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package java.io;


A PushbackInputStream adds
functionality to another input stream, namely
the  ability to "push back" or "unread" bytes,
by storing pushed-back bytes in an internal buffer.
This is useful in situations where
it is convenient for a fragment of code
to read an indefinite number of data bytes
that  are delimited by a particular byte
value; after reading the terminating byte,
the  code fragment can "unread" it, so that
the next read operation on the input stream
will reread the byte that was pushed back.
For example, bytes representing the  characters
constituting an identifier might be terminated
by a byte representing an  operator character;
a method whose job is to read just an identifier
can read until it  sees the operator and
then push the operator back to be re-read.

Author: David Connelly,  Jonathan Payne
Since:  1.0
/**
 * A <code>PushbackInputStream</code> adds
 * functionality to another input stream, namely
 * the  ability to "push back" or "unread" bytes,
 * by storing pushed-back bytes in an internal buffer.
 * This is useful in situations where
 * it is convenient for a fragment of code
 * to read an indefinite number of data bytes
 * that  are delimited by a particular byte
 * value; after reading the terminating byte,
 * the  code fragment can "unread" it, so that
 * the next read operation on the input stream
 * will reread the byte that was pushed back.
 * For example, bytes representing the  characters
 * constituting an identifier might be terminated
 * by a byte representing an  operator character;
 * a method whose job is to read just an identifier
 * can read until it  sees the operator and
 * then push the operator back to be re-read.
 *
 * @author  David Connelly
 * @author  Jonathan Payne
 * @since   1.0
 */
public
class PushbackInputStream extends FilterInputStream {
    
The pushback buffer.

Since:  1.1
/**
     * The pushback buffer.
     * @since   1.1
     */
    protected byte[] buf;

    
The position within the pushback buffer from which the next byte will
be read.  When the buffer is empty, pos is equal to
buf.length; when the buffer is full, pos is
equal to zero.

Since:  1.1
/**
     * The position within the pushback buffer from which the next byte will
     * be read.  When the buffer is empty, <code>pos</code> is equal to
     * <code>buf.length</code>; when the buffer is full, <code>pos</code> is
     * equal to zero.
     *
     * @since   1.1
     */
    protected int pos;

    
Check to make sure that this stream has not been closed

/**
     * Check to make sure that this stream has not been closed
     */
    private void ensureOpen() throws IOException {
        if (in == null)
            throw new IOException("Stream closed");
    }

    
Creates a PushbackInputStream
with a pushback buffer of the specified size,
and saves its argument, the input stream
in, for later use. Initially,
the pushback buffer is empty.

Params:
  • in –    the input stream from which bytes will be read.
  • size –  the size of the pushback buffer.
Throws:
Since: 1.1
/**
     * Creates a <code>PushbackInputStream</code>
     * with a pushback buffer of the specified <code>size</code>,
     * and saves its argument, the input stream
     * <code>in</code>, for later use. Initially,
     * the pushback buffer is empty.
     *
     * @param  in    the input stream from which bytes will be read.
     * @param  size  the size of the pushback buffer.
     * @exception IllegalArgumentException if {@code size <= 0}
     * @since  1.1
     */
    public PushbackInputStream(InputStream in, int size) {
        super(in);
        if (size <= 0) {
            throw new IllegalArgumentException("size <= 0");
        }
        this.buf = new byte[size];
        this.pos = size;
    }

    
Creates a PushbackInputStream
with a 1-byte pushback buffer, and saves its argument, the input stream
in, for later use. Initially,
the pushback buffer is empty.

Params:
  • in –   the input stream from which bytes will be read.
/**
     * Creates a <code>PushbackInputStream</code>
     * with a 1-byte pushback buffer, and saves its argument, the input stream
     * <code>in</code>, for later use. Initially,
     * the pushback buffer is empty.
     *
     * @param   in   the input stream from which bytes will be read.
     */
    public PushbackInputStream(InputStream in) {
        this(in, 1);
    }

    
Reads the next byte of data from this input stream. The value
byte is returned as an int in the range
0 to 255. If no byte is available
because the end of the stream has been reached, the value
-1 is returned. This method blocks until input data
is available, the end of the stream is detected, or an exception
is thrown.

