Java vs Rust

Java and Rust occupy different niches in the programming world. Java aims for developer productivity with automatic memory management and a massive ecosystem. Rust prioritizes performance and safety, giving programmers fine-grained control without sacrificing memory safety.

Rust has topped Stack Overflow’s “most loved language” survey for years. Java remains one of the most widely used languages in production. This comparison explores their differences and helps you decide which fits your project.

Quick Comparison

Aspect	Java	Rust
Released	1995	2015 (1.0)
Created by	Sun Microsystems (now Oracle)	Mozilla (now Rust Foundation)
Typing	Static, strong	Static, strong
Compilation	Bytecode (JVM)	Native binary (LLVM)
Memory management	Garbage collected	Ownership system (no GC)
Null handling	Nullable references	Option type (no null)
Concurrency	Threads, locks	Ownership prevents data races
OOP support	Full (classes, inheritance)	Structs, traits (no inheritance)
Learning curve	Moderate	Steep

Syntax Comparison

Hello World

Java:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Rust:

fn main() {
    println!("Hello, World!");
}

Rust’s syntax is more concise. No class wrapper required for a simple program. The exclamation mark in println! indicates it’s a macro, not a function.

Variables

Java:

int count = 10;
final int maxSize = 100;  // Immutable
String name = "Alice";
var items = new ArrayList<String>();  // Type inference (Java 10+)

Rust:

let count = 10;           // Immutable by default
let mut count = 10;       // Mutable
let max_size: i32 = 100;  // Explicit type
let name = "Alice";       // Type inferred
let items: Vec<String> = Vec::new();

Rust variables are immutable by default. You must explicitly opt into mutability with mut. This prevents accidental modifications and makes code intent clearer.

Functions

Java:

public int add(int a, int b) {
    return a + b;
}

public String greet(String name) {
    return "Hello, " + name;
}

Rust:

fn add(a: i32, b: i32) -> i32 {
    a + b  // No semicolon = return value
}

fn greet(name: &str) -> String {
    format!("Hello, {}", name)
}

// With explicit return
fn divide(a: f64, b: f64) -> Option<f64> {
    if b == 0.0 {
        return None;
    }
    Some(a / b)
}

Rust functions return the last expression without return or semicolon. The -> syntax specifies the return type. &str is a string reference (borrowed), String is an owned string.

Structs and Classes

Java:

public class User {
    private String name;
    private int age;
    
    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String greet() {
        return "Hi, I'm " + name;
    }
    
    public String getName() { return name; }
    public int getAge() { return age; }
}

Rust:

struct User {
    name: String,
    age: u32,
}

impl User {
    // Associated function (like static method)
    fn new(name: String, age: u32) -> User {
        User { name, age }
    }
    
    // Method (takes &self)
    fn greet(&self) -> String {
        format!("Hi, I'm {}", self.name)
    }
}

// Usage
let user = User::new(String::from("Alice"), 30);
println!("{}", user.greet());

Rust has no classes, only structs with associated functions. Methods take &self (borrow), &mut self (mutable borrow), or self (take ownership). There’s no inheritance, but traits provide polymorphism.

Memory Management

This is the fundamental difference between the languages.

Java: Garbage Collection

Java allocates objects on the heap and tracks references. When no references remain, the garbage collector reclaims memory.

public void processData() {
    List<String> data = new ArrayList<>();
    data.add("item");
    // When method ends, 'data' reference goes away
    // GC eventually frees the ArrayList
}

Benefits: Simple, safe, no manual memory management. Costs: GC pauses, higher memory usage, less predictable latency.

Rust: Ownership System

Rust tracks ownership at compile time. Each value has exactly one owner. When the owner goes out of scope, the value is dropped.

fn process_data() {
    let data = vec!["item"];
    // 'data' owns the Vec
} // data goes out of scope, memory freed immediately

No GC runs. Memory is freed deterministically. But you must satisfy the borrow checker.

Ownership and Borrowing

Rust’s ownership rules:

1. Each value has one owner
2. When the owner goes out of scope, the value is dropped
3. You can have either one mutable reference OR any number of immutable references

fn main() {
    let s1 = String::from("hello");
    let s2 = s1;  // s1 is MOVED to s2, s1 is now invalid
    
    // println!("{}", s1);  // Compile error: s1 was moved
    println!("{}", s2);     // Works
    
    // Borrowing instead of moving
    let s3 = String::from("world");
    let len = calculate_length(&s3);  // Borrow s3
    println!("'{}' has length {}", s3, len);  // s3 still valid
}

fn calculate_length(s: &String) -> usize {
    s.len()
}  // s goes out of scope, but doesn't drop what it refers to

This compile-time checking eliminates entire categories of bugs: use-after-free, double-free, dangling pointers. The cost is a steeper learning curve.

