scala-for-the-impatient | Exercises of Scala for the Impatient | Build Tool library

defs are called when asked for, but vals are kept in memory, so in this case the val version is still set to zero because it hasn’t been initialised, where the def version is called and therefore can never give a misleading value.

It’s only because the val here is overloaded that it isn’t initialised in time, because classes must be initialised starting at the top of the inheritance hierarchy and building down. If env were a def you’d be fine also, because it wouldn’t be created until called, by which time the vals would all be initialised. Of course this way you’d get different lists every time you called env so you might prefer to use lazy val, which is initialised the first time it’s called, but then remains the same and is kept in memory.