 This method returns the most recently pushed-back byte, if there is
one, and otherwise calls the read method of its underlying
input stream and returns whatever value that method returns.

Throws:
  • IOException –  if this input stream has been closed by invoking its close() method, or an I/O error occurs.
See Also:
Returns:    the next byte of data, or -1 if the end of the
            stream has been reached.
/**
     * Reads the next byte of data from this input stream. The value
     * byte is returned as an <code>int</code> in the range
     * <code>0</code> to <code>255</code>. If no byte is available
     * because the end of the stream has been reached, the value
     * <code>-1</code> is returned. This method blocks until input data
     * is available, the end of the stream is detected, or an exception
     * is thrown.
     *
     * <p> This method returns the most recently pushed-back byte, if there is
     * one, and otherwise calls the <code>read</code> method of its underlying
     * input stream and returns whatever value that method returns.
     *
     * @return     the next byte of data, or <code>-1</code> if the end of the
     *             stream has been reached.
     * @exception  IOException  if this input stream has been closed by
     *             invoking its {@link #close()} method,
     *             or an I/O error occurs.
     * @see        java.io.InputStream#read()
     */
    public int read() throws IOException {
        ensureOpen();
        if (pos < buf.length) {
            return buf[pos++] & 0xff;
        }
        return super.read();
    }

    
Reads up to len bytes of data from this input stream into
an array of bytes.  This method first reads any pushed-back bytes; after
that, if fewer than len bytes have been read then it
reads from the underlying input stream. If len is not zero, the method
blocks until at least 1 byte of input is available; otherwise, no
bytes are read and 0 is returned.

Params:
  • b –     the buffer into which the data is read.
  • off –   the start offset in the destination array b
  • len –   the maximum number of bytes read.
Throws:
See Also:
Returns:    the total number of bytes read into the buffer, or
            -1 if there is no more data because the end of
            the stream has been reached.
/**
     * Reads up to <code>len</code> bytes of data from this input stream into
     * an array of bytes.  This method first reads any pushed-back bytes; after
     * that, if fewer than <code>len</code> bytes have been read then it
     * reads from the underlying input stream. If <code>len</code> is not zero, the method
     * blocks until at least 1 byte of input is available; otherwise, no
     * bytes are read and <code>0</code> is returned.
     *
     * @param      b     the buffer into which the data is read.
     * @param      off   the start offset in the destination array <code>b</code>
     * @param      len   the maximum number of bytes read.
     * @return     the total number of bytes read into the buffer, or
     *             <code>-1</code> if there is no more data because the end of
     *             the stream has been reached.
     * @exception  NullPointerException If <code>b</code> is <code>null</code>.
     * @exception  IndexOutOfBoundsException If <code>off</code> is negative,
     * <code>len</code> is negative, or <code>len</code> is greater than
     * <code>b.length - off</code>
     * @exception  IOException  if this input stream has been closed by
     *             invoking its {@link #close()} method,
     *             or an I/O error occurs.
     * @see        java.io.InputStream#read(byte[], int, int)
     */
    public int read(byte[] b, int off, int len) throws IOException {
        ensureOpen();
        if (b == null) {
            throw new NullPointerException();
        } else if (off < 0 || len < 0 || len > b.length - off) {
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return 0;
        }

        int avail = buf.length - pos;
        if (avail > 0) {
            if (len < avail) {
                avail = len;
            }
            System.arraycopy(buf, pos, b, off, avail);
            pos += avail;
            off += avail;
            len -= avail;
        }
        if (len > 0) {
            len = super.read(b, off, len);
            if (len == -1) {
                return avail == 0 ? -1 : avail;
            }
            return avail + len;
        }
        return avail;
    }

    
Pushes back a byte by copying it to the front of the pushback buffer.
After this method returns, the next byte to be read will have the value
(byte)b.

