Hrm, just learning and this is puzzling me: ```val...

# announcements

[Steve Ramage]

Hrm, just learning and this is puzzling me:

val moduleCache : java.util.Map<ModuleInfo, WeakReference<ModuleInfo>> = Collections.synchronizedMap(WeakHashMap<ModuleInfo, WeakReference<ModuleInfo>>())

[TYPE_INFERENCE_EXPECTED_TYPE_MISMATCH] Type inference failed. Expected type mismatch: 
required:
java.util.Map<ModuleInfo, WeakReference<ModuleInfo>>
found:
(MutableMap<ModuleInfo!, WeakReference<ModuleInfo>!>..kotlin.collections.Map<ModuleInfo!, WeakReference<ModuleInfo>!>?)

[Quy D X Nguyen]

Remove java.util from the Map type

[Steve Ramage]

So if I do that, and look at this full object:

object Cache {

    val moduleCache = Collections.synchronizedMap(WeakHashMap<ModuleInfo, WeakReference<ModuleInfo>>())
    val optionCache = Collections.synchronizedMap(WeakHashMap<OptionInfo, WeakReference<OptionInfo>>())

    
    fun <K> intern(map : Map<K, WeakReference<K>>, obj: K) : K {
        val newRef = WeakReference(obj);

        val ref : K? = map.putIfAbsent(obj, newRef)?.get()

        if(ref == null) {
            return obj;
        } else {
            return ref;
        }
    }

    
}

The putIfAbsent method isn't found:

[UNRESOLVED_REFERENCE] Unresolved reference: putIfAbsent

[Steve Ramage]

What's odd is that with the java.util.Map in there, the following code (that is not generized) does work.

fun ModuleInfo.intern() : ModuleInfo {

    val obj = this;

    val newRef = WeakReference(obj);

    val ref : ModuleInfo? = Cache.moduleCache.putIfAbsent(obj, newRef)?.get()

    if(ref == null) {
        return obj;
    } else {
        return ref;
    }
}

[Steve Ramage]

Essentially I'm trying to replace that method implementation with:

fun ModuleInfo.intern() : ModuleInfo {
 Cache.intern(Cache.moduleCache, this);
}

[nkiesel]

putIfAbsent isn't found because you have a Map which in Kotlin is immutable (and thus does not have the methods that modify the map). Try to replace Map with MutableMap

[Steve Ramage]

So I was reading "Kotlin in Action" and maybe it's out of date because it says that "collections in Kotlin are the same classes as in Java" is that something that changed?

val moduleCache : java.util.Map<ModuleInfo, WeakReference<ModuleInfo>> = java.util.Collections.synchronizedMap(java.util.WeakHashMap<ModuleInfo, WeakReference<ModuleInfo>>())

If something more recently changed so it's more like Scala with it's own collection classes then I 'half' understand why the above doesn't work, hoewevr even with every class name fully qualified I don't understand where the kotlin type is leaking in from. IntelliJ also seems to think that every method is the Java version.

[pniederw]

Nothing has changed. Java collection classes are used under the hood, but one typically programs against Kotlin interfaces such as kotlin.collections.(Mutable)Map , which only contain a subset of methods present in the Java classes.

[Steve Ramage]

I guess something is rewriting one of those methods return types, I think Collections.synchronizedMap is not returning the same thing as it would in Java.

[pniederw]

Why would you think that?

[pniederw]

The only magic here is the illusion (offered by kotlinc) that the Java collection classes really implement the Kotlin interfaces.

[Steve Ramage]

Oh I was trying to understand the simplest example of something weird going on.

val map : java.util.Map<String, String> = java.util.HashMap<String, String>()

[Steve Ramage]

Kotlin doesn't even let that compile, and I guess IntelliJ gives me a warning, saying that I shouldn't use java.util.Map.

[Steve Ramage]

Oh I found the section in the book that talks about it, I'm going to re-read it. It's been a few months and I haven't done anything with it.

[Steve Ramage]

Thanks everyone for your help.

[pniederw]

The compile error looks like a bug (or weird limitation) to me. The warning makes sense in that one should typically use the Kotlin rather than Java collection interfaces.

[Steve Ramage]

Yeah I'm still just bootstrapping Kotlin in a project and just need to be able to code and slowly absorb the idioms at his point 🙂