Java 8 – Find Duplicate Elements in a Stream

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Introduction

Finding duplicate elements in a collection is a common task in many applications. Java 8 provides a simple and efficient way to find duplicates using the Stream API. By leveraging Collectors, Set, and filtering operations, we can easily identify the duplicate elements in a Stream.

In this guide, we will learn how to find duplicate elements in a stream using Java 8.

Solution Steps

Define the Input Stream: Create a Stream of elements to be processed.
Use a Set to Track Seen Elements: Use a Set to store elements that have been encountered.
Filter Duplicates: Filter elements that are already present in the set to identify duplicates.
Collect and Display the Duplicates: Collect the duplicates into a List or print them.

Java Program

import java.util.Arrays;
import java.util.List;
import java.util.Set;
import java.util.HashSet;
import java.util.stream.Collectors;

public class FindDuplicatesInStream {
    public static void main(String[] args) {
        // Step 1: Define the input list and create a Stream
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 3, 2, 6, 7, 2, 4);

        // Step 2-3: Use a Set to track seen elements and filter duplicates
        Set<Integer> seen = new HashSet<>();
        List<Integer> duplicates = numbers.stream()
            .filter(n -> !seen.add(n))  // Add element to Set and check if it's already there
            .collect(Collectors.toList());  // Collect duplicates into a List

        // Step 4: Display the duplicates
        System.out.println("Duplicate elements: " + duplicates);
    }
}

Output

Duplicate elements: [3, 2, 2, 4]

Explanation

Step 1: Define the Input Stream

We begin by defining a list of integers:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 3, 2, 6, 7, 2, 4);

This list contains several duplicate elements, including 2, 3, and 4.

Step 2-3: Use a Set to Track Seen Elements and Filter Duplicates

We use a HashSet to keep track of elements that we have already encountered:

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

The filter() method checks if adding the current element to the Set is successful. If seen.add(n) returns false, it means the element is already present, and thus it is a duplicate:

.filter(n -> !seen.add(n))

If an element is found to be a duplicate, it is collected using Collectors.toList().

Step 4: Display the Duplicates

The resulting list of duplicates is printed to the console:

System.out.println("Duplicate elements: " + duplicates);

Conclusion

In Java 8, finding duplicate elements in a Stream is easy and efficient using the filter() method and a Set to track previously seen elements. By using the Stream API, you can quickly identify duplicates and perform further operations on them, making this approach highly flexible for handling collections.

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