Params:
  • b –   the int value whose low-order
                 byte is to be pushed back.
Throws:
  • IOException – If there is not enough room in the pushback buffer for the byte, or this input stream has been closed by invoking its close() method.
/**
     * Pushes back a byte by copying it to the front of the pushback buffer.
     * After this method returns, the next byte to be read will have the value
     * <code>(byte)b</code>.
     *
     * @param      b   the <code>int</code> value whose low-order
     *                  byte is to be pushed back.
     * @exception IOException If there is not enough room in the pushback
     *            buffer for the byte, or this input stream has been closed by
     *            invoking its {@link #close()} method.
     */
    public void unread(int b) throws IOException {
        ensureOpen();
        if (pos == 0) {
            throw new IOException("Push back buffer is full");
        }
        buf[--pos] = (byte)b;
    }

    
Pushes back a portion of an array of bytes by copying it to the front
of the pushback buffer.  After this method returns, the next byte to be
read will have the value b[off], the byte after that will
have the value b[off+1], and so forth.

Params:
  • b – the byte array to push back.
  • off – the start offset of the data.
  • len – the number of bytes to push back.
Throws:
  • IOException – If there is not enough room in the pushback buffer for the specified number of bytes, or this input stream has been closed by invoking its close() method.
Since:    1.1
/**
     * Pushes back a portion of an array of bytes by copying it to the front
     * of the pushback buffer.  After this method returns, the next byte to be
     * read will have the value <code>b[off]</code>, the byte after that will
     * have the value <code>b[off+1]</code>, and so forth.
     *
     * @param b the byte array to push back.
     * @param off the start offset of the data.
     * @param len the number of bytes to push back.
     * @exception IOException If there is not enough room in the pushback
     *            buffer for the specified number of bytes,
     *            or this input stream has been closed by
     *            invoking its {@link #close()} method.
     * @since     1.1
     */
    public void unread(byte[] b, int off, int len) throws IOException {
        ensureOpen();
        if (len > pos) {
            throw new IOException("Push back buffer is full");
        }
        pos -= len;
        System.arraycopy(b, off, buf, pos, len);
    }

    
Pushes back an array of bytes by copying it to the front of the
pushback buffer.  After this method returns, the next byte to be read
will have the value b[0], the byte after that will have the
value b[1], and so forth.

Params:
  • b – the byte array to push back
Throws:
  • IOException – If there is not enough room in the pushback buffer for the specified number of bytes, or this input stream has been closed by invoking its close() method.
Since:    1.1
/**
     * Pushes back an array of bytes by copying it to the front of the
     * pushback buffer.  After this method returns, the next byte to be read
     * will have the value <code>b[0]</code>, the byte after that will have the
     * value <code>b[1]</code>, and so forth.
     *
     * @param b the byte array to push back
     * @exception IOException If there is not enough room in the pushback
     *            buffer for the specified number of bytes,
     *            or this input stream has been closed by
     *            invoking its {@link #close()} method.
     * @since     1.1
     */
    public void unread(byte[] b) throws IOException {
        unread(b, 0, b.length);
    }

    
Returns an estimate of the number of bytes that can be read (or
skipped over) from this input stream without blocking by the next
invocation of a method for this input stream. The next invocation might be
the same thread or another thread.  A single read or skip of this
many bytes will not block, but may read or skip fewer bytes.

 The method returns the sum of the number of bytes that have been pushed back and the value returned by available. 
Throws:
  • IOException –  if this input stream has been closed by invoking its close() method, or an I/O error occurs.
See Also:
Returns:    the number of bytes that can be read (or skipped over) from
            the input stream without blocking.
/**
     * Returns an estimate of the number of bytes that can be read (or
     * skipped over) from this input stream without blocking by the next
     * invocation of a method for this input stream. The next invocation might be
     * the same thread or another thread.  A single read or skip of this
     * many bytes will not block, but may read or skip fewer bytes.
     *
     * <p> The method returns the sum of the number of bytes that have been
     * pushed back and the value returned by {@link
     * java.io.FilterInputStream#available available}.
     *
     * @return     the number of bytes that can be read (or skipped over) from
     *             the input stream without blocking.
     * @exception  IOException  if this input stream has been closed by
     *             invoking its {@link #close()} method,
     *             or an I/O error occurs.
     * @see        java.io.FilterInputStream#in
     * @see        java.io.InputStream#available()
     */
    public int available() throws IOException {
        ensureOpen();
        int n = buf.length - pos;
        int avail = super.available();
        return n > (Integer.MAX_VALUE - avail)
                    ? Integer.MAX_VALUE
                    : n + avail;
    }

    
Skips over and discards n bytes of data from this
input stream. The skip method may, for a variety of
reasons, end up skipping over some smaller number of bytes,
possibly zero.  If n is negative, no bytes are skipped.

