Find all duplicates in an array in Java

This post will discuss how to report all duplicates in an array in Java.

1. Using a Set

The idea is to iterate through the array and keep track of the encountered items in a Set. If an element is seen before, mark it as duplicate and report all duplicates at the end of the loop. This can be easily done using Java 8 Stream:

import java.util.Arrays;

import java.util.HashSet;

import java.util.Set;

import java.util.stream.Collectors;

public class Main {

public static Set<Integer> findDuplicates(int[] arr) {

Set<Integer> seen = new HashSet<>();

return Arrays.stream(arr).filter(i -> !seen.add(i))

.boxed().collect(Collectors.toSet());

}

public static void main(String[] args) {

int[] values = {4, 3, 2, 3, 1, 4, 5};

Set<Integer> duplicates = findDuplicates(values);

System.out.println(duplicates);

}

Download Run Code

Output:

[3, 4]

Here’s a version without using streams:

import java.util.HashSet;

import java.util.Set;

public class Main {

public static Set<Integer> findDuplicates(int[] arr) {

Set<Integer> seen = new HashSet<>();

Set<Integer> duplicates = new HashSet<>();

for (int i: arr) {

if (!seen.add(i)) {

duplicates.add(i);

}

return duplicates;

}

public static void main(String[] args) {

int[] values = {4, 3, 2, 3, 1, 4, 5};

Set<Integer> duplicates = findDuplicates(values);

System.out.println(duplicates);

}

Download Run Code

Output:

[3, 4]

2. Using a List

Another solution is to convert the array to a list and filter duplicates in the list using Stream API. The following code uses the Collections.frequency() method to get the frequency of each element in the collection.

import java.util.Arrays;

import java.util.Collections;

import java.util.List;

import java.util.Set;

import java.util.stream.Collectors;

public class Main

{

public static Set<Integer> findDuplicates(int[] arr) {

List<Integer> nums = Arrays.stream(arr).boxed().collect(Collectors.toList());

return nums.stream().filter(i -> Collections.frequency(nums, i) > 1)

.collect(Collectors.toSet());

}

public static void main(String[] args) {

int[] values = {4, 3, 2, 3, 1, 4, 5};

Set<Integer> duplicates = findDuplicates(values);

System.out.println(duplicates);

}

Download Run Code

Output:

[3, 4]

3. Using a Frequency Map

The above implementation is not efficient, as it is getting the frequency of each element. We can improve its performance by creating a frequency map and then filtering the values having a frequency more than 1.

import java.util.*;

import java.util.function.Function;

import java.util.stream.Collectors;

public class Main

{

public static Set<Integer> findDuplicates(int[] arr)

{

Map<Integer, Long> frequencies = Arrays.stream(arr).boxed()

.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

return frequencies.keySet().stream()

.filter(key -> frequencies.get(key) > 1)

.collect(Collectors.toSet());

}

public static void main(String[] args) {

int[] values = {4, 3, 2, 3, 1, 4, 5};

Set<Integer> duplicates = findDuplicates(values);

System.out.println(duplicates);

}

Download Run Code

Output:

[3, 4]

Here’s a version without using streams:

import java.util.HashMap;

import java.util.HashSet;

import java.util.Map;

import java.util.Set;

public class Main

{

public static Set<Integer> findDuplicates(int[] arr)

{

Map<Integer, Long> frequencies = new HashMap<>();

for (int i: arr) {

frequencies.merge(i, 1L, Long::sum);

}

Set<Integer> duplicates = new HashSet<>();

for (Integer key : frequencies.keySet()) {

if (frequencies.get(key) > 1) {

duplicates.add(key);

}

return duplicates;

}

public static void main(String[] args) {

int[] values = {4, 3, 2, 3, 1, 4, 5};

Set<Integer> duplicates = findDuplicates(values);

System.out.println(duplicates);

}

Download Run Code

Output:

[3, 4]

That’s all about finding all duplicates in an array in Java.

Find all duplicates in an array in Java

1. Using a Set

2. Using a List

3. Using a Frequency Map

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