Kotlin multiple variables Null checker method implementation - kotlin

Question

I have multiple variables that can be nullable and i need to check them ( Strings and Dates ) .
I need a method where i pass it X number of variables and it returns me a list of the variables that are null.
I was thinking something that i can call like this :
internal fun checkNullVariables ( var x, var y , ..... ) : MutableList<String>{
// yada yada
return listOfNamesOfNullVariables
}

Answer 1

This definitely requires reflection, since you want parameter names. You need to add reflection as a dependency as explained in the documentation to use the below code.
private fun listNullProperties (vararg props: KProperty0<Any?>) : List<String> {
val list = mutableListOf<String>()
for (prop in props)
if (param.get() == null)
list.add(param.name)
return list
}
Usage:
val nullPropertiesByName = listNullParameters(
::myProperty,
::myOtherProperty,
::myDateProperty
)
println(nullPropertiesByName.joinToString())

Answer 2

If it is just about logging, you could:
fun <T> T?.logNull(name: String) {
when(this) {
null -> //log '$name' was null
else -> //do nothing
}
}
and call it like
var a: String? = null
a.logNull("my a variable") // "'my a variable' was null"

Answer 3

I still recommend the Map-approach. You may want to use properties stored in a map to overcome the use of reflection.
Here is an example using a type with 2 dates and 2 strings, both having a nullable and a non-null variant:
class YourData(internal val backedMap : Map<String, Any?>) {
val beginDate : Date by backedMap
val endDate : Date? by backedMap
val maybeString : String? by backedMap
val string : String by backedMap
constructor(beginDate : Date, string : String, endDate: Date? = null, maybeString : String? = null) : this(mapOf(
"beginDate" to beginDate,
"endDate" to endDate,
"maybeString" to maybeString,
"string" to string
))
}
While it may seem more complicated having that additional constructor in place, it just helps to easily create new objects the way you are most comfortable with.
Now you can either supply that function I placed in the comment or any variant of it. I now use an extension function for YourData instead:
fun YourData.getKeysWithNullValues() = backedMap.filterValues { it == null }.keys
Usage then may look as follows:
YourData(Date(), "test string")
.getKeysWithNullValues()
.forEach(::println)
which for this example would print:
endDate
maybeString