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.

Syntax:

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

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

Examples of Stream.reduce() in Java

Example 1: Get the Longest String

Java

// 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); 
    }
}

Output

GeeksforGeeks

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.

Example 2: Combine Strings

Java

// 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); 
    }
}

Output

Geeks-for-Geeks

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.

Example 3: Sum of All Elements

Java

// 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); 
    }
}

Output

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.

Example 4: Product of All Numbers in a Range

Java

// 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); 
    }
}

Output

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.

Key Points:

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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