Scala Buffer: Size or Length?

Question

I am using a mutable Buffer and need to find out how many elements it has.

Both size and length methods are defined, inherited

Answer 1

As of Scala-2.11, these methods may have different performance. For example, consider this code:

val bigArray = Array.fill(1000000)(0)
val beginTime = System.nanoTime()
var i = 0
while (i < 2000000000) {
  i += 1
  bigArray.length
}
val endTime = System.nanoTime()
println(endTime - beginTime)
sys.exit(-1)

Running this on my amd64 computer gives about 2423834 nanos time (varies from time to time).

Now, if I change the length method to size, it will become about 70764719 nanos time.

This is more than 20x slower.

Why does it happen? I didn't dig it through, I don't know. But there are scenarios where length and size perform drastically different.