Guide to Java 8 Stream.reduce() method

In the world of Java programming, Java 8 introduced the Stream API, which revolutionized the way we work with collections. Among the numerous features Stream offers, the reduce() method stands out as a powerful tool for performing aggregation operations on a stream of elements. This method allows you to perform operations such as summing, multiplying, finding the maximum or minimum, and more, with ease and elegance. In this article, we’ll dive deep into the world of Stream.reduce(), exploring its syntax, common use cases, and best practices.

Understanding Stream.reduce()

The Stream.reduce() method is used to reduce the elements of a stream to a single value. It combines elements in a way defined by a binary operator and an optional identity value. Here’s its signature:

T reduce(T identity, BinaryOperator<T> accumulator)
  • identity: An initial value, often the identity element of the operation (e.g., 0 for addition or 1 for multiplication).
  • accumulator: A binary operator that combines two elements of the stream.

Simple Example: Summing Elements

Let’s start with a simple example that demonstrates how to use reduce() to sum elements in a stream of integers:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

int sum =
    .reduce(0, (a, b) -> a + b);"Sum: {}", sum);

In this example, we initialize the sum with 0 and then use the lambda expression (a, b) -> a + b as the accumulator to sum the elements in the stream. The result will be 15, which is the sum of all the numbers in the list.

Simple Example: Finding Maximum and Minimum

You can use reduce() to find the maximum and minimum values in a stream:

List<Integer> numbers = Arrays.asList(3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5);

// Maximum
int max =, Integer::max);

// Minimum
int min =, Integer::min);"Max: {}", max);"Min: {}", min);

In this example, we use Integer::max and Integer::min as the binary operators to find the maximum and minimum values, respectively.


In this guide, we’ve explored the Stream.reduce() method in Java 8. This versatile method allows you to perform aggregation operations on streams with ease. Whether you need to sum elements, find the maximum or minimum, or perform more complex reduction tasks, reduce() provides an elegant solution. Just remember to choose the appropriate identity element and binary operator for your specific use case, and you’ll harness the full power of this valuable Java feature in your projects.