# ArrayIndexOutOfBoundsException

• 02-01-2010, 04:10 AM
Corey
ArrayIndexOutOfBoundsException
Hey, I'm new to java and I'm working on a simple program to try and run a simulated card game. I'm trying to construct a deck of 52 cards that will be in random order. Cards have been defined and list is an array of 52 cards, in order. For some reason I am getting an error on my randomizer method, even though I've double checked to ensure the indices are valid yet i'm still getting the ArrayIndexOutOfBoundsException thrown at me. Perhaps i've over looked something, here is the code that contains the error.

private Card[] randomizer()
{
int count = 1;
int num;
int index = 1;
boolean located;
Card[] randomDeck = new Card[52];
int nums[] = new int[52];
for (int x = 1; x <= 52; x++)
nums[x - 1] = x;
while (count <= 52)
{
index = 1;
located = false;
num = (int)(Math.random()*52+1);
while (index < 52 || located == false)
{
if (nums[index - 1] == num) //Error: ArrayIndexOutOfBoundsException
{
randomDeck[count - 1] = list[num -1];
nums[index - 1] = 0;
located = true;
count++;
}
else
index++;
}
}
return randomDeck;
}

Thank you for any help
• 02-01-2010, 04:40 AM
collin389
Here is an easy shuffle algorithm from Wikipedia called the Fisher-Yates shuffle.
Code:

```public static void shuffle(int[] array) {     Random rng = new Random();      // java.util.Random.     // n is the number of items left to shuffle     for (int n = array.length; n > 1; n--)     {         // Pick a random element to move to the end         int k = rng.nextInt(n);  // 0 <= k <= n - 1.         // Simple swap of variables         int tmp = array[k];         array[k] = array[n - 1];         array[n - 1] = tmp;     } }```
• 02-01-2010, 05:26 AM
Corey
• 02-01-2010, 05:45 AM
Corey
Hmm, that seems to have fixed the IndexOutOfBounds exception, but now I get the NullPointerException when I try to use that method in one of my constructors. Im pasting below: the constructor, the revised randomizer method and, the static list array, in case that helps.

public Deck()
{
Card[] random = randomizer();
for (int i = 1; i <= 26; i++)
}

private Card[] randomizer()
{
Random rng = new Random(); // java.util.Random.
// n is the number of items left to shuffle
Card[] randomDeck = list;
for (int i = 1; i <= 52; i++)
randomDeck[i - 1] = list[i - 1];
for (int n = randomDeck.length; n > 1; n--)
{
// Pick a random element to move to the end
int k = rng.nextInt(n); // 0 <= k <= n - 1.
// Simple swap of variables
Card tmp = randomDeck[k];
randomDeck[k] = randomDeck[n - 1];
randomDeck[n - 1] = tmp;
}
return randomDeck;
}

private static Card[] list; // list is a static array, so im not sure if that is affecting the constructor through the randomizer method. If so, i need the data in list, so should I simply copy the data manually, and am i creating an alias and really shuffling list (which i would like to remain in order)?
• 02-01-2010, 06:49 AM
collin389
First of all, in your loop:
Code:

```for (int i = 1; i <= 52; i++) randomDeck[i - 1] = list[i - 1];```
```for (int i = 0; i < 52; i++) randomDeck[i] = list[i];```