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Stream.reduce() in Java with examples

Last Updated : 28 Aug, 2024

In Java, the Stream.reduce() method is used to perform a reduction on the elements of a stream using an associative accumulation function and returns an Optional. It is commonly used to aggregate or combine elements into a single result, such as computing the maximum, minimum, sum, or product.

T reduce(T identity, BinaryOperator<T> accumulator);

• identity: An initial value of type T.
• accumulator: A function that combines two values of type T.

// Implementation of reduce method
// to get the longest String
import java.util.*;

class GFG {
    public static void main(String[] args) {
        // Creating a list of Strings
        List<String> words = Arrays.asList("GFG", "Geeks", "for", "GeeksQuiz", "GeeksforGeeks");

        // Using reduce to find the longest string in the list
        Optional<String> longestString = words.stream()
            .reduce((word1, word2) -> word1.length() > word2.length() ? word1 : word2);

        // Displaying the longest String
        longestString.ifPresent(System.out::println); 
    }
}

Explanation:

• Purpose: Finds the longest string in the list.
• Lambda Expression: Compares two strings and returns the longer one.
• Result: An Optional because the list might be empty.

// Implementation of reduce method
// to get the combined String
import java.util.*;

class GFG {
    public static void main(String[] args) {
        // String array to combine
        String[] array = { "Geeks", "for", "Geeks" };

        // Using reduce to concatenate strings with a hyphen
        Optional<String> combinedString = Arrays.stream(array)
            .reduce((str1, str2) -> str1 + "-" + str2);

        // Displaying the combined String
        combinedString.ifPresent(System.out::println); 
    }
}

Explanation:

• Purpose: Concatenates all strings in the array, separated by hyphens.
• Lambda Expression: Concatenates two strings with a hyphen.
• Result: An Optional because the stream might be empty.

// Implementation of reduce method
// to get the sum of all elements
import java.util.*;

class GFG {
    public static void main(String[] args) {
        // Creating list of integers
        List<Integer> numbers = Arrays.asList(-2, 0, 4, 6, 8);

        // Using reduce to find the sum of all elements
        int sum = numbers.stream()
            .reduce(0, (element1, element2) -> element1 + element2);

        // Displaying the sum of all elements
        System.out.println("The sum of all elements is " + sum); 
    }
}

The sum of all elements is 16

Explanation:

• Purpose: Computes the sum of all integers in the list.
• Lambda Expression: Adds each element to the accumulator.
• Identity Value: 0, which serves as the starting value.

// Implementation of reduce method
// to get the product of all numbers
// in given range.
import java.util.*;
import java.util.stream.IntStream;

class GFG {
    public static void main(String[] args) {
        // Calculating the product of all numbers in the range [2, 8)
        int product = IntStream.range(2, 8)
            .reduce((num1, num2) -> num1 * num2)
            .orElse(-1); // Provides -1 if the stream is empty

        // Displaying the product
        System.out.println("The product is : " + product); 
    }
}

The product is : 5040

Explanation:

• Purpose: Calculates the product of integers in the range [2, 8).
• Lambda Expression: Multiplies two numbers.
• Default Value: -1, used if the stream is empty.

• The reduce method can be used for various reduction operations.
• The result of reduce is an Optional because the stream might be empty.
• reduce requires an associative operation to combine elements.
• When using reduce with a non-empty stream, specifying an identity value (like 0 for sum) simplifies the operation without needing to handle Optional.

These examples and explanations demonstrate how to effectively use Stream.reduce() for different operations in Java.

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