Scala: Appending to the end of an array which is a value in a HashMap

Question

var temp = 5
val map = new HashMap[String,Array[Integer]]()
if(map contains str1){
   map(str1) = map(str1) :+ temp
}else{
   map(str1) = Array(temp)
}

Basically if the key is not in the map, I want to set the value to a new array with the value temp and if the key is already in the map I want to append temp to the end of the existing array. temp is an Integer.

I'm getting the following error when I execute the code above:

helloworld.scala:21: error: type mismatch;
 found   : Array[Any]
 required: Array[Integer]
Note: Any >: Integer, but class Array is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: Integer`. (SLS 3.2.10)

                        map(str1) = (map(str1) :+ temp)
                                               ^

I am new scala, so I'm completely lost as to why this is happening. Thanks for any help.

Answer 1

There are two problems with this code. I'm assuming you're using a mutable HashMap first of all, because you'd otherwise not be able to update it.

First, you're getting a compile error because you've declared a HashMap[String, Array[Integer]], but you're trying to append an Int to the Array[Integer]. They are not the same thing. Therefore the compiler infers the type as Any, which isn't allowed because the type parameter of Array is invariant.

So change your declaration to val map = new HashMap[String,Array[Int]](). There's isn't much of a reason to use Integer in scala. And if you really need to then use val temp = new Integer(5), instead.

The second problem that you haven't been able to see yet from the first compiler is that this won't work:

map(str1) = Array(temp)

If the str1 key does not exist on the HashMap, then calling map(str1) will throw an exception, so a new key cannot be assigned this way. Instead, you have to add the key-value pair to the map like this:

map += (str1 -> Array(temp))