How does Scala's mutable Map update [map(key) = newValue] syntax work?

Question

I\'m working through Cay Horstmann\'s Scala for the Impatient book where I came across this way of updating a mutable map.

scala> val scores = scala.coll         


        

Answer 1

Can you try this: => to update list of Map

import java.util.concurrent.ConcurrentHashMap
import scala.collection.JavaConverters._
import scala.collection.concurrent

val map: concurrent.Map[String, List[String]] = new ConcurrentHashMap[String, List[String]].asScala

def updateMap(key: String, map: concurrent.Map[String, List[String]], value: String): Unit = {
map.get(key) match {
case Some(list: List[String]) => {
val new_list = value :: list
map.put(key, new_list)
}
case None => map += (key -> List(value))
}
}

Answer 2

The problem is you're trying to update immutable map. I had the same error message when my map was declared as

var m = new java.util.HashMap[String, Int]

But when i replaced the definition by

var m = new scala.collection.mutable.HashMap[String, Int]

the m.update worked.

Answer 3

This is an example of the apply, update syntax.

When you call map("Something") this calls map.apply("Something") which in turn calls get.

When you call map("Something") = "SomethingElse" this calls map.update("Something", "SomethingElse") which in turn calls put.

Take a look at this for a fuller explanation.