https://openjdk.java.net/ 
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package java.security;

import java.io.IOException;
import java.io.EOFException;
import java.io.InputStream;
import java.io.FilterInputStream;
import java.io.PrintStream;
import java.io.ByteArrayInputStream;


A transparent stream that updates the associated message digest using
the bits going through the stream.

To complete the message digest computation, call one of the digest methods on the associated message digest after your calls to one of this digest input stream's read methods. 
It is possible to turn this stream on or off (see on). When it is on, a call to one of the read methods results in an update on the message digest. But when it is off, the message digest is not updated. The default is for the stream to be on. 
Note that digest objects can compute only one digest (see MessageDigest), so that in order to compute intermediate digests, a caller should retain a handle onto the digest object, and clone it for each digest to be computed, leaving the original digest untouched. 
Author:Benjamin Renaud
See Also:
Since:1.2
/**
 * A transparent stream that updates the associated message digest using
 * the bits going through the stream.
 *
 * <p>To complete the message digest computation, call one of the
 * {@code digest} methods on the associated message
 * digest after your calls to one of this digest input stream's
 * {@link #read() read} methods.
 *
 * <p>It is possible to turn this stream on or off (see
 * {@link #on(boolean) on}). When it is on, a call to one of the
 * {@code read} methods
 * results in an update on the message digest.  But when it is off,
 * the message digest is not updated. The default is for the stream
 * to be on.
 *
 * <p>Note that digest objects can compute only one digest (see
 * {@link MessageDigest}),
 * so that in order to compute intermediate digests, a caller should
 * retain a handle onto the digest object, and clone it for each
 * digest to be computed, leaving the original digest untouched.
 *
 * @see MessageDigest
 *
 * @see DigestOutputStream
 *
 * @author Benjamin Renaud
 * @since 1.2
 */

public class DigestInputStream extends FilterInputStream {

    /* NOTE: This should be made a generic UpdaterInputStream */

    /* Are we on or off? */
    private boolean on = true;

    
The message digest associated with this stream.

/**
     * The message digest associated with this stream.
     */
    protected MessageDigest digest;

    
Creates a digest input stream, using the specified input stream
and message digest.

Params:
  • stream – the input stream.
  • digest – the message digest to associate with this stream.
/**
     * Creates a digest input stream, using the specified input stream
     * and message digest.
     *
     * @param stream the input stream.
     *
     * @param digest the message digest to associate with this stream.
     */
    public DigestInputStream(InputStream stream, MessageDigest digest) {
        super(stream);
        setMessageDigest(digest);
    }

    
Returns the message digest associated with this stream.

See Also:
Returns:the message digest associated with this stream.
/**
     * Returns the message digest associated with this stream.
     *
     * @return the message digest associated with this stream.
     * @see #setMessageDigest(java.security.MessageDigest)
     */
    public MessageDigest getMessageDigest() {
        return digest;
    }

    
Associates the specified message digest with this stream.

Params:
  • digest – the message digest to be associated with this stream.
See Also:
/**
     * Associates the specified message digest with this stream.
     *
     * @param digest the message digest to be associated with this stream.
     * @see #getMessageDigest()
     */
    public void setMessageDigest(MessageDigest digest) {
        this.digest = digest;
    }

    
Reads a byte, and updates the message digest (if the digest function is on). That is, this method reads a byte from the input stream, blocking until the byte is actually read. If the digest function is on (see on), this method will then call update on the message digest associated with this stream, passing it the byte read. 
Throws:
See Also:
Returns:the byte read.
/**
     * Reads a byte, and updates the message digest (if the digest
     * function is on).  That is, this method reads a byte from the
     * input stream, blocking until the byte is actually read. If the
     * digest function is on (see {@link #on(boolean) on}), this method
     * will then call {@code update} on the message digest associated
     * with this stream, passing it the byte read.
     *
     * @return the byte read.
     *
     * @exception IOException if an I/O error occurs.
     *
     * @see MessageDigest#update(byte)
     */
    public int read() throws IOException {
        int ch = in.read();
        if (on && ch != -1) {
            digest.update((byte)ch);
        }
        return ch;
    }

    
Reads into a byte array, and updates the message digest (if the digest function is on). That is, this method reads up to len bytes from the input stream into the array b, starting at offset off. This method blocks until the data is actually read. If the digest function is on (see on), this method will then call update on the message digest associated with this stream, passing it the data. 
Params:
  • b – the array into which the data is read.
  • off – the starting offset into b of where the data should be placed.
  • len – the maximum number of bytes to be read from the input stream into b, starting at offset off.
Throws:
See Also:
Returns: the actual number of bytes read. This is less than len if the end of the stream is reached prior to reading len bytes. -1 is returned if no bytes were read because the end of the stream had already been reached when the call was made.
/**
     * Reads into a byte array, and updates the message digest (if the
     * digest function is on).  That is, this method reads up to
     * {@code len} bytes from the input stream into the array
     * {@code b}, starting at offset {@code off}. This method
     * blocks until the data is actually
     * read. If the digest function is on (see
     * {@link #on(boolean) on}), this method will then call {@code update}
     * on the message digest associated with this stream, passing it
     * the data.
     *
     * @param b the array into which the data is read.
     *
     * @param off the starting offset into {@code b} of where the
     * data should be placed.
     *
     * @param len the maximum number of bytes to be read from the input
     * stream into b, starting at offset {@code off}.
     *
     * @return  the actual number of bytes read. This is less than
     * {@code len} if the end of the stream is reached prior to
     * reading {@code len} bytes. -1 is returned if no bytes were
     * read because the end of the stream had already been reached when
     * the call was made.
     *
     * @exception IOException if an I/O error occurs.
     *
     * @see MessageDigest#update(byte[], int, int)
     */
    public int read(byte[] b, int off, int len) throws IOException {
        int result = in.read(b, off, len);
        if (on && result != -1) {
            digest.update(b, off, result);
        }
        return result;
    }

    
Turns the digest function on or off. The default is on. When it is on, a call to one of the read methods results in an update on the message digest. But when it is off, the message digest is not updated. 
Params:
  • on – true to turn the digest function on, false to turn
it off.
/**
     * Turns the digest function on or off. The default is on.  When
     * it is on, a call to one of the {@code read} methods results in an
     * update on the message digest.  But when it is off, the message
     * digest is not updated.
     *
     * @param on true to turn the digest function on, false to turn
     * it off.
     */
    public void on(boolean on) {
        this.on = on;
    }

    
Prints a string representation of this digest input stream and
its associated message digest object.

/**
     * Prints a string representation of this digest input stream and
     * its associated message digest object.
     */
     public String toString() {
         return "[Digest Input Stream] " + digest.toString();
     }
}