Convert Stream to Set in Java

Below given are some methods which can be used to convert Stream to Set in Java.

Method 1 : Using Collectors

Stream collect() method takes elements from a stream and stores them in a collection.collect(Collector.toSet()) collects elements from a stream to a Set.

Stream.collect() method can be used to collect elements of a stream in a container. The Collector which is returned by Collectors.toSet() can be passed that accumulates the elements of stream into a new Set.

// Java code for converting
// Stream to Set using Collectors
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));
    }
}

Output:

-1
0
-2
1
2

Method 2 : Converting Stream to Array and then to Set

The problem of converting Stream into Set can be divided into two parts :

1) Convert Stream to an Array
2) Convert Array to a Set

// Java code for converting
// Stream to Set using Divide
// and Conquer
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 Strings
    Stream<String> stream = Stream.of("G", "E", "K", "S");

    // Converting Stream into an Array
    String[] arr = stream.toArray(String[] :: new);

    // Creating a HashSet
    Set<String> set = new HashSet<>();

    // Converting Array to set
    Collections.addAll(set,arr);

    // Displaying the elements
    set.forEach(str -> System.out.println(str));
    }
}

Output:

S
E
G
K

Note : Output is random because HashSet takes input in random order as generated hash value.

Method 3 : Using forEach

Stream can be converted into Set using forEach(). Loop through all elements of the stream using forEach() method and then use set.add() to add each elements into an empty set.

// Java code for converting
// Stream to Set using forEach
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(5, 10, 15, 20, 25);

    // Creating a HashSet
    Set<Integer> set = new HashSet<>();

    // using set.add() to add elements
    // of stream into empty set
    stream.forEach(set::add);

    // Displaying the elements
    set.forEach(num -> System.out.println(num));
    }
}

Output:

20
5
25
10
15

Note : If Stream is parallel, then elements may not be processed in original order using forEach() method. To preserve the original order, forEachOrdered() method is used.

Convert a Set to Stream in Java