You won't get NumberFormatException multiplying large numbers. If the number produced is too large, you will get a cryptic NegativeArraySizeException as the size of the array overflows.
You are more likely to get an out of memory error.
The limit is 32 * 2^32-1 bits for BigInteger or about 2^(4 billion).
You can get a NumberFormatException if you
- create a BigInteger from an empty byte[]
- use a signum < -1 or > +1
- try to parse a number in base >36 or < 2
- have a string with illegal digits.
When you get an exception you should also look at the message and the stack trace as this usually gives you the real cause.