Java 8 - Convert Stream to Set

Introduction

In Java 8, converting a stream to a set is useful when you want to collect unique elements. By using Collectors.toSet(), you can easily convert a stream into a set, ensuring that any duplicate elements are removed automatically. The Stream API provides a flexible way to process and then collect the results into a set.

In this guide, we will learn how to convert a stream into a set in Java 8.

Solution Steps

1. Create or Obtain a Stream: Generate or obtain a stream from a collection or another data source.
2. Process the Stream (Optional): Perform any filtering, mapping, or other stream operations as needed.
3. Use collect(Collectors.toSet()): Collect the elements of the stream into a set.
4. Display or Use the Set: Use or print the resulting set.

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Java Program

Example 1: Convert Stream of Integers to a Set

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

public class StreamToSetExample {
    public static void main(String[] args) {
        // Step 1: Create a stream of integers
        Stream<Integer> numberStream = Stream.of(1, 2, 3, 4, 5, 3, 2);

        // Step 2: Convert the stream to a set
        Set<Integer> numberSet = numberStream.collect(Collectors.toSet());

        // Step 3: Display the set
        System.out.println(numberSet);  // Output: [1, 2, 3, 4, 5]
    }
}

Output

[1, 2, 3, 4, 5]

Explanation

Step 1: Create a Stream

We create a stream of integers, where some elements are duplicates:

Stream<Integer> numberStream = Stream.of(1, 2, 3, 4, 5, 3, 2);

Step 2: Convert the Stream to a Set

We use collect(Collectors.toSet()) to convert the stream into a set. The set automatically removes duplicate elements:

Set<Integer> numberSet = numberStream.collect(Collectors.toSet());

Step 3: Display the Set

The resulting set of integers is printed:

System.out.println(numberSet);

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Example 2: Convert Stream of Strings to a Set

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

public class StreamOfStringsToSet {
    public static void main(String[] args) {
        // Step 1: Create a stream of strings
        Stream<String> fruitStream = Stream.of("Apple", "Banana", "Orange", "Mango", "Apple");

        // Step 2: Convert the stream to a set
        Set<String> fruitSet = fruitStream.collect(Collectors.toSet());

        // Step 3: Display the set
        System.out.println(fruitSet);  // Output: [Apple, Banana, Mango, Orange]
    }
}

Output

[Apple, Banana, Mango, Orange]

Explanation

Step 1: Create a Stream

We create a stream of strings that contains duplicates:

Stream<String> fruitStream = Stream.of("Apple", "Banana", "Orange", "Mango", "Apple");

Step 2: Convert the Stream to a Set

We convert the stream into a set using collect(Collectors.toSet()), which automatically removes duplicates:

Set<String> fruitSet = fruitStream.collect(Collectors.toSet());

Step 3: Display the Set

We print the resulting set of unique strings:

System.out.println(fruitSet);

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Example 3: Convert Filtered Stream to a Set

In this example, we filter the stream of numbers to retain only even numbers and then collect the result into a set.

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

public class FilteredStreamToSet {
    public static void main(String[] args) {
        // Step 1: Create a stream of integers
        Stream<Integer> numberStream = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

        // Step 2: Filter and convert the stream to a set of even numbers
        Set<Integer> evenNumbers = numberStream
            .filter(n -> n % 2 == 0)  // Filter even numbers
            .collect(Collectors.toSet());  // Collect into a set

        // Step 3: Display the set of even numbers
        System.out.println(evenNumbers);  // Output: [2, 4, 6, 8, 10]
    }
}

Output

[2, 4, 6, 8, 10]

Explanation

Step 1: Create a Stream

We create a stream of integers from 1 to 10:

Stream<Integer> numberStream = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

Step 2: Filter the Stream and Convert to a Set

We filter the stream to retain only even numbers and collect them into a set:

Set<Integer> evenNumbers = numberStream
    .filter(n -> n % 2 == 0)
    .collect(Collectors.toSet());

Step 3: Display the Set

The filtered set of even numbers is printed:

System.out.println(evenNumbers);

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Conclusion

In Java 8, converting a stream to a set is simple and efficient using the collect(Collectors.toSet()) method. This approach is useful when you want to collect unique elements from the stream, as the set automatically handles duplicate removal. Whether you are working with numbers, strings, or custom objects, this method is a great way to handle collections in a functional manner.