Remove duplicates from a List in Java

This post will discuss how to remove duplicates from a list in Java without destroying the original ordering of the list elements.

1. Plain Java

We know that a set doesn’t allow any duplicate elements. So if we convert the given list with duplicates to a set, we’ll get a set of elements from the list without duplicates. If we convert the set back to a list, we’ll get a list without duplicates.

Please note that HashSet will destroy the ordering of the elements. To preserve the original order, we can use LinkedHashset instead.

import java.util.*;

class Main

{

public static void main(String[] args)

{

// input list with duplicates

List<String> listWithDuplicates = new ArrayList<>(Arrays.asList("C++", "Java", "C++"));

// construct a set from elements of the list

Set<String> set = new LinkedHashSet<>(listWithDuplicates);

// construct a new list from a set and print it

List<String> listWithoutDuplicates = new ArrayList<>(set);

System.out.println(listWithoutDuplicates);

}

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Output:

[C++, Java]

2. Using Java 8 Stream

Here’s how we can do this in Java 8 and above using Stream:

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

// input list with duplicates

List<String> listWithDuplicates = new ArrayList<>(Arrays.asList("C++", "Java", "C++"));

// Construct a new list from the set constructed from elements

// of the original list

List<String> listWithoutDuplicates = listWithDuplicates.stream()

.collect(Collectors.toSet())

.stream()

.collect(Collectors.toList());

System.out.println(listWithoutDuplicates);

}

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Output:

[Java, C++]

The above approach destroys the ordering of the list elements. To preserve the original order, we can use Stream.distinct(), as shown below:

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

// input list with duplicates

List<String> listWithDuplicates = new ArrayList<>(Arrays.asList("C++", "Java", "C++"));

// Removing duplicates from the list using Java 8 Stream

List<String> listWithoutDuplicates = listWithDuplicates.stream()

.distinct()

.collect(Collectors.toList());

System.out.println(listWithoutDuplicates);

}

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Output:

[C++, Java]

This is equivalent to:

import java.util.*;

class Main

{

public static void main(String[] args)

{

// input list with duplicates

List<String> listWithDuplicates = new ArrayList<>(Arrays.asList("C++", "Java", "C++"));

List<String> listWithoutDuplicates = new ArrayList<>();

Set<String> uniqueValues = new HashSet<>();

for (String listWithDuplicate : listWithDuplicates) {

if (uniqueValues.add(listWithDuplicate)) {

listWithoutDuplicates.add(listWithDuplicate);

}

System.out.println(listWithoutDuplicates);

}

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Output:

[C++, Java]

3. Using Guava Library

Guava also provides static methods to create mutable instances of List and Set. These can be used with Java 7 or before to remove duplicates from a list.

import com.google.common.collect.Lists;

import com.google.common.collect.Sets;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

class Main

{

public static void main(String[] args)

{

// input list containing duplicates

List<String> listWithDuplicates = new ArrayList<>(Arrays.asList("C++", "Java", "C++"));

// Using Guava – mutable list

List<String> listWithoutDuplicates =

Lists.newArrayList(Sets.newHashSet(listWithDuplicates));

System.out.println(listWithoutDuplicates);

}

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Output:

[Java, C++]

If mutability is not required, we can go for immutable lists:

1	listWithoutDuplicates = ImmutableList.copyOf(ImmutableSet.copyOf(listWithDuplicates));

The above approach also destroys the ordering of the list elements. To preserve the original order, we can use ImmutableSet.copyOf(), as shown below:

import com.google.common.collect.ImmutableList;

import com.google.common.collect.ImmutableSet;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

class Main

{

public static void main(String[] args)

{

// input list containing duplicates

List<String> listWithDuplicates = new ArrayList<>(Arrays.asList("C++", "Java", "C++"));

// Using Guava – Immutable list

List<String> listWithoutDuplicates = ImmutableSet.copyOf(listWithDuplicates).asList();

System.out.println(listWithoutDuplicates);

}

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Output:

[C++, Java]

That’s all about removing duplicates from a List in Java.

Remove duplicates from a List in Java

1. Plain Java

2. Using Java 8 Stream

3. Using Guava Library

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