07: Classes and Instances
Audience: All (with beginner sidebar) Time: 120 minutes Prerequisites: 01-06 You'll learn: Define classes, instantiate objects, instance methods, fields, initialization
The Big Picture
Classes are blueprints for objects. Instead of a loose collection of variables, a class groups related data and behavior together.
Real world: A car has properties (color, speed) and behaviors (accelerate, brake)
Code: A Car class has fields (color, speed) and methods (accelerate, brake)
Defining Classes
Simple Class
// file: 07_class_basic.zbr
// teaches: class definition // chapter: 07-Classes-and-Instances
class Person var name as str var age as int
def greet print "Hi, I'm ${name}"
class Main shared def main var person = Person() person.name = "Alice" person.age = 30 person.greet() // Hi, I'm Alice
Breakdown: - class Person — Define a class named Person - var name as str — Field (property) of the class - def greet — Method of the class - Person() — Create an instance
Fields and Initialization
// file: 07_init.zbr
// teaches: field initialization // chapter: 07-Classes-and-Instances
class Rectangle var width as int = 0 var height as int = 0
def area as int return width * height
class Main shared def main var rect = Rectangle() rect.width = 10 rect.height = 5 print rect.area() // 50
Instance Methods
// file: 07_methods.zbr
// teaches: instance methods // chapter: 07-Classes-and-Instances
class Counter var count as int = 0
def increment count = count + 1
def decrement count = count - 1
def reset count = 0
def get_count as int return count
class Main shared def main var counter = Counter() counter.increment() counter.increment() counter.increment() print counter.get_count() // 3 counter.reset() print counter.get_count() // 0
Shared Methods (Class Methods)
// file: 07_shared.zbr
// teaches: shared (class) methods // chapter: 07-Classes-and-Instances
class Math shared def abs(x as int) as int if x < 0 return 0 - x return x
def max(a as int, b as int) as int if a > b return a return b
class Main shared def main print Math.abs(-5) // 5 print Math.max(10, 20) // 20
If you're new to programming
> A class is like a template. When you create an instance (with Person()), you're making a specific copy from that template. > > Fields are the properties (like name, age) > > Methods are the behaviors (like greet()) > > Instance methods work on a specific object (person.greet()) > > Shared methods belong to the class itself (Math.abs())
Real World: User Management
// file: 07_user_system.zbr
// teaches: realistic class design // chapter: 07-Classes-and-Instances
class User var username as str = "" var email as str = "" var created_at as str = "" var is_active as bool = true
def is_valid as bool return username.len > 0 and email.contains("@")
def deactivate is_active = false
def display_profile print "User: ${username}" print "Email: ${email}" print "Active: ${is_active}"
class UserManager shared var users as List(User) = List()
def add_user(user as User) as bool if not user.is_valid return false users.add(user) return true
def find_user(username as str) as User? for user in users if user.username == username return user return nil
def user_count as int return users.count()
class Main shared def main var user1 = User() user1.username = "alice" user1.email = "alice@example.com"
if UserManager.add_user(user1) print "User added"
var found = UserManager.find_user("alice") if found != nil found.display_profile()
Common Patterns
Value Object
class Point
var x as int = 0 var y as int = 0
def distance_from_origin as float return 0.0 # sqrt(xx + yy)
Service Class
class EmailService
shared def send(to as str, subject as str, body as str) as bool # Implementation return true
Builder Pattern
class UserBuilder
var username as str = "" var email as str = "" var age as int = 0
def set_username(name as str) username = name
def set_email(addr as str) email = addr
def build as User var user = User() user.username = username user.email = email return user
Common Mistakes
> ❌ Mistake: Forgetting to initialize fields > >
> class Person > var name as str # No default value > var p = Person() > print p.name # ❌ Uninitialized! > > class Person > var name as str = "" # Has default > > ❌ Mistake: Modifying shared fields unintentionally > >
> class Counter > shared var count as int = 0 > > def reset > count = 0 # Affects ALL instances! > > class Counter > var count as int = 0 # Instance field > > def reset > count = 0 # Only this instance > Exercises
Exercise 1: Bank Account
Create a BankAccount class with deposit and withdraw methods:
Solution
class BankAccount
var balance as float = 0.0 var account_number as str = ""
def deposit(amount as float) balance = balance + amount
def withdraw(amount as float) as bool if amount > balance return false balance = balance - amount return true
def get_balance as float return balance
class Main shared def main var account = BankAccount() account.account_number = "1234567890" account.deposit(1000.0) print "Balance: ${account.get_balance()}" account.withdraw(100.0) print "Balance: ${account.get_balance()}"
Exercise 2: Product Catalog
Create a Product class and a simple store:
Solution
class Product
var name as str = "" var price as float = 0.0 var quantity as int = 0
def total_value as float return price * quantity
class Store var products as List(Product) = List()
def add_product(product as Product) products.add(product)
def total_inventory_value as float var total = 0.0 for product in products total = total + product.total_value() return total
class Main shared def main var store = Store()
var apple = Product() apple.name = "Apple" apple.price = 0.50 apple.quantity = 100 store.add_product(apple)
var orange = Product() orange.name = "Orange" orange.price = 0.75 orange.quantity = 80 store.add_product(orange)
print "Total value: ${store.total_inventory_value()}"
Next Steps
- → 08-Interfaces — Contracts for classes - → 09-Inheritance — Class hierarchies - 🏋️ Project-2-HTTP-Server — Use classes extensively
Key Takeaways
- Classes group data and behavior — Fields + methods - Instances are copies — Each Person() is independent - Instance methods work on specific objects - Shared methods belong to the class, not instances - Initialize fields — Don't leave them undefined - Methods should have clear names — deposit() not d()
Next: Head to 08-Interfaces to define contracts between classes.