Classes, objects, fields and methods

Python 3.14 · main.py

class Ticket:
    def __init__(self, seat: str, price: int) -> None:
        # Each new object gets its own seat and price; the price is in cents.
        self.seat = seat
        self.price = price

    def describe(self) -> str:
        return f"seat {self.seat}, {self.price // 100}.{self.price % 100:02d}"

    def upgrade(self, seat: str, extra: int) -> None:
        self.seat = seat
        self.price += extra


a = Ticket("C7", 900)
b = Ticket("C8", 900)
print(f"a: {a.describe()}")
print(f"b: {b.describe()}")
print("upgrade a to A1, 6.50 more")
a.upgrade("A1", 650)
print(f"a: {a.describe()}")
print(f"b: {b.describe()}")

In Python: self is written out as the first parameter of every method, and a.describe() passes a as self. The fields come into being when __init__ assigns them; nothing declares them.

Recorded with CPython 3.14.8 on macOS 26 arm64. It runs in your browser with Pyodide 314.0.7 (CPython 3.14.2). To run it yourself: python3 main.py

JavaScript Node 24 · main.mjs

class Ticket {
  constructor(seat, price) {
    // Each new object gets its own seat and price; the price is in cents.
    this.seat = seat;
    this.price = price;
  }

  describe() {
    const cents = String(this.price % 100).padStart(2, "0");
    return `seat ${this.seat}, ${Math.trunc(this.price / 100)}.${cents}`;
  }

  upgrade(seat, extra) {
    this.seat = seat;
    this.price += extra;
  }
}

const a = new Ticket("C7", 900);
const b = new Ticket("C8", 900);
console.log(`a: ${a.describe()}`);
console.log(`b: ${b.describe()}`);
console.log("upgrade a to A1, 6.50 more");
a.upgrade("A1", 650);
console.log(`a: ${a.describe()}`);
console.log(`b: ${b.describe()}`);

In JavaScript: The constructor runs when new Ticket(…) is evaluated, with this as the new object, and the fields appear when it assigns them.

Recorded with Node.js 24.21.0 LTS (V8 13.6) on macOS 26 arm64. It runs in your browser with QuickJS (quickjs-emscripten 0.32.0). To run it yourself: node main.mjs

Java 25 LTS · Main.java

import java.util.Locale;

public class Main {
  public static void main(String[] args) {
    Locale.setDefault(Locale.ROOT);
    Ticket a = new Ticket("C7", 900);
    Ticket b = new Ticket("C8", 900);
    System.out.println("a: " + a.describe());
    System.out.println("b: " + b.describe());
    System.out.println("upgrade a to A1, 6.50 more");
    a.upgrade("A1", 650);
    System.out.println("a: " + a.describe());
    System.out.println("b: " + b.describe());
  }
}

final class Ticket {
  private String seat;
  private int price;

  Ticket(String seat, int price) {
    // Each new object gets its own seat and price; the price is in cents.
    this.seat = seat;
    this.price = price;
  }

  String describe() {
    return String.format("seat %s, %d.%02d", seat, price / 100, price % 100);
  }

  void upgrade(String seat, int extra) {
    this.seat = seat;
    price += extra;
  }
}

In Java: this.seat = seat is needed because the parameter seat hides the field of the same name. String.format runs after main sets Locale.ROOT, so the digits are the same in every locale.

Recorded with OpenJDK 25.0.4.1 LTS (Eclipse Temurin) on macOS 26 arm64. It runs in your browser with a TeaVM 0.16.0 build in QuickJS (quickjs-emscripten 0.32.0). To run it yourself: java Main.java

C++23 · main.cpp

#include <format>
#include <print>
#include <string>
#include <utility>

class Ticket {
public:
  // Each new object gets its own seat and price; the price is in cents.
  Ticket(std::string seat, int price) : seat_(std::move(seat)), price_(price) {}

  std::string describe() const {
    return std::format("seat {}, {}.{:02}", seat_, price_ / 100, price_ % 100);
  }

  void upgrade(std::string seat, int extra) {
    seat_ = std::move(seat);
    price_ += extra;
  }

private:
  std::string seat_;
  int price_;
};

int main() {
  Ticket a("C7", 900);
  Ticket b("C8", 900);
  std::println("a: {}", a.describe());
  std::println("b: {}", b.describe());
  std::println("upgrade a to A1, 6.50 more");
  a.upgrade("A1", 650);
  std::println("a: {}", a.describe());
  std::println("b: {}", b.describe());
}

In C++: The member initialiser list, : seat_(…), price_(…), sets the fields before the body of the constructor runs. describe() is const: it promises not to change the object.

