Implementation of a Hash Collision with SHA512/t

Collision.java

import java.security.NoSuchProviderException;
import java.security.Security;
import java.security.NoSuchAlgorithmException;
import java.util.HashMap;
import java.math.BigInteger;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.digests.SHA512tDigest;
import org.bouncycastle.jce.provider.BouncyCastleProvider;

public class Collision {

	/**
	 * @param args digestSize in Bit
	 */
	public static void main(String[] args) throws NoSuchAlgorithmException, NoSuchProviderException {
		
		// init digest
		int digestSize = Integer.parseInt(args[0]);
		Security.addProvider(new BouncyCastleProvider());
		Digest messageDigest = new SHA512tDigest(digestSize);
		
		// init variables
		HashMap<BigInteger, String> HM = new HashMap<BigInteger, String>();
		String FirstPart = "529252-";
		int count = 0;
		String ReturnString = null;
		byte[] retValue = new byte[messageDigest.getDigestSize()];
		BigInteger Hash = null;
		
		// add <hash, string> to hashmap until collision
		while (ReturnString == null) {
			String TestString = FirstPart + count;
			
			messageDigest.update(TestString.getBytes(), 0, TestString.length());
			messageDigest.doFinal(retValue, 0);
			Hash = new BigInteger(retValue);
			
			// when collision ReturnString = old value
			ReturnString = HM.put(Hash, TestString);
			
			count++;
		}
		
		System.out.println("----------------");
		System.out.println("Collision found!");
		System.out.println("----------------");
		System.out.println("Hash: " + byteArrayToHexString(retValue));
		System.out.println("String1: " + ReturnString);
		System.out.println("String2: " + HM.get(Hash));
		
	}
	
    //Converting a bytes array to string of hex character
    public static String byteArrayToHexString(byte[] b) {
    	int len = b.length;
    	String data = new String();

    	for (int i = 0; i < len; i++){
    			data += Integer.toHexString((b[i] >> 4) & 0xf);
    				data += Integer.toHexString(b[i] & 0xf);
    	}
    	return data;
    }

}