Starting Size for an ArrayList

I want to use an ArrayList (or some other collection) like how I would use a standard array.

Specifically, I want it to start with an intial size (say, SIZE), and be able to set elements explicitly right off the bat,

e.g.

array[4] = "stuff";

could be written

array.set(4, "stuff");

However, the following code throws an IndexOutOfBoundsException:

ArrayList<Object> array = new ArrayList<Object>(SIZE);
array.set(4, "stuff"); //wah wahhh

I know there are a couple of ways to do this, but I was wondering if there was one that people like, or perhaps a better collection to use. Currently, I'm using code like the following:

ArrayList<Object> array = new ArrayList<Object>(SIZE);

for(int i = 0; i < SIZE; i++) {
   array.add(null);
}

array.set(4, "stuff"); //hooray...

The only reason I even ask is because I am doing this in a loop that could potentially run开发者_如何学C a bunch of times (tens of thousands). Given that the ArrayList resizing behavior is "not specified," I'd rather it not waste any time resizing itself, or memory on extra, unused spots in the Array that backs it. This may be a moot point, though, since I will be filling the array (almost always every cell in the array) entirely with calls to array.set(), and will never exceed the capacity?

I'd rather just use a normal array, but my specs are requiring me to use a Collection.

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The initial capacity means how big the array is. It does not mean there are elements there. So size != capacity.

In fact, you can use an array, and then use Arrays.asList(array) to get a collection.

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I recomend a HashMap

HashMap hash = new HasMap();
hash.put(4,"Hi");

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Considering that your main point is memory. Then you could manually do what the Java arraylist do, but it doesn't allow you to resize as much you want. So you can do the following:

1) Create a vector.
2) If the vector is full, create a vector with the old vector size + as much you want.
3) Copy all items from the old vector to your new vector.

This way, you will not waste memory.

Or you can implement a List (not vector) struct. I think Java already has one.

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Yes, hashmap would be a great ideia. Other way, you could just start the array with a big capacity for you purpose.