Recorded with Apple clang 21.0.0 (C++23) and GCC 16.2.0 on macOS 26 arm64. It runs in your browser with Clang in WebAssembly (YoWASP) 22.0.0-git20542-10. To run it yourself: clang++ -std=c++23 -O2 -Wall -Wextra -Werror -pedantic -fsanitize=address,undefined main.cpp -o main && ./main

C# 14 · .NET 10 · app.cs

using System.Globalization;

CultureInfo.CurrentCulture = CultureInfo.InvariantCulture;

var a = new Ticket("C7", 900);
var b = new Ticket("C8", 900);
Console.WriteLine($"a: {a.Describe()}");
Console.WriteLine($"b: {b.Describe()}");
Console.WriteLine("upgrade a to A1, 6.50 more");
a.Upgrade("A1", 650);
Console.WriteLine($"a: {a.Describe()}");
Console.WriteLine($"b: {b.Describe()}");

// Each new object gets its own seat and price; the price is in cents.
sealed class Ticket(string seat, int price)
{
    public string Seat { get; private set; } = seat;
    public int Price { get; private set; } = price;

    public string Describe() => $"seat {Seat}, {Price / 100}.{Price % 100:D2}";

    public void Upgrade(string seat, int extra)
    {
        Seat = seat;
        Price += extra;
    }
}

In C#: Ticket(string seat, int price) is a primary constructor, new in C# 12, whose parameters initialise the two properties. {Price % 100:D2} pads the cents to two digits.

Recorded with C# 14 on .NET 10.0.12 (SDK 10.0.401) on macOS 26 arm64. It runs in your browser with the .NET runtime for WebAssembly 10.0.12. To run it yourself: dotnet run app.cs

C23 · main.c

#include <stddef.h>
#include <stdio.h>

struct ticket {
  char seat[8];
  int price;
};

static struct ticket ticket_new(const char *seat, int price) {
  // Each new object gets its own seat and price; the price is in cents.
  struct ticket ticket = {.price = price};
  snprintf(ticket.seat, sizeof ticket.seat, "%s", seat);
  return ticket;
}

static void ticket_describe(const struct ticket *ticket, char *out,
                            size_t size) {
  snprintf(out, size, "seat %s, %d.%02d", ticket->seat, ticket->price / 100,
           ticket->price % 100);
}

static void ticket_upgrade(struct ticket *ticket, const char *seat, int extra) {
  snprintf(ticket->seat, sizeof ticket->seat, "%s", seat);
  ticket->price += extra;
}

int main(void) {
  char text[32];
  struct ticket a = ticket_new("C7", 900);
  struct ticket b = ticket_new("C8", 900);
  ticket_describe(&a, text, sizeof text);
  printf("a: %s\n", text);
  ticket_describe(&b, text, sizeof text);
  printf("b: %s\n", text);
  puts("upgrade a to A1, 6.50 more");
  ticket_upgrade(&a, "A1", 650);
  ticket_describe(&a, text, sizeof text);
  printf("a: %s\n", text);
  ticket_describe(&b, text, sizeof text);
  printf("b: %s\n", text);
  return 0;
}

In C: C has no methods. ticket_describe(&a, …) plays the method, and its first parameter is the object that the other languages pass for you as self or this.

Recorded with Apple clang 21.0.0 (C23) and GCC 16.2.0 on macOS 26 arm64. It runs in your browser with Clang in WebAssembly (YoWASP) 22.0.0-git20542-10. To run it yourself: clang -std=c23 -O2 -Wall -Wextra -Werror -pedantic -fsanitize=address,undefined main.c -o main && ./main

PHP 8.5 · main.php

<?php

declare(strict_types=1);

final class Ticket
{
    // Each new object gets its own seat and price; the price is in cents.
    public function __construct(
        private string $seat,
        private int $price,
    ) {}

    public function describe(): string
    {
        $cents = $this->price % 100;

        return sprintf('seat %s, %d.%02d', $this->seat, intdiv($this->price, 100), $cents);
    }

    public function upgrade(string $seat, int $extra): void
    {
        $this->seat = $seat;
        $this->price += $extra;
    }
}

$a = new Ticket('C7', 900);
$b = new Ticket('C8', 900);
echo "a: {$a->describe()}\n";
echo "b: {$b->describe()}\n";
echo "upgrade a to A1, 6.50 more\n";
$a->upgrade('A1', 650);
echo "a: {$a->describe()}\n";
echo "b: {$b->describe()}\n";

In PHP: Constructor promotion, private string $seat in the parameter list, declares the field and assigns it in one step.

