Performance difference between def and val

Question

Consider the below code where I am passing a method and a function as a parameter to map()

  val list1:List[Int]=List(10,20,30)

  def func1(x:Int):Int={
      x+10
  }

  list1.map(func1)
  list1.map(_+10)

I have few questions about ETA expansion:

Is there a performance difference in using a method in place of a function, especially since the method is internally getting converted into a function?
Is there a performance difference between def x:Int=10 and val x:Int=10?
I have read that the call-by-name parameter is actually a method which does not accept any parameter. Now, if methods are not objects, how are we using a method as a parameter value?

score 2 · Accepted Answer · answered Feb 08 '17 at 05:32

2

There is no significant difference between the expressions you're asking about.

val x incurs a private field.

Note that vs.map(_+10) inlines the function, as compared to vs.map(x => f(x)). But you have to create a function object in any case.

A call-by-name argument => X is a () => X under the hood.

From the REPL, use javap to show code. -c for code, -v for verbose.

scala> vs.map(f)
res0: List[Int] = List(2, 3, 4, 5, 6, 7, 8, 9, 10, 11)

scala> :javap -pv -
[snip]

answered Feb 08 '17 at 05:32

som-snytt

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=>X is actually () =>X then how are we invoking the function without () – codingsplash Feb 09 '17 at 06:42
The definition of call-by-name is just that a reference evaluates the expression; it does that by invoking the function. – som-snytt Feb 09 '17 at 06:53

score 0 · Answer 2 · answered Feb 08 '17 at 10:20

One of the differences is, val values are measured when class is loaded whilst def are measured when called.

A simple example is, say you have 100K val variables in the class (for argument's sake), the system might take long time to start. But if you have a def A in which declares 100K val. The performance will be hit only when the A is called.

Performance difference between def and val

2 Answers2