I have a Kotlin/Spring Boot application in which I...

# server

[Nolan]

I have a Kotlin/Spring Boot application in which I am doing a simple string comparison against a string literal within a list filter. Initially I used === , and all of my unit tests passed. However, once deployed, the filter function did not work as expected. I wrote an simple JVM app to see if it was an issue with JVM specifically, but in that case "str" === "str" resolved to true, which is not the behavior I was seeing in my Spring Boot app. Any ideas why I'd be seeing this discrepancy in the Spring Boot app?

[Dariusz Kuc]

see https://kotlinlang.org/docs/equality.html

[Nolan]

Thank you, I have read that documentation, which I interpreted to mean that it === "str" would always be false, which is what I was seeing in my Spring Boot app. However, this is not the behavior I was seeing in my unit tests or in the simple JVM console app.

My simple JVM console app:

class StringListWrapper(val stringList: List<String>) {
    fun goodWordsOnly() = stringList.filter { "GOOD" === it }
}

fun main() {
    val stringWrapper = StringListWrapper(listOf("BAD", "BADDER", "BADDEST", "GOOD"))
    println("${stringWrapper.stringList.joinToString(",")} is unfiltered")
    println("${stringWrapper.goodWordsOnly().joinToString(",")} is filtered")
}

Output:

BAD,BADDER,BADDEST,GOOD is unfiltered
GOOD is filtered

[Dariusz Kuc]

=== is referential integrity check meaning that both references point to the same object in memory

[Dariusz Kuc]

you most likely want to use == which is structural integrity (i.e. whether object contents are equal) -> note: in Java == is a referential integrity check vs .equals using structural integrity check

[Dariusz Kuc]

google JVM String pools to see whats happening with the String equality

val x = "foo"
val y = "foo"

println(x == y)
println(x === y)

val z = String("foo".toByteArray())
println(x == z)
println(x === z)

[Nolan]

i didn't even think to do the equivalent in Java. i replicated it there. it's a Java thing entirely. the strings are being interned in the test run in a way they are not when coming in from an http request. that's ... annoying. 🙂

[molikuner]

Here is an explanation on why this is happening 😅: https://www.geeksforgeeks.org/string-constant-pool-in-java/

[CLOVIS]

Basically: just use == on strings. Always. If they point to the same reference, it will shortcut and be 0(1). If they don't point to the same reference, it will check the contents, which is O(n).