import java.util.*

class Animal {
    var age = 0
        get() = field
        set(value) {
            if(value >= 0) {
                field = value
            } else {
                throw Error("bad number")
            }
        }
}


fun main() {
    val animal = Animal()
    animal.age = 8
    println(animal.age) // 8
    
    animal.age = -2 // Error: bad number
}