PrimesFromArray3.java

import java.util.Arrays;
import java.util.ArrayList;

public class PrimesFromArray3 {

    /**
    *   Program:    PrimesFromArray3.java
    *   Purpose:    How to get prime numbers from an array using function
    *   Creator:    Chris Clarke
    *   Created:    22.05.2015
    *   Version:    Function getPrimes() returns array
    */

    public static boolean isPrime(int k) {
        if (k<2) {
            return false;       // 1 has only 1 unique divisor
        } // if             // so it's not prime

        int sqrt = (int) Math.sqrt(k);  // calculate square root of k

        for (int j=2; j<=sqrt; j++) {   // iterate from 2 to root of k
            if (k%j==0) {       // if j is a divisor of k...
                return false;   // ...k is not prime
            } // end if
        } // for

        return true;            // k is prime
    } // isPrime()

    public static int[] getPrimes(int[] theArray) {
        // create array list
        ArrayList<Integer> arl = new ArrayList<Integer>();

        // populate arraylist with primes
        for (int i: theArray) {
            if (isPrime(i)) {
                arl.add(i);
            } // if
        } // for

        // create primes array
        int[] primes = new int[arl.size()];

        // copy values from arraylist to array
        for (int i=0; i<arl.size(); i++) {
            primes[i] = arl.get(i);
        } // for

        // return array
        return primes;

    } // getPrimes()

    public static void main(String[] args) {
        int[] theArray = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        int[] primes = getPrimes (theArray);
        System.out.println (Arrays.toString (primes));
    } // main()

} // class PrimesFromArray3