Convert Stream to Set in Java

A Stream in Java is used to process collections of data in a functional style. However, after performing stream operations, you often need to collect the processed elements into a Set to eliminate duplicates and store unique values. Java provides multiple ways to convert a Stream into a Set.

• Converts a Stream into a Set containing unique elements.
• Automatically removes duplicate values during collection.
• Supports collecting elements into different implementations such as HashSet, LinkedHashSet, and TreeSet.

Methods to Convert a Stream to a Set

Java provides the following ways to convert a Stream into a Set.

1. Convert a Stream to a Set Using Collectors.toSet()

The Collectors.toSet() method is the most commonly used way to collect stream elements into a Set. It automatically removes duplicate elements.

Syntax:

stream.collect(Collectors.toSet());

import java.util.*;
import java.util.stream.Stream;
import java.util.stream.Collectors;

class GFG {
    
    // Driver code
    public static void main(String[] args) {
    
    // Creating a Stream of Integers
    Stream<Integer> stream = Stream.of(-2, -1, 0, 1, 2);
    
    // Using Stream.collect() to collect the 
    // elements of stream in a container.
    Set<Integer> streamSet = stream.collect(Collectors.toSet());
    
    // Displaying the elements
    streamSet.forEach(num -> System.out.println(num));
    }
}

-1
0
-2
1
2

2. Convert a Stream to a Set Using Collectors.toCollection()

The Collectors.toCollection() method allows collecting stream elements into a specific Set implementation such as LinkedHashSet or TreeSet.

Syntax:

stream.collect(Collectors.toCollection(LinkedHashSet::new));

import java.util.*;
import java.util.stream.*;

class GFG {

    public static void main(String[] args)
    {
        Stream<String> stream
            = Stream.of("Java", "Python", "Java", "C++");

        Set<String> set = stream.collect(
            Collectors.toCollection(LinkedHashSet::new));

        System.out.println(set);
    }
}

[Java, Python, C++]

3. Convert a Stream to an Unmodifiable Set Using Collectors.toUnmodifiableSet()

The Collectors.toUnmodifiableSet() method (introduced in Java 10) creates an immutable Set. Any attempt to modify the resulting set throws an UnsupportedOperationException.

Syntax:

stream.collect(Collectors.toUnmodifiableSet());

import java.util.*;
import java.util.stream.*;

class GFG {

    public static void main(String[] args)
    {
        Stream<String> stream
            = Stream.of("Java", "Python", "Java", "C++");

        Set<String> set
            = stream.collect(Collectors.toUnmodifiableSet());

        System.out.println(set);
    }
}

[Java, Python, C++]

Benefits After Conversion

Converting a Stream to a Set is useful when only unique elements are required after performing stream operations. It also enables efficient searching and prevents duplicate entries in the resulting collection.

• Eliminates duplicate elements automatically.
• Provides efficient lookup operations.
• Supports different Set implementations based on ordering requirements.