class Ideone{
	public static void main (String[] args)	{
		System.out.println(isPrime(9));
		System.out.println(isPrime(15));
		System.out.println(isPrime(21));
	}
 
	static boolean isPrime(int num) {
	    if (num < 1) throw new IllegalArgumentException(
	            "Argument (" + num + ") must be a positive integer.");
 
	    if (num <= 3) return num > 1;
	    if ((num & 1) == 0) return false;
 
	    for (int i = 5; i * i <= num; i += 2)
	        if (num % i == 0) return false;
 
	    return true;
	}
}

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