"+=" will reassign or not?

Question

See the following code:

val names = Set("Mike", "Jack")
names += "Jeff"

There will be an error:

error: reassignment to val 

I see in some books, it said += is actually a method, and the code can be:

val names = Set("Mike", "Jack")
names.+=("Jeff")

If += is a method, why will it assign the "names"?

Answer 1

scala.collection.mutable.Set has += method. So irrespective of val or var, you are just invoking a method on the underlying set. But scala.collection.immutable.Set has no += method but has + method. += has special meaning in Scala; It can be applied like this, names = names + "Jeff" and since this is reassignment to a val 'names', the compiler is reporting the error.

Example (+ is applied and reassignment is done in place of +=)

class Test(num: Int) {
def +(n: Int) = new Test(num + n);
override def toString = num.toString
}

defined class Test

val foo = new Test(5)

foo: Test = 5

foo += 4

error: reassignment to val
       foo += 4
           ^

var bar = new Test(5)

bar: Test = 5

bar += 4

bar

res12: Test = 9

Answer 2

A short answer:

scala> val names = collection.mutable.Set("Mike", "Jack")
names: scala.collection.mutable.Set[java.lang.String] = Set(Jack, Mike)

scala> names += "Jeff"
res23: names.type = Set(Jack, Jeff, Mike)

or you can import the mutable Set like this:

import scala.collection.mutable.Set