Kotlin data class copy method not deep copying all members

Question

Could someone explain how exactly the copy method for Kotlin data classes work? It seems like for some members, a (deep) copy is not actually created and the references are still to the original.

fun test() {
    val bar = Bar(0)
    val foo = Foo(5, bar, mutableListOf(1, 2, 3))
    println("foo    : $foo")

    val barCopy = bar.copy()
    val fooCopy = foo.copy()
    foo.a = 10
    bar.x = 2
    foo.list.add(4)

    println("foo    : $foo")
    println("fooCopy: $fooCopy")
    println("barCopy: $barCopy")
}

data class Foo(var a: Int,
               val bar: Bar,
               val list: MutableList<Int> = mutableListOf())

data class Bar(var x: Int = 0)

Output:
foo : Foo(a=5, bar=Bar(x=0), list=[1, 2, 3])
foo : Foo(a=10, bar=Bar(x=2), list=[1, 2, 3, 4])
fooCopy: Foo(a=5, bar=Bar(x=2), list=[1, 2, 3, 4])
barCopy: Bar(x=0)

Why is barCopy.x=0 (expected), but fooCopy.bar.x=2 (I would think it would be 0). Since Bar is also a data class, I would expect foo.bar to also be a copy when foo.copy() is executed.

To deep copy all members, I can do something like this:

val fooCopy = foo.copy(bar = foo.bar.copy(), list = foo.list.toMutableList())

fooCopy: Foo(a=5, bar=Bar(x=0), list=[1, 2, 3])

But am I missing something or is there a better way to do this without needing to specify that these members need to force a deep copy?

Answer 1

The copy method of Kotlin is not supposed to be a deep copy at all. As explained in the reference doc (https://kotlinlang.org/docs/reference/data-classes.html), for a class such as:

data class User(val name: String = "", val age: Int = 0)

the copy implementation would be:

fun copy(name: String = this.name, age: Int = this.age) = User(name, age)

So as you can see, it's a shallow copy. The implementations of copy in your specific cases would be:

fun copy(a: Int = this.a, bar: Bar = this.bar, list: MutableList<Int> = this.list) = Foo(a, bar, list)

fun copy(x: Int = this.x) = Bar(x)