Understanding ArrayBuffer in Scala

Question

I'm trying to undersatend how removing elements from ArrayBuffer works. Here is it:

  override def remove(n: Int, count: Int) {
    if (count < 0) throw new IllegalArgumentException("removing negative number of elements: " + count.toString)
    else if (count == 0) return  // Did nothing
    if (n < 0 || n > size0 - count) throw new IndexOutOfBoundsException("at " + n.toString + " deleting " + count.toString)
    copy(n + count, n, size0 - (n + count))
    reduceToSize(size0 - count)
  }

The thing is copy is implemented as follows:

protected def copy(m: Int, n: Int, len: Int) {
  scala.compat.Platform.arraycopy(array, m, array, n, len)
}

It means it just copies the content of the new array to the same array without resizing it. In contrast, ArrayList in JDK resizes array as long as we delete elements from it.

Where is my understanding wrong?

aristotll · Accepted Answer · 2017-07-31 09:44:53Z

1

The reduceToSize method reduce the size of array I think.

  def reduceToSize(sz: Int) {
    require(sz <= size0)
    while (size0 > sz) {
      size0 -= 1
      array(size0) = null
    }
  }

answered Jul 31, 2017 at 9:44

aristotll

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2 Comments

St.Antario Over a year ago

Not quite clear. Is it exactly the same if we set null as recreating array with a new size?

aristotll Over a year ago

@St.Antario Recreating array is much more expensive, so just set the element to null and make GC can do its work is more appropriate. We don't really care about the real size of the inner array, but the value of private field size0.

chengpohi · Accepted Answer · 2017-07-31 10:19:04Z

1

For the Java ArrayList it also not shrink the data array, just set the null for the removed elements for GC them. Scala ArrayBuffer has done nearly same thing with ArrayList

public E remove(int index) {
    rangeCheck(index);

    modCount++;
    E oldValue = elementData(index);

    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; // clear to let GC do its work

    return oldValue;
}

edited Jul 31, 2017 at 10:19

answered Jul 31, 2017 at 9:59

chengpohi

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2 Comments

St.Antario Over a year ago

So it means we actually have an array of the same size, but with the elements set to null, right?

chengpohi Over a year ago

Yes, maybe this post is helpful for you: stackoverflow.com/questions/41933700/…

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