User.kt

package builder

class User private constructor(
    val firstName: String,
    val lastName: String,
    val mobileNumber: String,
    val age: Int
) {

    data class Builder(
        private var firstName: String = "",
        private var lastName: String = "",
        private var mobileNumber: String = "",
        private var age: Int = -1
    ) {

        fun firstName(firstName: String) = apply { this.firstName = firstName }
        fun lastName(lastName: String) = apply { this.lastName = lastName }
        fun mobileNumber(mobileNumber: String) = apply { this.mobileNumber = mobileNumber }
        fun age(age: Int) = apply { this.age = age }
        fun build() = User(firstName, lastName, mobileNumber, age)]
    }
}