User.kt

class User private constructor(
    val firstName: String,
    val middleName: String?, // optional
    val lastName: String,
    val address: Address,
    val contact: Contact?, // optional
    val company: Company?, // optional
    val educations: List<Education>,
)  {

    class Builder {
        var firstName: String? = null
        var middleName: String? = null // optional
        var lastName: String? = null
        var address: Address? = null
        var contact: Contact? = null // optional
        var company: Company? = null // optional
        var educations: List<Education>? = null

        fun setFirstName(firstName: String): Builder {
            this.firstName = firstName
            return this
        }

        // OR

        fun setMiddleName(middleName: String) = apply {
            this.middleName = middleName
        }

        fun setLastName(lastName: String) = apply {
            this.lastName = lastName
        }

        fun setAddress(address: Address) = apply {
            this.address = address
        }

        fun setContact(contact: Contact) = apply {
            this.contact = contact
        }

        fun setCompany(company: Company) = apply {
            this.company = company
        }

        fun setEducations(educations: List<Education>) = apply {
            this.educations = educations
        }

        fun build(): User {
            return User(
                firstName!!,
                middleName,
                lastName!!,
                address!!,
                contact,
                company,
                educations!!
            )
        }
    }
}