Please:

• add proper junit tests,
• your code does not work for negative y,
• reformat the code,
• please apply your function in other places, where this function is needed, e.g.

  • Java/src/main/java/com/thealgorithms/maths/PrimeCheck.java

    Line 74 in da21ffe

    private static long modPow(long a, long b, long c) {

  • Java/src/main/java/com/thealgorithms/maths/MillerRabinPrimalityCheck.java

    Line 75 in da21ffe

    private static long powerModP(long x, long y, long p) {

I also think that this could be a generic, to handle ints and longs.

Further more, what do you think about renaming it to modPow - this seems to be a more standard name in this context.