Java exercises:

Java
1

I received some java exercises on email, and I wanted to give them a try. I’m posting my solution to excercise “1. Prime Numbers“ since it the mail said “Feel free to share your solutions“ in this forum. I would like to see other persons solutions so I can improve.

  1. Prime Numbers

Create a function that returns a list of all prime numbers up to a given number. A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. For example, 2, 3, 5, 7, and 11 are prime numbers.

My solution:

public class App {
    public static void main(String[] args) throws Exception {
        System.out.println(primeNumbers(100));

    }

    public static List<Integer> primeNumbers(int toNum) {

        List<Integer> primeNumbers = new ArrayList<>();

        for (int i = 2; i <= toNum; i++) {
            int divisibleCount = 0;
            //System.out.println(i);

            for (int j = i; j >= 1; j--) {

                if (i % j == 0) {
                    //System.out.println(i + " is divisible by " + j);
                    divisibleCount++;
                }

                if (divisibleCount > 2) {
                    //System.out.println("divisible count is " + divisibleCount + " no need to count further");
                    break;
                }

            }
            if (divisibleCount <= 2) {
                primeNumbers.add(i);
            }
        }

        return primeNumbers;

    }
}

Output:
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]

2

I would use an algorithm that counts divisor integers up to half of the number being tested. - Bruce W.

2 Likes
3

Sorry - I meant the square root of the number.

2 Likes
5

Nice solution for a beginner exercise. One optimization could be checking divisibility only up to the square root of the number instead of all the way down to 1, which would make it much faster for larger numbers.