val-mutable versus var-immutable in Scala

Question

Are there any guidelines in Scala on when to use val with a mutable collection versus using var with an immutable collection? Or should you really aim for val with an immuta

Answer 1

If you work with immutable collections and you need to "modify" them, for example, add elements to them in a loop, then you have to use vars because you need to store the resulting collection somewhere. If you only read from immutable collections, then use vals.

In general, make sure that you don't confuse references and objects. vals are immutable references (constant pointers in C). That is, when you use val x = new MutableFoo(), you'll be able to change the object that x points to, but you won't be able to change to which object x points. The opposite holds if you use var x = new ImmutableFoo(). Picking up my initial advice: if you don't need to change to which object a reference points, use vals.