You're absolutely right. The obvious solution would have been to create a boolean table containing all the pre-computed answers, and then simply use the integer you are testing as the index of the correct answer in memory. Now your isEven code is just a simple array lookup! Such an obvious improvement, I can't believe the OP didn't see it.
And with a little extra work you can shrink the whole table's size in memory by a factor of eight, but I'll leave that as an exercise for the interested reader.
And with a little extra work you can shrink the whole table's size in memory by a factor of eight, but I'll leave that as an exercise for the interested reader.