stringbuffer

scala> val b = new StringBuilder    // empty stringbuffer
b: StringBuilder =

scala> b += 'L'
res22: b.type = L

scala> b += "aci"
<console>:9: error: type mismatch;
 found   : String("aci")
 required: Char
              b += "aci"
                   ^

scala> b ++= "aci"                  // "aci" is a collection of chars, that's why ++= is needed
res24: b.type = Laci

scala> b.toString
res25: String = Laci