Usage of reduce() in Java Streams

The reduce() method in Java Streams is used to perform aggregation operations like summing, finding the minimum or maximum, concatenation, etc. It reduces a stream to a single value.

1. Syntax of reduce()

reduce() has three main variations:

1. Identity + Accumulator

T reduce(T identity, BinaryOperator accumulator);

• Identity → Initial value
• Accumulator → Operation to combine elements

2. Accumulator Only (Returns Optional)

Optional reduce(BinaryOperator accumulator);

• No identity → returns Optional (empty if stream is empty).

3. Identity + Accumulator + Combiner (for Parallel Streams)

<U> U reduce(U identity, BiFunction<U, T, U> accumulator, BinaryOperator<U> combiner);

• Used in parallel processing.
• Combiner merges results from multiple threads.

2. Examples of reduce() Usage

Example 1: Sum of Numbers

import java.util.Arrays;
import java.util.List;

public class ReduceExample {
    public static void main(String[] args) {
        List numbers = Arrays.asList(10, 20, 30, 40);

        int sum = numbers.stream()
                         .reduce(0, (a, b) -> a + b); // Identity = 0, Summation

        System.out.println("Sum: " + sum);
    }
}

Output:

Sum: 100

Example 2: Find Maximum Value

import java.util.Arrays;
import java.util.List;
import java.util.Optional;

public class ReduceMaxExample {
    public static void main(String[] args) {
        List numbers = Arrays.asList(3, 7, 2, 8, 6);

        Optional max = numbers.stream()
                                       .reduce((a, b) -> a > b ? a : b); // No identity, returns Optional

        max.ifPresent(m -> System.out.println("Max Value: " + m));
    }
}

Output:

Max Value: 8

Example 3: Concatenate Strings

import java.util.Arrays;
import java.util.List;

public class ReduceStringExample {
    public static void main(String[] args) {
        List words = Arrays.asList("Java", "Streams", "Reduce");

        String result = words.stream()
                             .reduce("", (a, b) -> a + " " + b).trim();

        System.out.println("Concatenated String: " + result);
    }
}

Output:

Concatenated String: Java Streams Reduce

Example 4: Multiplication Using Parallel Streams

import java.util.Arrays;
import java.util.List;

public class ReduceParallelExample {
    public static void main(String[] args) {
        List numbers = Arrays.asList(2, 3, 4, 5);

        int product = numbers.parallelStream()
                             .reduce(1, (a, b) -> a * b, (a, b) -> a * b); // Combiner required for parallel

        System.out.println("Product: " + product);
    }
}

Output:

Product: 120

Key Points

• If identity is given, the result is always a non-null value.
• If identity is not given, it returns Optional<T> to handle empty streams.
• Parallel streams need a combiner to merge partial results efficiently.