了解 SHA-256 哈希

Question

I am using Sha-256 hashing in my Java program which is working as per behavior.

I am actually a bit confused in the function I have used for SHA-256.

Following is the code of the function:

// Function for generating to Hash of the file content..
    public static String generateHash( String fileContent ) 
     {
        String hashtext = EMPTY_STRING;

        try {

            // SHA - 256 Message Digest..
            MessageDigest shaDigest = MessageDigest.getInstance( "SHA-256" );

            // digest() method is called 
            // to calculate message digest of the input string 
            // returned as array of byte 
            byte[] messageDigest = shaDigest.digest( fileContent.getBytes() ); 

            // Convert byte array into signum representation 
            BigInteger no = new BigInteger( 1, messageDigest ); 

            // Convert message digest into hex value 
            hashtext = no.toString( 16 ); 

            // Add preceding 0s to make it 32 bit 
            while ( hashtext.length() < 32 ) { 
                hashtext = "0" + hashtext; 
            }

        }
        catch ( Exception hashingException ) {

            System.out.println( "Exception in Hashing of Content = " + hashingException );

        }
         // return the HashText 
         return hashtext; 
     }

Now, here I am confused in three statements; as I am unaware of what is their actual purpose since I have surfed them on the internet but didnt get any explanatory stuff. Can some one elaborate these three steps to me?

STATEMENT 1

BigInteger no = new BigInteger( 1, messageDigest ); 

STATEMENT 2

 hashtext = no.toString( 16 );

STATEMENT 3

while ( hashtext.length() < 32 ) { 
    hashtext = "0" + hashtext;
}

Answer 1

BigInteger no = new BigInteger( 1, messageDigest ); 

Covert the bytes to a Positive Sign-Magnitude representation. Read Javadoc for more info.

hashtext = no.toString( 16 );

Convert the BigInteger number to a Base 16 (Hex Decimal) String

while ( hashtext.length() < 32 ) { 
    hashtext = "0" + hashtext;
}

Prepend 0 until the hashtext has the size of 32.