Null Safety

Java: NullPointerException

Java references can be null, leading to runtime errors:

String name = null;
int length = name.length();  // NullPointerException at runtime

Java added Optional in version 8, but it’s not enforced:

Optional<String> maybeName = Optional.ofNullable(getName());
String name = maybeName.orElse("Unknown");

Rust: No Null

Rust has no null. Use Option for values that might be absent:

fn find_user(id: u32) -> Option<User> {
    if id == 1 {
        Some(User::new(String::from("Alice"), 30))
    } else {
        None
    }
}

// Must handle both cases
match find_user(1) {
    Some(user) => println!("Found: {}", user.name),
    None => println!("User not found"),
}

// Or use if let
if let Some(user) = find_user(1) {
    println!("Found: {}", user.name);
}

// Or unwrap (panics if None - use carefully)
let user = find_user(1).unwrap();

// Or provide default
let user = find_user(1).unwrap_or_else(|| User::new(String::from("Guest"), 0));

The compiler forces you to handle the None case. No null pointer exceptions at runtime.

Error Handling

Java Exceptions

public String readFile(String path) throws IOException {
    return Files.readString(Path.of(path));
}

try {
    String content = readFile("data.txt");
} catch (IOException e) {
    System.err.println("Error: " + e.getMessage());
}

Rust Result Type

use std::fs;
use std::io;

fn read_file(path: &str) -> Result<String, io::Error> {
    fs::read_to_string(path)
}

// Pattern matching
match read_file("data.txt") {
    Ok(content) => println!("{}", content),
    Err(e) => eprintln!("Error: {}", e),
}

// The ? operator propagates errors
fn process_file(path: &str) -> Result<usize, io::Error> {
    let content = fs::read_to_string(path)?;  // Returns early on error
    Ok(content.len())
}

Rust’s Result type makes error handling explicit. The ? operator reduces boilerplate while keeping error handling visible.

Concurrency

Java Threads

// Shared mutable state requires synchronization
class Counter {
    private int count = 0;
    
    public synchronized void increment() {
        count++;
    }
    
    public synchronized int getCount() {
        return count;
    }
}

Counter counter = new Counter();

Thread t1 = new Thread(() -> {
    for (int i = 0; i < 1000; i++) counter.increment();
});

Thread t2 = new Thread(() -> {
    for (int i = 0; i < 1000; i++) counter.increment();
});

t1.start();
t2.start();
t1.join();
t2.join();

Rust: Fearless Concurrency

Rust’s ownership system prevents data races at compile time:

use std::thread;
use std::sync::{Arc, Mutex};

fn main() {
    let counter = Arc::new(Mutex::new(0));
    let mut handles = vec![];
    
    for _ in 0..2 {
        let counter = Arc::clone(&counter);
        let handle = thread::spawn(move || {
            for _ in 0..1000 {
                let mut num = counter.lock().unwrap();
                *num += 1;
            }
        });
        handles.push(handle);
    }
    
    for handle in handles {
        handle.join().unwrap();
    }
    
    println!("Result: {}", *counter.lock().unwrap());
}

Arc (atomic reference counting) allows shared ownership across threads. Mutex ensures exclusive access. The compiler rejects code that could cause data races.

// This won't compile - can't share mutable data without synchronization
let mut data = vec![1, 2, 3];
thread::spawn(|| {
    data.push(4);  // Error: can't borrow `data` as mutable
});

Performance

Startup time: Rust wins significantly. Rust compiles to native code with no runtime. Java requires JVM startup.

Peak throughput: Both can be fast. Rust gives more predictable performance. Java’s JIT can optimize hot paths aggressively, sometimes matching or exceeding ahead-of-time compiled code.

Memory usage: Rust typically uses 2-10x less memory than equivalent Java code. No GC overhead, no object headers, value types on the stack.

Latency: Rust offers predictable latency with no GC pauses. Java’s modern collectors (ZGC, Shenandoah) achieve low pause times but can’t match Rust’s determinism for hard real-time requirements.

