ArrayList and ArrayList Positional Access Efficiency

Question

I just read this sample code: http://robaustin.wikidot.com/how-does-the-performance-of-arraylist-compare-to-array

which causes j = INT_ARRAY [i]; be three times faster than j = ARRAY_LIST.get (i)

I know that ArrayList internally uses an array. Therefore, I would like to know in detail what additional operations are added this time (calling methods, casting, other JVM considerations, etc.)

Thanks in advance.

Answer 1

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Answer 2

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object ArraySpeed {
  def ptime[A](f: => A) = {
    val t0 = System.nanoTime
    val ans = f
    printf("Elapsed: %.3f seconds\n",(System.nanoTime-t0)*1e-9)
    ans
  }

  val a = Array.range(0,1000000).map(x => new java.lang.Integer(x))
  val b = new java.util.ArrayList[java.lang.Integer]
  a.foreach(x => b.add(x))

  var j = 0

  def jfroma = {
    var i=0
    while (i<1000000) {
      j += a(i).intValue
      i += 1
    }
    j
  }

  def jfromb = {
    var i=0
    while (i<1000000) {
      j += b.get(i).intValue
      i += 1
    }
    j
  }

  def main(args: Array[String]) {
    for (i <- 1 to 5) {
      ptime(for (j <- 1 to 100) yield jfroma)
      ptime(for (j <- 1 to 100) yield jfromb)
      println
    }
  }
}

:

$ scalac ArraySpeed.scala
$ scala ArraySpeed
Elapsed: 0.324 seconds   // This is direct array access
Elapsed: 0.378 seconds   // This is ArrayList

Elapsed: 0.326 seconds
Elapsed: 0.389 seconds

Elapsed: 0.355 seconds
Elapsed: 0.349 seconds

Elapsed: 0.323 seconds
Elapsed: 0.333 seconds

Elapsed: 0.318 seconds
Elapsed: 0.331 seconds

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Answer 3

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readFromArrayList:

   6:   goto    25
   9:   getstatic       #47; //Field ARRAY_LIST:Ljava/util/List;
   12:  iload_3
   13:  invokeinterface #116,  2; //InterfaceMethod java/util/List.get:(I)Ljava/lang/Object;
   18:  checkcast       #17; //class java/lang/Integer
   21:  astore_0
   22:  iinc    3, 1
   25:  iload_3
   26:  getstatic       #25; //Field INT_ARRAY:[Ljava/lang/Integer;
   29:  arraylength
   30:  if_icmplt       9

readFromArray:

   6:   goto    18
   9:   getstatic       #25; //Field INT_ARRAY:[Ljava/lang/Integer;
   12:  iload_3
   13:  aaload
   14:  astore_0
   15:  iinc    3, 1
   18:  iload_3
   19:  getstatic       #25; //Field INT_ARRAY:[Ljava/lang/Integer;
   22:  arraylength
   23:  if_icmplt       9

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Answer 4

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Answer 5

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Answer 6

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 public void ensureCapacity(int minCapacity) {
    modCount++;
    int oldCapacity = elementData.length;
    if (minCapacity > oldCapacity) {
        Object oldData[] = elementData;
        int newCapacity = (oldCapacity * 3)/2 + 1;
            if (newCapacity < minCapacity)
        newCapacity = minCapacity;
            // minCapacity is usually close to size, so this is a win:
            elementData = Arrays.copyOf(elementData, newCapacity);
    }
    }

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