Question

So I’m getting an index out of bounds exception on some code i’m writing. What I don’t understand is that I know for a fact that the index element I’m trying to work with exists.

Here’s the code:

I have a constructor for an array list

    public StixBoard(int number)
{
    stixGame = new ArrayList<Integer>(number);

    for (int i = 0; i < number; i++)
    {
        stixGame.add(i);
    }

}

This block generates a random variable 1-3

public int computeMove()
{

    int numberOfStix = (int) (3.0 * Math.random()) + 1;

    return numberOfStix;
}

Really straight forward, now here I have a method that takes the parameter supplied and attempts to remove those number of elements from the array list. As you can see, the parameter must be between 1 and 3, and it must be less than or equal to the size of the array list. Otherwise, the user is prompted to enter another number

public boolean takeStix(int number)
{
    boolean logicVar = false;
    placeHolder = stixGame.size();

    if ((number >= 1 && number <= 3) && number <= placeHolder)
    {
        for (int i = 0; i < number; i++)
        {
            stixGame.remove(i);
            logicVar = true;
        }
    } else if (number > 3 || number > placeHolder)
    {
        do
        {
            System.out
                    .println("Please enter a different number, less than or equal to three.");
            Scanner numberScan = new Scanner(System.in);
            number = numberScan.nextInt();
        } while (number > 3 || number > placeHolder);
    }

    return logicVar;
}

So as this program runs, the computeMove() method generates a random int (assuming the role of a computerized player) and attempts to translate that value to the number of indexes to be removed from the array list.

This ultimately brings me to this:

How many stix on the table? 4
|||||||||| 4 stix on the table
It's the computer's turn!
The computer chose 3

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 2, Size: 2
at java.util.ArrayList.RangeCheck(ArrayList.java:547)
at java.util.ArrayList.remove(ArrayList.java:387)
at StixBoard.takeStix(StixBoard.java:38)
at StixGame.main(StixGame.java:55)

So as you can see, the array list is of size 4, but when the computer rolls a 3, (which should leave me with 1), I am left with this error. How does my array list go from being of size 4 indexes to size 2?

Answer 1

You iterate through your list, from beginning to end, and remove an element at each step. This makes all the elements in the list shift to the left.

First iteration: i = 0

[1, 2, 3]

Second iteration: i = 1

[2, 3]

Third iteration: i = 2

[2] -> IndexOutOfBoudsException. There is no index 2 in this list.

Iterate from the end to the beginning instead. That willmake it correct, and faster since the list won’t have to copy all the elements from right to left.