Scala - Vals and Defs - Decompiled

Question

If, in Scala, this val foo: Int = 0 gets decompiled to this:

private[this] val foo_: Int = 0
def foo: Int = foo_

Then how can defs evaluate each time they are called but vals are evaluated only once? Since the val is decompiled to a def anyway?

Answer 1

For a val, there is a private variable ("val"), and a getter method ("def"). The variable is initialized with a value, which is computed once.

For a def, there is no variable to "cache" the value, so it it is computed every time.

 class Foo {
    val bar = { println("BAR"); "bar" }
 }

Is roughly equivalent to

 class Foo {
     private var bar_ = { println("BAR"); "bar" }
     def bar = bar_
 }

The block initializing bar_ is only executed once, when the instance of the class is being created. So, something like

 val f = new Foo
 println("Created")
 f.bar
 f.bar
 f.bar

Will print out "BAR", then "Created", and nothing else.

But if you define Foo as

 class Foo {
    def bar = { println("BAR"); "bar" }
 }

Then the block is evaluated every time you access bar, and the code above, will first print "Created", and then "BAR" three times.