Comparing reduce in Java and fold in Rust: Declarative Approaches to Data Aggregation

Data aggregation is essential when working with collections. Both Java and Rust offer powerful methods for this purpose: reduce in Java’s Streams API and fold in Rust’s Iterators. In this article, we’ll compare how each language handles this functionality by looking at three practical examples to clarify their similarities, differences, and key considerations.

1. Summing a List of Values

A classic example of using reduce and fold is to sum the values in a list. In Java, reduce allows you to apply a binary operation to each element, combining them into a single aggregated result. In Rust, fold does the same but with a focus on efficiency and memory control.

Example in Java with reduce:

List<Integer> numbers = List.of(1, 2, 3, 4, 5);
int sum = numbers.stream().reduce(0, Integer::sum);
System.out.println("Sum in Java: " + sum);
        

Example in Rust with fold:

let numbers = vec![1, 2, 3, 4, 5];
let sum: i32 = numbers.iter().fold(0, |acc, &x| acc + x);
println!("Sum in Rust: {}", sum);
        

In both cases, we are summing the values in the list, starting from 0. The main difference is that Rust uses a closure in fold to define how values are combined, while Java uses a method reference (Integer::sum), which simplifies the syntax.

2. Concatenating Strings

Concatenating a list of strings is a great example to highlight reduce and fold with non-numeric data collections. In Java, we can use reduce with an initial empty value, while in Rust, fold also allows this initialization and leverages closures to concatenate efficiently.

Example in Java with reduce:

List<String> words = List.of("Rust", "and", "Java", "are", "powerful");
String sentence = words.stream()
                       .reduce("", (a, b) -> a + " " + b);
System.out.println("Sentence in Java: " + sentence.trim());
        

Example in Rust with fold:

let words = vec!["Rust", "and", "Java", "are", "powerful"];
let sentence = words.iter()
    .fold(String::new(), |acc, &word| acc + " " + word);
println!("Sentence in Rust: {}", sentence.trim());
        

Here, both approaches concatenate the words into a single string. In Rust, using String::new() as the initial value and a closure |acc, &word| ensures the code is safe and efficient, as Rust handles necessary memory allocation.

3. Finding the Maximum in a List

To find the maximum value in a list, reduce and fold are extremely useful, allowing us to compare each element with the accumulated value until finding the largest. This example shows how Java and Rust implement this functionality.

Example in Java with reduce:

List<Integer> numbers = List.of(10, 5, 30, 20);
int max = numbers.stream()
                 .reduce(Integer.MIN_VALUE, Integer::max);
System.out.println("Max in Java: " + max);
        

Example in Rust with fold:

let numbers = vec![10, 5, 30, 20];
let max = numbers.iter()
    .fold(i32::MIN, |acc, &x| acc.max(x));
println!("Max in Rust: {}", max);
        

In this example, both codes start with a minimum value (Integer.MIN_VALUE in Java and i32::MIN in Rust) and compare each subsequent element to update the maximum value. Rust’s fold approach ensures efficiency without additional allocations, keeping the code both safe and optimized.

Final Thoughts: reduce in Java vs. fold in Rust

While reduce and fold offer the same basic functionality, the implementations reflect each language's philosophy. Java emphasizes simplicity and readability with a more user-friendly API, while Rust shines in memory control and efficiency, allowing safe and optimized operations. Choosing between reduce and fold depends on your project context, but both are powerful tools that make data aggregation easier, cleaner, and more concise.

These differences make Java and Rust interesting options for developers, each with distinct advantages for different contexts—from high-level applications to low-level, performance-critical systems.