Implementation of the karatsuba multiplication

Karatsuba.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigInteger;


public class Karatsuba {

	private static final BigInteger ONE = new BigInteger("1");
	private static final BigInteger TWO = new BigInteger("2");

	public Karatsuba() {
		System.out.println("Karatsuba Multiplication Application");
		BigInteger a = readSingleValue("a");		
		BigInteger b = readSingleValue("b");
		BigInteger result = multiply(a, b);
		System.out.println("The result is: " + result);
	}		

	private BigInteger readSingleValue(String valueName) {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		do {
			try {
				System.out.print("Input numeric value " + valueName + ": ");				
				return new BigInteger(reader.readLine());
			} catch (Exception e) {
				System.err.println("Could not read your input. Please input a valid number");
			}					
		} while(true);
	}
	
	private BigInteger multiply(BigInteger a, BigInteger b) {
		//TODO Bits auffüllen
		int nHalf = a.bitCount()/2;
		BigInteger r = a.shiftRight(nHalf);
		// Bitsmaske = 2^(n+1)-1
		createBitmask(nHalf);
//		BigInteger s = a.and(val)
		return null;
	}

	private BigInteger createBitmask(int nHalf) {
		return TWO.pow(nHalf+1).subtract(ONE);
	}
	
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		new Karatsuba();
	}

}

Not yet finished