Recorded with PHP 8.5.11 (CLI) on macOS 26 arm64. It runs in your browser with php-wasm 0.2.0 (PHP 8.5.2). To run it yourself: php main.php

Rust 1.99 · main.rs

struct Ticket {
    seat: String,
    price: u32,
}

impl Ticket {
    fn new(seat: &str, price: u32) -> Self {
        // Each new object gets its own seat and price; the price is in cents.
        Ticket {
            seat: seat.to_string(),
            price,
        }
    }

    fn describe(&self) -> String {
        format!(
            "seat {}, {}.{:02}",
            self.seat,
            self.price / 100,
            self.price % 100
        )
    }

    fn upgrade(&mut self, seat: &str, extra: u32) {
        self.seat = seat.to_string();
        self.price += extra;
    }
}

fn main() {
    let mut a = Ticket::new("C7", 900);
    let b = Ticket::new("C8", 900);
    println!("a: {}", a.describe());
    println!("b: {}", b.describe());
    println!("upgrade a to A1, 6.50 more");
    a.upgrade("A1", 650);
    println!("a: {}", a.describe());
    println!("b: {}", b.describe());
}

In Rust: &self is the explicit receiver, a borrowed ticket. upgrade takes &mut self, so a is declared mut and b is not. Ticket::new is a naming convention, not a keyword.

Recorded with rustc 1.99.0 (edition 2024) on macOS 26 arm64. It runs in your browser with a WebAssembly build on the WASI runner (browser_wasi_shim 0.4.2). To run it yourself: rustc --edition 2024 -O main.rs && ./main

TypeScript 6.0 · main.ts

class Ticket {
  seat: string;
  price: number;

  constructor(seat: string, price: number) {
    // Each new object gets its own seat and price; the price is in cents.
    this.seat = seat;
    this.price = price;
  }

  describe(): string {
    const cents = String(this.price % 100).padStart(2, "0");
    return `seat ${this.seat}, ${Math.trunc(this.price / 100)}.${cents}`;
  }

  upgrade(seat: string, extra: number): void {
    this.seat = seat;
    this.price += extra;
  }
}

const a = new Ticket("C7", 900);
const b = new Ticket("C8", 900);
console.log(`a: ${a.describe()}`);
console.log(`b: ${b.describe()}`);
console.log("upgrade a to A1, 6.50 more");
a.upgrade("A1", 650);
console.log(`a: ${a.describe()}`);
console.log(`b: ${b.describe()}`);

In TypeScript: The fields are declared first, and strict mode checks that the constructor gives each one a value. Parameter properties would do both at once, but they are not erasable syntax.

Recorded with TypeScript 6.0.3 (strict), run by Node.js 24.21.0 on macOS 26 arm64. It runs in your browser with QuickJS (quickjs-emscripten 0.32.0) after Sucrase 3.35.1. To run it yourself: npx -p typescript@6.0.3 tsc --noEmit --strict main.ts && node main.ts

Go 1.27 · main.go

package main

import "fmt"

type Ticket struct {
	seat  string
	price int
}

func NewTicket(seat string, price int) *Ticket {
	// Each new object gets its own seat and price; the price is in cents.
	return &Ticket{seat: seat, price: price}
}

func (t *Ticket) Describe() string {
	return fmt.Sprintf("seat %s, %d.%02d", t.seat, t.price/100, t.price%100)
}

func (t *Ticket) Upgrade(seat string, extra int) {
	t.seat = seat
	t.price += extra
}

func main() {
	a := NewTicket("C7", 900)
	b := NewTicket("C8", 900)
	fmt.Println("a:", a.Describe())
	fmt.Println("b:", b.Describe())
	fmt.Println("upgrade a to A1, 6.50 more")
	a.Upgrade("A1", 650)
	fmt.Println("a:", a.Describe())
	fmt.Println("b:", b.Describe())
}

In Go: The receiver, (t *Ticket), is written before the method name. It is a pointer, so Upgrade changes the caller's ticket. NewTicket is Go's convention for a constructor.

