# reduce() method

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Published 2023-06-10

This tutorial explains how we can use reduce() method with Streams in Java.

reduce() method produce one single result (this is a terminal operation) from a sequence of elements, by repeatedly applying a combining operation to the elements in the stream.

Please take a look at the following example and read carefully the comments. The code is self-explanatory.

This example is created from a simple Spring Boot application created with Spring Initializr. I am using Maven, Java 17, Spring Boot 3.1.0.

From the base application downloaded from Spring Initializr, I updated the main class as below:

DemoApplication.java
package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationContext;

import java.util.*;
import java.util.concurrent.ExecutionException;

@SpringBootApplication
public class DemoApplication {

	public static void main(String[] args) throws InterruptedException, ExecutionException {

		ApplicationContext appContext = SpringApplication.run(DemoApplication.class, args);

		Integer[] numbers = {10, 20, 30};

		Integer sum = Arrays.stream(numbers)
				// Calculate the SUM of the number in the stream
				.reduce(0, (a,b) -> a+b);

		System.out.println("SUM="+sum);
	}
}

When we consider reduce(0, (a,b) -> a+b), "0" is the initial value, "a" is the partial result (accumulator) and "b" is the current element.

The same result could be done by using the method reference (Integer::sum):

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationContext;

import java.util.*;
import java.util.concurrent.ExecutionException;

@SpringBootApplication
public class DemoApplication {

	public static void main(String[] args) throws InterruptedException, ExecutionException {

		ApplicationContext appContext = SpringApplication.run(DemoApplication.class, args);

		Integer[] numbers = {10, 20, 30};

		Integer sum = Arrays.stream(numbers)
				// Calculate the SUM of the number in the stream
				.reduce(Integer::sum)
				.get();

		System.out.println("SUM="+sum);
	}
}

When I run this code I receive:

SUM=60

Process finished with exit code 0

When we are working with Streams, we can have:

INTERMEDIATE operations: filter, map, limit, sorted, peak, distinct, skip, flatMap
TERMINAL operations: forEach, count, sum, min, collect, reduce, anyMatch, noneMatch, allMatch