Create a HashMap in Scala from a list of objects without looping

Question

I have a List of objects, each object with two fields of interest which I'll call "key" and "value". From that I need to build a HashMap made up of entries where "key" maps to "value".

I know it can be done by looping through the list and calling hmap.put(obj.key, obj.value) for every item in the list. But somehow it "smells" like this can be done in one simple line of code using map or flatMap or some other mix of Scala's List operations, with a functional construct in there. Did I "smell" right, and how would it be done?

Solution 1

list.map(i => i.key -> i.value).toMap

Solution 2

Also:

Map(list map (i => i.key -> i.value): _*)

Solution 3

To create from a collection (remember NOT to have a new keyword)

val result: HashMap[Int, Int] = HashMap(myCollection: _*)

Comments

[Luigi Plinge]

I suppose the advantage of this is that you can specify the type of Map you need, so it's more general pattern

[Daniel C. Sobral]

@LuigiPlinge That is true. I much prefer using .toMap, as it is much more readable. If performance is of great concern, maybe use breakOut.

[Gigatron]

I'm lost with that last part : _* I get that the underscore is a placeholder for the functional variable, but I can't see how it fits in to that expression, nor the role of the asterisk and colon in that context.

[Gigatron]

OK, I got it now. Map.apply with varargs.

[samthebest]

OP wants a HashMap not a Map

[Richard Gomes]

Warning: It does not support duplicated keys!

[Richard Gomes]

Note: It does not support duplicated keys!