Classes & Properties

Declaring a class

class Person(val name: String, var age: Int)

val p = Person("Ada", 36)   // no `new` keyword
println(p.name)             // Ada
p.age = 37                  // var property can change

The parameters in parentheses form the primary constructor. Prefixing them with val/var declares them as properties in one line.

Initialization

Run setup code in an init block:

class Account(val owner: String) {
    val createdAt: Long

    init {
        createdAt = System.currentTimeMillis()
        require(owner.isNotBlank()) { "owner is required" }
    }
}

Properties with custom accessors

A property can compute its value or validate on assignment:

class Rectangle(val width: Int, val height: Int) {
    val area: Int
        get() = width * height          // computed, no backing field

    var label: String = ""
        set(value) {                    // custom setter
            field = value.trim()        // `field` is the backing field
        }
}

Secondary constructors

class Point(val x: Int, val y: Int) {
    constructor(value: Int) : this(value, value)   // delegates to primary
}

lateinit and lazy

Defer initialization when you cannot set a value up front:

class Service {
    lateinit var client: HttpClient        // assigned later; non-null var
    val config by lazy { loadConfig() }     // computed once, on first access
}

Use lateinit for var properties initialized after construction; use lazy for val properties you want computed on first use.

Exercises

1. Create a Temperature class with a celsius property and a computed fahrenheit property (c * 9/5 + 32).

2. Write a BankAccount class with a read-only balance, and deposit(amount) / withdraw(amount) methods that reject non-positive or overdrawn amounts.

   Solution

   class BankAccount(initial: Long = 0) {
       var balance: Long = initial
           private set

       fun deposit(amount: Long) {
           require(amount > 0) { "amount must be positive" }
           balance += amount
       }

       fun withdraw(amount: Long) {
           require(amount > 0) { "amount must be positive" }
           require(amount <= balance) { "insufficient funds" }
           balance -= amount
       }
   }
   

</details>

This solution is in examples/core/part3/ and is tested by CI.

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