Kotlin: How to show an instance’s data type (class) in the REPL

If you need to see the data type (or class) of an instance in the Kotlin REPL, you can use the javaClass method to see that type. Here are a few examples:

>>> val map = mapOf(
...     "A" to 1,
...     "B" to 2,
...     "C" to 3
... )
>>> map.javaClass
class java.util.LinkedHashMap

>>> listOf(1,2,3).javaClass
class java.util.Arrays$ArrayList

As shown, mapOf creates a LinkedHashMap, and listOf creates an ArrayList. At the time of this writing (August, 2018) you can’t see those data types in the REPL, so Kotlin’s javaClass method is a nice way of showing them.

More Kotlin javaClass methods

As I just learned, for more complicated problems you may need to go a little deeper. I just ran into a problem where the arrayOf and intArrayOf functions ended up behaving differently with a Kotlin varags parameter, so I had to experiment, like this.

arrayOf:

>>> val x = arrayOf(1,2,3)

>>> println(x.javaClass.name)
[Ljava.lang.Integer;

>>> println(x.javaClass.kotlin)
class kotlin.Array

>>> println(x.javaClass.kotlin.qualifiedName)
kotlin.Array

intArrayOf:


>>> val x = intArrayOf(1,2,3)

>>> println(x.javaClass.name)
[I

>>> println(x.javaClass.kotlin)
class kotlin.IntArray

>>> println(x.javaClass.kotlin.qualifiedName)
kotlin.IntArray

As shown, these approaches give different results — Array vs IntArray — and helped me see the (disappointing) differences between arrayOf and intArrayOf:

println(x.javaClass.name)
println(x.javaClass.kotlin)
println(x.javaClass.kotlin.qualifiedName)

I use the word “disappointing” there because unfortunately the two arrays work differently with a varargs parameter in one of my functions. I assumed they would both have the type Array<Int>, and that obviously isn’t the case.

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