Kotlin Class Functions

Defining Member Functions

A member function is declared with the fun keyword inside a class body, just like a top-level function, except it has automatic access to the class's properties. Member functions describe what an object can do.

class Circle(val radius: Double) {
    fun area(): Double {
        return Math.PI * radius * radius
    }
}

fun main() {
    val circle = Circle(3.0)
    println("Area: ${circle.area()}")
}

Using 'this' Inside a Class

Inside a member function, the keyword this refers to the current object instance. It's mostly optional in Kotlin because the compiler resolves property names automatically, but it becomes necessary when a parameter name shadows a property name.

class Wallet(var balance: Double) {
    fun setBalance(balance: Double) {
        this.balance = balance
    }
}

fun main() {
    val wallet = Wallet(10.0)
    wallet.setBalance(250.0)
    println(wallet.balance)
}

Functions That Mutate State

Member functions often update an object's own var properties rather than returning a brand-new value. This is a core OOP idea: behavior and the state it modifies live in the same place.

class ShoppingCart {
    var itemCount: Int = 0
    var total: Double = 0.0

    fun addItem(price: Double) {
        itemCount += 1
        total += price
    }
}

fun main() {
    val cart = ShoppingCart()
    cart.addItem(12.5)
    cart.addItem(7.25)
    println("${cart.itemCount} items, total: ${cart.total}")
}

• Member functions are declared with fun inside a class body
• They automatically see the class's properties without any extra passing
• this refers to the current instance and resolves naming conflicts
• Functions can return computed values or mutate the object's own state

Understanding member functions sets up the final core concept: inheritance, where classes can share and extend behavior.