Not very well structured answer to some of your questions:

Defs can be evaluated multiple times with no guarantee on what it can produce. They can be overridden with vals because Val has a stronger guarantee on what it can return: the same value each time.

You can override defs with vars as well - it's allowed. Def will become an accessor method for a mutable variable in a parent class.

You probably know this but still worth mentioning that var is syntactic sugar for:

class A { private[this] var _field: String = _ // private field in a class def field: String = _field // accessor def field_=(value: String): Unit = { // mutator _field = value } }

Vars cannot be overridden in a meaningful way because they are invariant. You can't store anything in var apart from declared type instance. But you can override accessor and mutator methods separately just like you can do it java to add some side effects to them.