Recorded with Go 1.27.1 on macOS 26 arm64. It runs in your browser with the Go compiler in WebAssembly 1.27.1. To run it yourself: go run main.go

Swift 6.4 · main.swift

final class Ticket {
  private(set) var seat: String
  private(set) var price: Int

  init(seat: String, price: Int) {
    // Each new object gets its own seat and price; the price is in cents.
    self.seat = seat
    self.price = price
  }

  func describe() -> String {
    let cents = price % 100
    return "seat \(seat), \(price / 100).\(cents < 10 ? "0" : "")\(cents)"
  }

  func upgrade(to seat: String, extra: Int) {
    self.seat = seat
    price += extra
  }
}

let a = Ticket(seat: "C7", price: 900)
let b = Ticket(seat: "C8", price: 900)
print("a: \(a.describe())")
print("b: \(b.describe())")
print("upgrade a to A1, 6.50 more")
a.upgrade(to: "A1", extra: 650)
print("a: \(a.describe())")
print("b: \(b.describe())")

In Swift: init must give every stored property a value before it returns, and the compiler checks it. Argument labels make the call read upgrade(to:extra:).

Recorded with Apple Swift 6.4 on macOS 26 arm64. It runs in your browser with a WebAssembly build on the WASI runner (browser_wasi_shim 0.4.2). To run it yourself: swiftc -O main.swift -o main && ./main

Ruby 4.0 · main.rb

# frozen_string_literal: true

# A seat and its price, in cents.
class Ticket
  def initialize(seat, price)
    # Each new object gets its own seat and price; the price is in cents.
    @seat = seat
    @price = price
  end

  def describe
    "seat #{@seat}, #{@price / 100}.#{(@price % 100).to_s.rjust(2, '0')}"
  end

  def upgrade(seat, extra)
    @seat = seat
    @price += extra
  end
end

a = Ticket.new('C7', 900)
b = Ticket.new('C8', 900)
puts "a: #{a.describe}"
puts "b: #{b.describe}"
puts 'upgrade a to A1, 6.50 more'
a.upgrade('A1', 650)
puts "a: #{a.describe}"
puts "b: #{b.describe}"

In Ruby: Instance variables such as @seat come into being when initialize assigns them, as Python's attributes do, and every method of the object can read them.

Recorded with CRuby 4.0.7 on macOS 26 arm64. It runs in your browser with ruby.wasm 2.10.1 (Ruby 4.0.0). To run it yourself: ruby main.rb

Kotlin 2.4 · Main.kt

// Each new object gets its own seat and price; the price is in cents.
class Ticket(
    seat: String,
    price: Int,
) {
    var seat = seat
        private set
    var price = price
        private set

    fun describe(): String {
        val cents = (price % 100).toString().padStart(2, '0')
        return "seat $seat, ${price / 100}.$cents"
    }

    fun upgrade(
        seat: String,
        extra: Int,
    ) {
        this.seat = seat
        price += extra
    }
}

fun main() {
    val a = Ticket("C7", 900)
    val b = Ticket("C8", 900)
    println("a: ${a.describe()}")
    println("b: ${b.describe()}")
    println("upgrade a to A1, 6.50 more")
    a.upgrade("A1", 650)
    println("a: ${a.describe()}")
    println("b: ${b.describe()}")
}

In Kotlin: The class header is the primary constructor. Each property starts from a parameter of the same name and keeps a private setter, so this.seat in upgrade means the property.

Recorded with Kotlin 2.4.20 on Java 25.0.4.1 on macOS 26 arm64. It runs in your browser with a WebAssembly build on the WASI runner (browser_wasi_shim 0.4.2). To run it yourself: kotlinc Main.kt -include-runtime -d main.jar && java -jar main.jar

Dart 3.13 · main.dart

class Ticket {
  // Each new object gets its own seat and price; the price is in cents.
  Ticket(this._seat, this._price);

  String _seat;
  int _price;

  String describe() =>
      'seat $_seat, ${_price ~/ 100}.${(_price % 100).toString().padLeft(2, '0')}';

  void upgrade(String seat, int extra) {
    _seat = seat;
    _price += extra;
  }
}

void main() {
  final a = Ticket('C7', 900);
  final b = Ticket('C8', 900);
  print('a: ${a.describe()}');
  print('b: ${b.describe()}');
  print('upgrade a to A1, 6.50 more');
  a.upgrade('A1', 650);
  print('a: ${a.describe()}');
  print('b: ${b.describe()}');
}

In Dart: Ticket(this._seat, this._price) uses initializing formals, which store each argument in the field of the same name before the body runs.

Recorded with Dart 3.13.5 on macOS 26 arm64. It runs in your browser with a WebAssembly build by dart2wasm 3.13.5. To run it yourself: dart run main.dart