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Constructors, init, and properties

Primary constructor​

class User(val id: Long, var name: String)

id and name are constructor parameters and properties because they use val/var. id cannot be reassigned; name can.

init: run validation during construction​

class Percentage(val value: Int) {
    init {
        require(value in 0..100) { "Percentage must be 0..100" }
    }
}

Order of construction follows source order: property initializer, init block, next property initializer, next init block.

Open in Kotlin Playground
Expected: lines 1 through 4 in source order.

Secondary constructor​

class User(val id: Long, val name: String) {
    constructor(id: Long) : this(id, "Unknown")
}

Every secondary constructor must eventually delegate to the primary constructor. Prefer default arguments or named factory functions when they explain intent better.

Custom getter and setter​

class Temperature(celsius: Double) {
    var celsius: Double = celsius
        set(value) {
            require(value >= -273.15)
            field = value
        }

    val fahrenheit: Double
        get() = celsius * 9 / 5 + 32
}

field refers to the property's backing field. Calling celsius = value inside its setter would call the setter again forever.

Open in Kotlin Playground
Expected: 32.0, followed by 212.0.

lateinit and lazy​

  • lateinit var promises a non-null property will be assigned before reading. Reading too early throws UninitializedPropertyAccessException.
  • val by lazy { ... } computes once on first access and then stores the result.

✕ Avoid lifecycle-unsafe lateinit access

lateinit var token: String
println(token) // crashes if assignment has not happened

✓ Prefer constructor injection when the value is required

class ApiClient(private val token: String)
Interview-ready answer

The primary constructor defines the main creation path. Property initializers and init blocks execute in source order. Secondary constructors delegate to the primary constructor. Use lateinit only when lifecycle-driven delayed assignment is unavoidable; use lazy for one-time computation on first access.