Java Program to Determine If a Number Is Prime

What Is a Prime Number?

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. Classic examples include 2, 3, 5, 7, 11, 13, 17, ….

Key points:

• 0 and 1 are not prime.
• 2 is the only even prime number.

Java Program to Determine If a Number Is Prime

Algorithm overview:

• Check divisibility of the target number by all integers from 2 up to its half (inclusive). If any division yields a remainder of 0, the number is composite.
• Once a divisor is found, the loop can exit early to save time.
• If no divisor is found, the number is prime.

public class PrimeChecker {
    public static void main(String[] args) {
        int numberToCheck = 17; // change this value to test other numbers
        boolean isPrime = true;

        // Edge cases: 0 and 1 are not prime
        if (numberToCheck < 2) {
            isPrime = false;
        } else {
            for (int i = 2; i <= numberToCheck / 2; i++) {
                int remainder = numberToCheck % i;
                System.out.println(numberToCheck + " ÷ " + i + " = " + (numberToCheck / i) + " remainder " + remainder);
                if (remainder == 0) {
                    isPrime = false;
                    break;
                }
            }
        }

        if (isPrime) {
            System.out.println(numberToCheck + " is a prime number.");
        } else {
            System.out.println(numberToCheck + " is not a prime number.");
        }
    }
}

Sample Output

17 ÷ 2 = 8 remainder 1
17 ÷ 3 = 5 remainder 2
17 ÷ 4 = 4 remainder 1
17 ÷ 5 = 3 remainder 2
17 ÷ 6 = 2 remainder 5
17 ÷ 7 = 2 remainder 3
17 ÷ 8 = 2 remainder 1
17 is a prime number.

Try the program above to check any integer, or extend it to find all prime numbers between 1 and 100.