trouble creating a method that returns the number of prime integers in the array

Question

I am having trouble creating a method that will find out the number of prime integers in the array , so far i have done this but when i run it , it doesn't work properly it counts 2 as prime number and also other numbers . Any suggestions please?

    int[] newSet = {8, 4, 22, 82, 12, 32, 18, 400, 3, 33, 401, -1, 1, 3, 9};
    System.out.println(primeIntegers(newSet));

}

public static int primeIntegers(int[] numbers) {
    int count = 0;
    for (int i = 0; i < numbers.length; i++) {
        if (numbers[i] % 3 == 0) {
            count++;
        }

    }

    return count;

}

}

Answer 1

if (numbers[i] % 3 == 0) definitely be wrong for determining a prime. Firstly , you need to know what a prime number is (2 is a prime number).

The easiest(not the best) way to determine if a number is a prime would be :

//checks whether an int is prime or not.
boolean static  isPrime(int n) {
    for(int i=2;i<n;i++) {//here,change i<n into 2*i<n or  Math.sqrt(i)<n will be better
        if(n%i==0)
            return false;//can be dividable by not one or itself.
    }
    return true;
}

//use above method to count the prime number
public static int primeIntegers(int[] numbers) {
    int count = 0;
    for (int i = 0; i < numbers.length; i++) {
        if(isPrime(numbers[i]))
            count++;
        }
    }
    return count;
}

Answer 2

The condition you are validating returns you only the numbers that are not divisible by 3 to make for finding the prime numbers you need to at least check for all the numbers less than its square root . ie.

boolean chkPrime(int num)
{
    for(int i=2;i<(sqrt(num));i++)
        if(num % i==0)
            return false;
    return true;
}