
    assertThat(listWithoutDuplicates, containsInRelativeOrder(5, 0, 3, 1, 2));
}

4. Remove Duplicates From a List Using Java 8 Lambdas

Finally, let's look at a new solution, using Lambdas in Java 8. We'll use the distinct() method from the Stream API, which returns a stream consisting of distinct elements based on the result returned by the equals() method.

Additionally, for ordered streams, the selection of distinct elements is stable. This means that for duplicated elements, the element appearing first in the encounter order is preserved:

public void 
  givenListContainsDuplicates_whenRemovingDuplicatesWithJava8_thenCorrect() {
    List<Integer> listWithDuplicates = Lists.newArrayList(5, 0, 3, 1, 2, 3, 0, 0);
    List<Integer> listWithoutDuplicates = listWithDuplicates.stream()
     .distinct()
     .collect(Collectors.toList());

    assertThat(listWithoutDuplicates, hasSize(5));
    assertThat(listWithoutDuplicates, containsInAnyOrder(5, 0, 3, 1, 2));
}

There we have it, three quick ways to clean up all the duplicate items from a List.

5. Conclusion

In this article, we demonstrated how easy it is to remove duplicates from a list using plain Java, Google Guava, and Java 8.

The implementation of all of these examples and snippets can be found in the GitHub project. This is a Maven-based project, so it should be easy to import and run.

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