Java IO.pdf - Nguyen Dang Binh

10.2.3 Feeding Data to the Digest
Java I/O
Once you have a MessageDigest object, you feed data into that digest by passing bytes into
one of three update() methods. If you're digesting some other form of data, like Unicode
text, you must first convert that data to bytes.
public void update(byte input)
public void update(byte[] input)
public void update(byte[] input, int offset, int length)
For example:
byte[] data = new byte[128];
int bytesRead = in.read(data);
sha.update(data, 0, bytesRead);
The first update() method takes a single byte as an argument. The second method takes an
entire array of bytes. The third method takes the subarray of input beginning at offset and
continuing for length bytes. You can call update() repeatedly, as long as you have more
data to feed it. Example 10.1 passed in bytes as they were read from the input stream. The
only restriction is that the bytes should not be reordered between calls to update().
10.2.4 Finishing the Digest
In general, digest algorithms cannot finish the calculation and return the digest until the last
byte is received. When you are ready to finish the calculation and receive the digest, you
invoke one of three overloaded digest() methods:
public byte[] digest()
public byte[] digest(byte[] input)
public int digest(byte[] output, int offset, int length)
throws DigestException
The first digest() method simply returns the digest as an array of bytes based on the data
that was already passed in through update(). For example:
byte[] result = sha.digest();
The second digest() method receives one last chunk of data in the input array, then returns
the digest. The third digest() method calculates the digest and places it in the array output
beginning at offset and continuing for at most length bytes, then returns the number of
bytes in the digest. If there are more than length bytes in the digest a DigestException is
thrown. After you've called digest(), the MessageDigest object is reset so it can be reused
to calculate a new digest.
10.2.5 Reusing Digests
There's some overhead associated with creating a new message digest with
MessageDigest.getInstance(). Therefore, if you want to use the same algorithm to
calculate digests for many different files, web pages, or other streams, you can reset the digest
and reuse it. The reset() method accomplishes this:
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