Custom Getters / Setters

// Custom getter
val propertyName: Type
    get() = expression

// Custom setter (requires var)
var propertyName: Type = initialValue
    get() = field
    set(value) {
        field = value
    }

class Circle(val radius: Double) {
    // Custom getter: calculates and returns the area.
    val area: Double
        get() = Math.PI * radius * radius
}

class Person(name: String) {
    // Validates the value in the setter.
    var name: String = name
        set(value) {
            // Rejects empty strings.
            field = if (value.isNotBlank()) value else field
        }

    // Custom getter: returns the name converted to uppercase.
    val upperName: String
        get() = name.uppercase()
}

fun main() {
    val circle = Circle(5.0)
    println(circle.area) // 78.53...

    val person = Person("Tanaka")
    println(person.upperName) // TANAKA (converted to uppercase)
    person.name = ""           // Empty string is ignored.
    println(person.name)       // Tanaka (unchanged)
    person.name = "Suzuki"
    println(person.name)       // Suzuki
}

A custom getter lets you compute and return a property's value on every access. Since it requires no backing field, it is useful for expressing values derived from other properties.

In a custom setter, you can validate or transform the new value received via the value parameter before assigning it to field. Never assign directly using the property name inside a setter, as that causes infinite recursion — always use field instead.