Demystifying Java Streams: Step‑by‑Step Code Walkthrough

1. Stream Example Code Analysis

This Stream code consists of five lines; we examine each line.

Line 1

The line contains two parts: a lambda expression and the Stream.iterate() call. num -> num + 4 This lambda adds 4 to its input, equivalent to:

int addFour(int num) {
    return num + 4;
}

Stream.iterate(4, num -> num + 4)

creates an infinite stream starting at 4, where each subsequent element adds 4, producing:

4, 8, 12, 16, 20, 24, 28, …

Line 2

The limit() operation works like SQL's LIMIT, restricting the infinite stream to a finite number of elements. With total = 5, the stream becomes:

4, 8, 12, 16, 20

Line 3

The map() operation applies a lambda that squares each element, transforming the stream to:

4*4, 8*8, 12*12, 16*16, 20*20 => 16, 64, 144, 256, 400

Line 4

The filter() step keeps only elements divisible by 10; in this case only 400 remains:

400

Line 5

The reduce() operation aggregates the stream. Starting from 0, it sums all elements, yielding the final result:

From 0, calculate the sum of all stream elements and return the accumulated value.

The Stream therefore returns 400 .

2. Code Comparison for Understanding

To make the Stream logic clearer, we rewrite the same computation using ordinary loops:

Key correspondences: Stream.iterate() initial value ↔ loop initialization.

Second argument (lambda) ↔ loop body operation. limit() ↔ loop termination condition (based on element count). map() ↔ square calculation inside the loop. filter() ↔ if statement inside the loop. reduce() ↔ accumulation variable, update, and final return.

With this side‑by‑side analysis, the behavior of Java Streams becomes easier to grasp.