Checksum of a Java Object

CheckSumTest.java

package ee.ut.cs.mc.example;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Random;

/*
 * author Huber Flores
 */

public class CheckSumTest implements Serializable{

	public void bubbleSort(int randValue, int sizeArray) throws NoSuchAlgorithmException, IOException{
		CheckObject l = new CheckObject();
		BigInteger sum = checksum(l);
		
		System.out.println("Size of the object: " + sizeOf(l));
		System.out.println("Checking integrity of the object, checksum= " + sum);
		
		l.bubbleFunction(randValue, sizeArray);
		
	}
	
		
	public static void main(String[] args) {
		CheckSumTest test = new CheckSumTest();
		try {
			test.bubbleSort(1000000, 9999);
		} catch (NoSuchAlgorithmException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		System.out.println("Done");
	}

		
	class CheckObject implements Serializable{
		
		public int[] bubbleFunction(int randValue, int sizeArray){
			int[] num = new int[sizeArray];
	        Random r = new Random();
	        for (int i = 0; i < num.length; i++) {
	                num[i] = r.nextInt(randValue);
	        } 
	        int j;
	        boolean flag = true; 
	        int temp; 

	        while (flag) {
	                flag = false; 
	                for (j = 0; j < num.length - 1; j++) {
	                        if (num[j] < num[j + 1]) 
	                        {
	                                temp = num[j]; 
	                                num[j] = num[j + 1];
	                                num[j + 1] = temp;
	                                flag = true; 
	                        }
	                }
	        }
	        
	       return num;
		}

	}
	
	/*
	 * target object to apply checksum should be Serializable
	 */
	private BigInteger checksum(Object obj) throws IOException, NoSuchAlgorithmException {

	    if (obj == null) {
	      return BigInteger.ZERO;   
	    }

	    ByteArrayOutputStream bos = new ByteArrayOutputStream();
	    ObjectOutputStream oos = new ObjectOutputStream(bos);
	    oos.writeObject(obj);
	    oos.close();
	        
	    
	    MessageDigest m = MessageDigest.getInstance("SHA1");
	    m.update(bos.toByteArray());

	    return new BigInteger(1, m.digest());
	}
	
	/*
	 * size of the object.
	 * some code replicated to be removed.
	 */
	public double sizeOf(Object obj) throws IOException {

	    ByteArrayOutputStream bos = new ByteArrayOutputStream();
	    ObjectOutputStream oos = new ObjectOutputStream(bos);

	    oos.writeObject(obj);
	    oos.flush();
	    oos.close();

	    return bos.toByteArray().length;
	}

}