Java- Print all duplicate elements from a List

By Atul Rai | Last Updated: November 1, 2022 Previous       Next

In Java, we have multiple ways to find duplicate elements in a List. We can loop through the List and add each element to a Set which stores the only unique elements.

1. Filter & Set.add()

Using filter(Predicate<? super T> predicate) method of Stream API and adding each element to Set will give us the duplicate element.

// List contains duplicate elements
List<Integer> list = Arrays.asList(8, 49, 25, 98, 98, 32, 15, 15,
							10, 30, 20, 20, 50, 10, 12,	10 
							);

Set<Integer> set = new HashSet<>();
Set<Integer> output1 = list.stream()
							.filter(e -> !set.add(e))
							.collect(Collectors.toSet());
		
output1.forEach(System.out::println); // 98,20,10,15

2. Collections.frequency()

Collections.frequency(Collection<?> c, Object o) returns the number of elements in the specified collection equal to the specified object. And its return type is int.

// List contains duplicate elements
List<Integer> list = Arrays.asList(8, 49, 25, 98, 98, 32, 15, 15,
							10, 30, 20, 20, 50, 10, 12,	10 
							);

Set<Integer> output2 = list.stream()
						   .filter(e -> Collections.frequency(list, e) > 1)
				           .collect(Collectors.toSet());
		
output2.forEach(System.out::println); // 98,20,10,15

3. Collectors.groupingBy()

Collectors.groupingBy() provide us with functionality similar to the ‘GROUP BY’ clause in the SQL language. We use them for grouping objects by some property and storing results in a Map instance.

// List contains duplicate elements
List list = Arrays.asList(8, 49, 25, 98, 98, 32, 15, 15,
							10, 30, 20, 20, 50, 10, 12,	10 
							);

Set output3 = list.stream()
						   .collect(Collectors.groupingBy(Function.identity(), 
                                    Collectors.counting()))
						   .entrySet()
						   .stream()
						   .filter(e -> e.getValue() > 1)
						   .map(Map.Entry::getKey)
						   .collect(Collectors.toSet());

output3.forEach(System.out::println); // 98,20,10,15

Check out the complete example.

package org.websparrow;

import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;

public class DuplicateElements {

    public static void main(String[] args) {

        // List contains duplicate elements
        List list = Arrays.asList(8, 49, 25, 98, 98, 32, 15, 10,
                30, 20, 20, 50, 10, 12, 10, 15);

        System.out.println("Result using filter and add");

        Set output1 = filterAndAdd(list);
        output1.forEach(System.out::println);

        System.out.println("Result using Collections.frequency");

        Set output2 = collectionsFrequency(list);
        output2.forEach(System.out::println);

        System.out.println("Result using Collectors.groupingBy");

        Set output3 = collectorsGroupingBy(list);
        output3.forEach(System.out::println);

    }

    private static Set filterAndAdd(List list) {
        Set set = new HashSet<>();
        return list.stream().filter(e -> !set.add(e))
                .collect(Collectors.toSet());
    }

    private static Set collectionsFrequency(List list) {
        return list.stream().filter(e -> Collections.frequency(list, e) > 1)
                .collect(Collectors.toSet());
    }

    private static Set collectorsGroupingBy(List list) {
        return list.stream().collect(Collectors.groupingBy(Function.identity(),
                                     Collectors.counting()))
                .entrySet()
                .stream().filter(e -> e.getValue() > 1)
                .map(Map.Entry::getKey)
                .collect(Collectors.toSet());
    }
}

Output

Result using filter and add
98
20
10
15
Result using Collections.frequency
98
20
10
15
Result using Collectors.groupingBy
98
20
10
15

References

1. How to count the frequency of a character in a string in Java
2. Java 8 Stream filter() Method Example
3. How to find distinct elements in a list in Java