Question

I’ve an unexpected trouble calling put on an old-school hashtable. What’s going on here?

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scala> import com.ibm.mq._                 
import com.ibm.mq._

scala> MQEnvironment.properties                                                    
res1: java.util.Hashtable[_, _] = {}

scala> res1.put("transport", "MQSeries")
<console>:10: error: type mismatch;
 found   : java.lang.String("transport")
 required: ?0 where type ?0
       res1.put("transport", "MQSeries")
            ^

PS, the question still stands as is, but I have a nasty workaround:

scala> new java.util.Hashtable[String, String]
res10: java.util.Hashtable[String,String] = {}

scala> res10.put("transport", "MQSeries")      
res11: String = null

scala> MQEnvironment.properties = res10

scala> MQEnvironment.properties        
res13: java.util.Hashtable[_, _] = {transport=MQSeries}

Answer 1

That properties interface appears to be one of those old-school APIs that pre-date Java generics. Those underscores in java.util.HashTable[_, _] are shorthands for existential types, the first of which (the key type) corresponds to the ?0 appearing in the diagnostic. These old Java “raw” types are an unfortunate, visible seam in Scala’s Java interoperability, albeit one that usually shows up only in very old APIs.