Binary size: Rust produces self-contained binaries (typically 1-10MB). Java requires the JVM (200MB+) unless using GraalVM native images.

Ecosystem

Java’s ecosystem is massive. Enterprise frameworks (Spring, Jakarta EE), build tools (Maven, Gradle), ORMs (Hibernate), testing frameworks (JUnit, Mockito). Decades of libraries for every domain.

Rust’s ecosystem is younger but growing rapidly. Cargo (build tool) and crates.io (package registry) are excellent. Strong libraries exist for web (Actix, Axum), async runtime (Tokio), serialization (serde), and systems programming.

Key Rust crates:

• tokio – Async runtime
• serde – Serialization/deserialization
• actix-web, axum – Web frameworks
• diesel, sqlx – Database access
• clap – Command-line parsing

Use Cases

Choose Java When:

Enterprise applications: Java dominates enterprise software with mature frameworks, tooling, and developer availability.

Android development: The Android ecosystem is built on Java (and Kotlin).

Big data: Hadoop, Spark, Kafka, Flink run on the JVM. Java integrates naturally.

Rapid development: Less time fighting the compiler, faster iteration. Garbage collection simplifies memory management.

Large teams: More Java developers available. Easier to hire and onboard.

Choose Rust When:

Systems programming: Operating systems, drivers, embedded systems. Rust provides C/C++ level control with memory safety.

Performance-critical services: When every millisecond matters. Low latency, predictable performance, minimal resource usage.

WebAssembly: Rust compiles to WebAssembly efficiently. Popular for browser-based computation.

Command-line tools: Fast startup, small binaries, no runtime dependencies.

Security-sensitive code: Memory safety guarantees prevent entire classes of vulnerabilities.

Infrastructure: Container runtimes, databases, networking tools. Where reliability and performance intersect.

Industry Adoption

Java: Powers most Fortune 500 companies. Banks, healthcare, retail, government. Netflix, LinkedIn, Amazon, and countless enterprises rely on it.

Rust: Adopted by major tech companies for specific use cases. Amazon (Firecracker), Microsoft (Windows components), Google (Android, Chrome), Discord (performance-critical services), Cloudflare (edge computing). Mozilla, Dropbox, and Facebook use it for systems work.

The Linux kernel now accepts Rust code for new drivers, a significant endorsement for systems programming.

Learning Curve

Java is more approachable. The concepts are familiar from other OOP languages. Garbage collection means not worrying about memory. The ecosystem is documented extensively.

Rust is harder to learn. The ownership system requires a mental shift. The compiler rejects code that compiles fine in other languages. Fighting the borrow checker is a common frustration for newcomers.

But Rust’s difficulty is front-loaded. Once code compiles, many bug categories are eliminated. “If it compiles, it usually works” is a common sentiment.

Code Example: HTTP Server

Java (Spring Boot)

@RestController
public class HelloController {
    
    @GetMapping("/hello")
    public String hello(@RequestParam(defaultValue = "World") String name) {
        return "Hello, " + name + "!";
    }
}

Rust (Actix-web)

use actix_web::{get, web, App, HttpServer, Responder};

#[get("/hello")]
async fn hello(name: web::Query<NameQuery>) -> impl Responder {
    format!("Hello, {}!", name.name.as_deref().unwrap_or("World"))
}

#[derive(serde::Deserialize)]
struct NameQuery {
    name: Option<String>,
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    HttpServer::new(|| App::new().service(hello))
        .bind("127.0.0.1:8080")?
        .run()
        .await
}

Both achieve the same result. Java with Spring Boot requires less setup code. Rust is more explicit about async, error handling, and types.

Summary

Java and Rust serve different purposes well.

Java offers productivity, a mature ecosystem, and widespread adoption. It’s the pragmatic choice for enterprise applications, large teams, and projects where time-to-market matters more than bare-metal performance.

Rust offers performance, memory safety, and fine-grained control. It’s the right choice for systems programming, performance-critical services, and applications where reliability and efficiency are paramount.

Many organizations use both: Java for business applications, Rust for performance-critical components. They complement rather than compete.

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Related: Java vs C++ | Java vs Go | Java vs Python | Java Multithreading Basics

Sources

• Rust. “The Rust Programming Language.” doc.rust-lang.org/book
• Oracle. “Java Documentation.” docs.oracle.com/en/java
• Stack Overflow. “Developer Survey 2024.” stackoverflow.com/survey
• Rust Foundation. “Rust in Production.” rustfoundation.org