strange ArrayBuffer behavior

Question

Can someone please explain to me why the padTo method of ArrayBuffer doesn't work as I would expect it to? In this example, I would expect the array created by toArray to have a length of 10.

scala> val b = new scala.collection.mutable.ArrayBuffer[Byte]
b: scala.collection.mutable.ArrayBuffer[Byte] = ArrayBuffer()

scala> b.append(2)

scala> b
res1: scala.collection.mutable.ArrayBuffer[Byte] = ArrayBuffer(2)

scala> b.append(2)

scala> b
res3: scala.collection.mutable.ArrayBuffer[Byte] = ArrayBuffer(2, 2)

scala> b.padTo(10,0)
res4: scala.collection.mutable.ArrayBuffer[AnyVal] = ArrayBuffer(2, 2, 0, 0, 0, 0, 0, 0, 0, 0)

scala> b.toArray
res5: Array[Byte] = Array(2, 2)

Answer 1

Because padTo returns a new sequence (it doesn't mutate the existing one):

Try

var c = b.padTo(10,0)

c.toArray

See also: https://issues.scala-lang.org/browse/SI-2257

Answer 2

From scaladoc:

returns: a new collection of type That consisting of all elements of this arraybuffer followed by the minimal number of occurrences of elem so that the resulting collection has a length of at least len.

So, b, even mutable, does not changes.

Answer 3

If you look at the documentation, you'll see the difference:

def append (elems: A*): Unit

Use case (append): Appends the given elements to this buffer.

def padTo (len: Int, elem: A): ArrayBuffer[A]

Use case (padTo): Appends an element value to this arraybuffer until a given target length is reached.

Append returns Unit, while padTo returns new ArrayBuffer.