Reduce Sum is a common problem faced by many developers and programming enthusiasts alike. It requires fundamental understandings of programming concepts. Although Java offers a variety of methods to solve this problem, it can still prove to be a confusing task for some. In this article, we will delve into one of the most efficient solutions to the Reduce Sum problem in Java, explaining each part of the code step-by-step, thereby making it easily comprehensible even for beginners.

## The Reduce Sum Problem

The reduce sum problem essentially boils down to summing the numbers in a numeric stream. The problem can have different variations. It could be summing elements of an array, elements of a LinkedList or summing elements of a Stream in a multi-thread environment which uses the `stream.reduce()` function. It utilizes lambda expressions and functional interfaces, which are essential features of Java 8 and above.

## Java Solution

Java provides an array of libraries and functions to easily handle and resolve the problem. We would be using the Stream API that was introduced in Java 8. In particular, the `reduce()` method which combines the elements of a stream to yield a single summary result.

Here is the sample Java code to do so.

public int sumOfArray(Integer[] numbers) { return Arrays.stream(numbers) .reduce(0, Integer::sum); }

## Step-by-step Explanation of the Code

**The above code represents a method that takes an array of integers as an argument and returns the sum of these integers.**

- First, `Arrays.stream(numbers)` converts the array into a Stream.
- The `reduce()` method is then called on this stream. This method takes two parameters: the initial value of the sum and the method to be applied for calculating the sum.
- The initial value in this case is set as ‘0’ and the method used is `Integer::sum`. `Integer::sum` is a reference to the static method `sum` in the Integer class. This method returns the sum of its arguments. It’s being passed as a method reference into the reduce method.
- The `reduce()` function then performs the sum operation on each of the elements in the stream and the sum is then returned as the result of the function.

## Java Libraries and Similar Functions

Java offers an extensive range of libraries that can help you manipulate and operate on streams. Similar functions to `reduce` include `collect`, `count`, `match`, `find`, `iterate` among others, based on what operation needs to be performed on the stream.

Through this illustration, we are able to see that Java provides powerful and flexible tools to solve complex problems, even like the reduce sum problem. It’s a testament to the versatility and robustness of the language.