Kotlin data class copy function not working

Question

Maybe I'm misinterpreting how the copy function of a data class works or maybe there's a bug, but the following is example of the copy function not working as expected:

Kotlin:

data class A {
    public var x: String? = null
    public var y: String? = null
    public var z: B = B.ONE
}

enum class B {
    ONE
    TWO
    THREE
}

Java

A a1 = new A()
a1.setX("Hello")
a1.setY("World")
a1.setZ(B.TWO)

A a2 = a1.copy()
// a2.x is null
// a2.y is null
// a2.z is B.ONE

It seems that copy is just making a new instance of A and not copying the values. If I put the variables in the constructor, the values are assigned, but then it's no different than constructing a new instance.

Answer 1

Okay, I missed this sentence in the docs:

If any of these functions is explicitly defined in the class body or inherited from the base types, it will not be generated.

Which, infact, makes copy no better than a constructor for Java interop.

Answer 2

What you can do to get around the limitations of Kotlin's copy(), is to create your own copy function inside your data class. Example below:

data class User(val name : String, val property: String) {

    fun copy() : User {
      //uses the fields name and property defined in the constructor
      return User(name,property)
    }

    //or if you need a copy with a changed field
    fun copy(changedProperty : String) : User {
      return User(name, changedProperty)
    }

}

Answer 3

For interop with java you can make function that use kotlin generated .copy

@Entity
data class User(@PrimaryKey var id: Int = 0,
            var firstName: String? = null,
            var lastName: String? = null,
            var phone: String? = null,
            var email: String? = null,
            var phoneCode: String? = null,
            var tokenId: String? = null,
            var provider: SocialProvider? = null) : Serializable {


var countryCodeIso: String? = null
    set(countryCodeIso) {
        if (countryCodeIso != null) {
            field = countryCodeIso.toLowerCase()
        }
    }

fun javaCopy(): User {
    val user = copy()
    user.countryCodeIso = countryCodeIso
    return user
}}