isPrime.java

public static int isPrime(int x) {
    if(x <= 2) return 1;

    int num = 2, k = 0, sqrt = (int) Math.sqrt(x);
    boolean[] arr = new boolean[x-1];
    while(num <= sqrt) {
        if(arr[num-2]) {
            num++;
            continue;
        }

        int mul = 2;
        while((k = (num * mul)) <= x) {
            if(!arr[k-2]) {
                arr[k-2] = true;
            }
            mul++;
        }
        num++;
    }

    for(int i = 0; i < arr.length && arr[x-2]; i++) {
        if(!arr[i] && x % (i+2) == 0) {
            return i+2;
        }
    }

    return 1;
}