Innumeracy is my first attempt at a Java library. It adds some extra math features such as isPrime and isPalindromic. I decided to write this API for a couple of reasons, one being that I enjoy using maths and problem solving within programming and so by writing a library, I was 'cutting to the chase', therefore not needing to create any GUIs or interfaces which a standard program would need to have (E.g A calculator application). The other reason was, I knew it would be a good learning experience - not only had I not created an API before, but I was not familiar with many algorithms before starting this project. So this project has introduced me to algorithms such as the Euclidean algorithm for finding the Greatest Common Divisor of 2 numbers, which has helped me make my code much, much more efficient.
I have tested the library out by doing some Project Euler problems (https://projecteuler.net/) and it seems to work pretty well, reducing the amount of code needed to solve a problem, while keeping the program running fast and efficiently!
EDIT (25/03/2021):
TL;DR This is a Math library with 0 jUnit tests to back it up.
It Just Works™.