NoSuchMethodException when retrieve method by reflection

I am trying to get a class method by is name, e.g.:

case class A(i : Int) {def geti = i}
val a = A(123)

val met = a.getClass.getMethod("geti")

This seems to work, but if I try to do the same on a Double:

val a:Double = 3.0
a.getClass.getMethod("toString")

I run into this error:

java.lang.NoSuchMethodException: double.toString()
  at java.lang.Class.getMethod(Class.java:1786)

Could you help me figuring out what I am missing?