New to Java

I keep getting this error.

First call to assembleArray(): 
	Enter the first integer: 
42
	Enter the second integer: 
427

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
	at filename.assembleArray(filename.java:151)
	at filename.main(filename.java:18)

but when I put something like the following it works:

First call to assembleArray(): 
	Enter the first integer: 
1328
	Enter the second integer: 
456
The resulting array has 4 elements as follows,
  [ 0 2 3 0 ]

line 151:

finalArray[ finalIndex ] = finalNum;

line 18:

lastArray = assembleArray (firstA, firstB);

I know it has something to do with the index but I dont know how to fix it because I can't set the index since it depends on user input.
For the resulting array, the first number is the sum of all odd one-digits, second number is the sum of all even 10-digits, and the third number is the sum of all multiples of 3.