Convert values of data class

August 8, 2024

Assuming I have a data class like this

data class PersonDTO(
    val id: Long?,
    val firstName: String,
    val lastName: String,
)

which I then persist in the database on this entity

@Entity
@Table(name = "person")
class Person(
    @Column(name = "first_name", nullable = true)
    var firstName: String? = null,

    @Column(name = "last_name", nullable = true)
    var lastName: String? = null,
) {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    val id: Long = 0
}

like so

personRepository.save(
    Person(
        firstName = personDto.firstName,
        lastName = personDto.lastName
    )
)

Since firstName and lastName are nullable, I want the data class PersonDTO to behave the following way: If firstName is of length === 0, return null for firstName (and the same for lastName).

Is there a way to achieve this?

>Solution :

I would just add new properties to the data class:

data class PersonDTO(
    val id: Long?,
    val firstName: String,
    val lastName: String,
) {
    val nonEmptyFirstName: String?
        get() = firstName.takeUnless { it.isEmpty() }
    val nonEmptyLastName: String?
        get() = lastName.takeUnless { it.isEmpty() }
}

personRepository.save(
    Person(
        firstName = personDto.nonEmptyFirstName,
        lastName = personDto.nonEmptyLastName
    )
)

Or write a function to convert to a Person directly.

fun PersonDTO.toPerson() = Person(
    firstName = firstName.takeUnless { it.isEmpty() },
    lastName = lastName.takeUnless { it.isEmpty() }
)

If this doesn’t have to be a data class, you can achieve this without using any extra properties. The properties firstName and lastName are nullable, but the constructor does not accept null.

class PersonDTO(
    val id: Long?,
    firstName: String,
    lastName: String,
) {
    val firstName: String? = firstName
        get() = field?.takeUnless { it.isEmpty() }
    val lastName: String? = lastName
        get() = field?.takeUnless { it.isEmpty() }
}