puzzle equilibrium indexes

//2012-05-21 johnpfeiffer
// An equilibrium point in an array of integers is an Index where the sum of the left values equals the sum of the right values
// If there are no values to the left or right the sum is arbitrarily set to 0
// an O(n) solution maintains the running lefthand and righthand sums

public class Solution
{
    public static void main(String[] args)
    {
        int a[] = { 1 ,  2 , 0 , 3 };
        int b[] = { -7 , 1 ,  5 , 2 , -4 , 3,  0 };

        displayArrayValues( a );
        System.out.println( sum(a) + " = total Sum with no index selected" );
        equi( a );

        displayArrayValues( b );
        System.out.println( sum(b) + " = total Sum with no index selected" );
        equi( b );
    }


    private static void equi( int[] arrayOfIntegers )
    {
        if( arrayOfIntegers == null ){  throw new IllegalArgumentException();       }

        int leftSum = 0;
        int rightSum = sum( arrayOfIntegers );
        int previousIndex = -1;

        for( int currentIndex = 0; currentIndex < arrayOfIntegers.length; currentIndex++ , previousIndex++ )
        {
            if( previousIndex != -1 )
            {       leftSum = leftSum + arrayOfIntegers[previousIndex];
            }else
            {       leftSum = 0;
            }

            rightSum = rightSum - arrayOfIntegers[currentIndex];

            System.out.print( currentIndex + " : " + leftSum + " vs " + rightSum );
            if( rightSum == leftSum )
            {               System.out.print( " EQUILIBRIUM POINT!" );
            }
            System.out.println();
        }
    }

    private static int sum( int[] arrayOfIntegers ) throws IllegalArgumentException
    {
        if( arrayOfIntegers == null ){  throw new IllegalArgumentException();       }

        int sumOfIntegers = 0;
        for( int i = 0; i < arrayOfIntegers.length; i++ )
        {       sumOfIntegers += arrayOfIntegers[i];
        }
        return sumOfIntegers;
    }

    private static void displayArrayValues( int [] arrayOfIntegers )
    {
        if( arrayOfIntegers == null ){  throw new IllegalArgumentException();       }

        for( int i = 0; i < arrayOfIntegers.length; i++ )
        {       System.out.print( arrayOfIntegers[i] + " , " );
        }
        System.out.println();
    }

} //end class