8-next-prime.apex

public Integer nextPrime(Integer num) {
    if (num < 2) return 2; 
    while (!isPrime(++num));
    return num;
}
public Boolean isPrime(Integer num) {
    for (Integer i=2; i<num; i++) {
        if (Math.mod(num, i) == 0) return false;
    }
    return true;
}