An implementation of a divide-and conquer method of calculating modular exponents

ModularExponent.java

public class ModularExponent {
  //ModularExponent
  //An implementation of divide-and-conquer method of calculating modular exponents
  //See Introduction to Algorithms, Corment et al.
	
	//tested correct to (a,b,n) = (256,256,256)
	public static int modularExponent(int a, int b, int n) {
		if (a < 0 || b < 0 || n < 0) {
			throw new IllegalArgumentException("Arguments must be positive integers");
		}
		int c = 0;
		int d = 1;
		char [] bits = Integer.toBinaryString(b).toCharArray();
		for (int k = 0 ; k < bits.length; k++) {
			c = c * 2;
			d = Math.multiplyExact(d,d) % n;
			Integer i = new Integer(3);
			if (bits[k] == '1') ) {
				c++;
				d = Math.multiplyExact(d,a) % n;
			}
		}
		return d;
	}
}