what does MessageDigest update do
This MessageDigest class provides applications the functionality of a message digest algorithm, such as SHA-1 or SHA-256. Message digests are secure one-way hash functions that take arbitrary-sized data and output a fixed-length hash value.digest method can be called once for a given number of updates.erforms a final update on the digest using the specified array of bytes, then completes the digest computation. That is, this method first calls update(input) , passing the input array to the update method, then calls digest() . Returns a string representation of this message digest object.