ArrayList.ensureCapacity

Have you ever wanted to initialize a java ArrayList to a certain size? You peruse the javadoc and you see what appear to be two different options, one is a constructor option and the other is a chained method.

Here’s the constructor:

ArrayList (int initialCapacity)

Constructs an empty list with the specified initial capacity.

And here’s the method:

void ensureCapacity(int minCapacity)

Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.

And here’s the thing: Neither one changes the logical size of the array. They each change its capacity. That is they change the size it can reach before it has to start copying its values to resize itself. If you do something like the following, you’ll get an IndexOutOfBounds Exception:

ArrayList<String> list = new ArrayList<String>();
list.ensureCapacity(200);
list.add(190, "JoeDog");

Exception in thread "main" java.lang.IndexOutOfBoundsException

If you want to initialize an ArrayList to a particular size, you’ll have to roll your own method:

 private static void ensureSize(ArrayList<?> list, int size) {
   list.ensureCapacity(size);
   while (list.size() < size) {
     list.add(null);
   }
 }

You can call it like this:

ArrayList<Thing> list = new ArrayList<Thing>();
ensureSize(list, 64);

 




It Knows Me Better Than I Know Myself….

robotI write a lot of software with which I interact. If it’s easy for me, then it’s easy for you. I try to keep it easy for me. JoeDog’s Pinochle is the first program against which I’ve competed. It’s been a surreal experience.

The program was designed to be competitive against me. Tonight it took two out of three games. The damn thing knows me inside and out. And why not? I wrote it. And while I can exploit some knowledge of its inter-workings, I can’t predict all its behavior. It was designed to learn bidding from experience.

Bidding is the hardest aspect of this game. The team that wins the bid has an incredible opportunity to earn a lot of points. At the same time, overbids come at a large price. A failure to make the bid means the bid is deducted from your score.

When the game was first released, its bids were implemented programmatically. I like to think I’m a pretty good programmer but that version of the game played like a moran. To improve it, I had the game play itself hundreds of thousands of times. It would store those results and use them to generate future bids.

This implementation has resulted in a much more competitive program. Now it bids more aggressively — much more aggressively. It bids like me which is odd because I didn’t tell it to do that. I told it to learn from its experience and as a result of that experience, its personality morphed into mine.