Question

What are the implications of using def vs. val in Scala to define a constant, immutable value? I obviously can write the following:

val x = 3;
def y = 4;
var a = x + y; // 7

What’s the difference between those two statements? Which one performs better / is the recommended way / more idiomatic? When would I use one over the other?

Answer 1

Assuming these are class-level declarations:

The compiler will make a val final, which can lead to better-optimised code by the VM.

A def won’t store the value in the object instance, so will save memory, but requires the method to be evaluated each time.

For the best of both worlds, make a companion object and declare constants as vals there.

i.e. instead of

class Foo {
  val MyConstant = 42
}

this:

class Foo {}

object Foo {
  val MyConstant = 42
}