Rabin-Karp string matching algorithm in java

RabinKarpAlgorithm.java

public class RabinKarpAlgorithm {
	
	private static long getNumber(char[] chars) {
		long number = 0;
		
		for (char ch: chars) {
			number = (number * 10) + ((int) ch - (int)'a' + 1);
		}
		
		return number;
	}
	
	private static boolean contains(String a, String b) {
		String smaller = a.length() <= b.length() ? a : b;
		String larger = a.length() > b.length() ? a : b;
		char[] ch1 = larger.toCharArray();
		char[] ch2 = smaller.toCharArray();
		
		long smallerNumber = getNumber(ch2);
		
		int i = 0;
		int j = smaller.length() - 1;
		long number = 0;
		while (true) {
			if (i == 0) {
				for (int k = 0; k <= j; k++) {
					number = (number * 10) + ((int) ch1[k] - (int)'a' + 1);
				}
			}
			
			if (number == smallerNumber) {
				return true;
			} else {
				j++;
				
				if (j == ch1.length) {
					break;
				} else {
					number = ((number - ((int) ch1[i] - (int) 'a' + 1) * (long) Math.pow(10, ch2.length - 1)) * 10) + ((int) ch1[j] - (int) 'a' + 1);
					i++;
				}
			}
		}
		
		return false;
	}
	
	public static void main(String[] args) {
		String a = "abcdeabdeabcdf";
		String b = "abcdf";
		
		System.out.println(contains(a, b));
	}
}