Today I was converting a servlet from a Java implementation to a Scala implementation. I needed to print out the parameters (getParameterMap) and headers (getHeaderNames). These are implemented as java.util.Map collections with the following definition.

java.util.Map<java.lang.String,java.lang.String[]>

Using the implicit conversion of Scala did not work as straight forward as I thought. I started of with the following code.

def makeKeyValuePair(key: String, values: Array[String], length: Int) : String = {
  if (length == 0) s"$key=$values" else s", $key=$values"
}

def parameters(request: HttpServletRequest) : String = {
  var s = ""
  for ((k, v) <- request.getParameterMap()) s += makeKeyValuePair(k, v, s.length())
}

That would give me a not very helpful error message (at least I think the error message is rather cryptic)

type mismatch; found : ?0 where type ?0 required: String

It took me a while, but I finally figured out I needed to explicitly declare the k and v variable types. So this is the final code.

def makeKeyValuePair(key: String, values: Array[String], length: Int) : String {
  if (length == 0) s"$key=$values" else s", $key=$values"
}

def parameters(request: HttpServletRequest) : String = {
  var s = ""
  for ((k: String, v: Array[String]) <- request.getParameterMap()) s += makeKeyValuePair(k, v, s.length())
}

I hope this helps someone else from having to solve the explicit conversion problem.