Why does the Elvis operator/null coalescing operator work this way in Kotlin?

Question

I have the following code:

   companion object {
        private const val DEFAULT_LANGUAGE_CODE = "en-us"
    }

   val currentLanguageCode: String
   get() {
        return selectedLanguage?.code ?: configManager.get().agency?.language ?: DEFAULT_LANGUAGE_CODE
    }

Which returns null to the caller accessing currentLanguageCode..
Note, selectedLanguage is null and configManager.get().agency is null as well at that point. The following code works correctly though:

    val currentLanguageCode: String
    get() {
        val selectedCode = selectedLanguage?.code
        val configCode = configManager.get().agency?.language
        return selectedCode ?: configCode ?: DEFAULT_LANGUAGE_CODE
    }

I end up getting the DEFAULT_LANGUAGE_CODE "en-us" returned to the caller when accessing currentLanguageCode in the exact same scenario otherwise.

Any ideas why this is?

Using Kotlin 1.8.0

I checked converting this to bytecode -> Java to see if I could spot the problem but I think should have worked based on what I got:

  //working version:
   public final String getCurrentLanguageCode() {
      CommunityLanguage var10000 = this.selectedLanguage;
      String selectedCode = var10000 != null ? var10000.getCode() : null;
      ConfigAgency var3 = ((CaptureConfig)this.configManager.get()).getAgency();
      String configCode = var3 != null ? var3.getLanguage() : null;
      String var4 = selectedCode;
      if (selectedCode == null) {
         var4 = configCode;
      }

      if (var4 == null) {
         var4 = "en-us";
      }

      return var4;
   }


   //broken version which returns null
   public final String getCurrentLanguageCode() {
      String var1;
      label18: {
         CommunityLanguage var10000 = this.selectedLanguage;
         if (var10000 != null) {
            var1 = var10000.getCode();
            if (var1 != null) {
               break label18;
            }
         }

         ConfigAgency var2 = ((CaptureConfig)this.configManager.get()).getAgency();
         var1 = var2 != null ? var2.getLanguage() : null;
      }

      if (var1 == null) {
         var1 = "en-us";
      }

      return var1;
   }

How can it return null when it's declared as `String` instead of nullable `String?`? Can you create a minimal self-contained reproducible example? — Klitos Kyriacou, Mar 01 '23 at 18:24
I tried but it returns the correct value when I made a little dummy test.. For whatever reason the compiled code seems to have skipped creating some variables or what not. I checked the compiled code (java converted) in Android studio for both variations and they should work (however there are differences between the two). Not sure if that's giving me the same code that gets onto the actual phone though.. — Matt Wolfe, Mar 01 '23 at 18:48
What means are you using to determine it returns null? Is it throwing an NPE the first time you use it? — Tenfour04, Mar 01 '23 at 20:19
Debugger. I had the following: if (selectedLanguage == null) { val code = currentLanguageCode selectedLanguage = CommunityLanguage(code, getFriendlyName(code)) } I had the debugger on the line after code = currentLanguageCode — Matt Wolfe, Mar 01 '23 at 22:26

score 0 · Answer 1 · answered Mar 03 '23 at 00:07

For me, it returns "en-us":

fun main() {
    print(test().currentLanguageCode)
}

class test {
    companion object {
        private const val DEFAULT_LANGUAGE_CODE = "en-us"
    }

    val selectedLanguage: SelectLanguage? = null
    val configManager: ConfigManager = ConfigManager()

    val currentLanguageCode: String
        get() {
            return selectedLanguage?.code ?: configManager.get().agency?.language ?: DEFAULT_LANGUAGE_CODE
        }
}

class SelectLanguage {
    val code: String = ""
}

class ConfigManager {
    fun get(): Foo {
        return Foo()
    }
}

class Foo {
    val agency: Agency? = null
}

class Agency {
    val language: String = ""
}

Why does the Elvis operator/null coalescing operator work this way in Kotlin?

1 Answers1