I am reading a text file which contains numbers in the range [1, 10^100]. I am then performing a sequence of arithmetic operations on each number. I would like to use a BigInteger only if the number is out of the int/long range. One approach would be to count how many digits there are in the string and switch to BigInteger if there are too many. Otherwise I'd just use primitive arithmetic as it is faster. Is there a better way?

Is there any reason why Java could not do this automatically i.e. switch to BigInteger if an int was too small? This way we would not have to worry about overflows.