 The skip method of PushbackInputStream
first skips over the bytes in the pushback buffer, if any.  It then
calls the skip method of the underlying input stream if
more bytes need to be skipped.  The actual number of bytes skipped
is returned.

Params:
  • n –  {@inheritDoc}
Throws:
  • IOException –  if the stream has been closed by invoking its close() method, in.skip(n) throws an IOException, or an I/O error occurs.
See Also:
Returns:    {@inheritDoc}
Since:     1.2
/**
     * Skips over and discards <code>n</code> bytes of data from this
     * input stream. The <code>skip</code> method may, for a variety of
     * reasons, end up skipping over some smaller number of bytes,
     * possibly zero.  If <code>n</code> is negative, no bytes are skipped.
     *
     * <p> The <code>skip</code> method of <code>PushbackInputStream</code>
     * first skips over the bytes in the pushback buffer, if any.  It then
     * calls the <code>skip</code> method of the underlying input stream if
     * more bytes need to be skipped.  The actual number of bytes skipped
     * is returned.
     *
     * @param      n  {@inheritDoc}
     * @return     {@inheritDoc}
     * @throws     IOException  if the stream has been closed by
     *             invoking its {@link #close()} method,
     *             {@code in.skip(n)} throws an IOException,
     *             or an I/O error occurs.
     * @see        java.io.FilterInputStream#in
     * @see        java.io.InputStream#skip(long n)
     * @since      1.2
     */
    public long skip(long n) throws IOException {
        ensureOpen();
        if (n <= 0) {
            return 0;
        }

        long pskip = buf.length - pos;
        if (pskip > 0) {
            if (n < pskip) {
                pskip = n;
            }
            pos += pskip;
            n -= pskip;
        }
        if (n > 0) {
            pskip += super.skip(n);
        }
        return pskip;
    }

    
Tests if this input stream supports the mark and
reset methods, which it does not.

See Also:
Returns:  false, since this class does not support the
          mark and reset methods.
/**
     * Tests if this input stream supports the <code>mark</code> and
     * <code>reset</code> methods, which it does not.
     *
     * @return   <code>false</code>, since this class does not support the
     *           <code>mark</code> and <code>reset</code> methods.
     * @see     java.io.InputStream#mark(int)
     * @see     java.io.InputStream#reset()
     */
    public boolean markSupported() {
        return false;
    }

    
Marks the current position in this input stream.

 The mark method of PushbackInputStream
does nothing.

Params:
  • readlimit –   the maximum limit of bytes that can be read before
                     the mark position becomes invalid.
See Also:
/**
     * Marks the current position in this input stream.
     *
     * <p> The <code>mark</code> method of <code>PushbackInputStream</code>
     * does nothing.
     *
     * @param   readlimit   the maximum limit of bytes that can be read before
     *                      the mark position becomes invalid.
     * @see     java.io.InputStream#reset()
     */
    public synchronized void mark(int readlimit) {
    }

    
Repositions this stream to the position at the time the
mark method was last called on this input stream.

 The method reset for class
PushbackInputStream does nothing except throw an
IOException.

Throws:
  • IOException –  if this method is invoked.
See Also:
/**
     * Repositions this stream to the position at the time the
     * <code>mark</code> method was last called on this input stream.
     *
     * <p> The method <code>reset</code> for class
     * <code>PushbackInputStream</code> does nothing except throw an
     * <code>IOException</code>.
     *
     * @exception  IOException  if this method is invoked.
     * @see     java.io.InputStream#mark(int)
     * @see     java.io.IOException
     */
    public synchronized void reset() throws IOException {
        throw new IOException("mark/reset not supported");
    }

    
Closes this input stream and releases any system resources
associated with the stream.
Once the stream has been closed, further read(), unread(),
available(), reset(), or skip() invocations will throw an IOException.
Closing a previously closed stream has no effect.

Throws:
  • IOException –  if an I/O error occurs.
/**
     * Closes this input stream and releases any system resources
     * associated with the stream.
     * Once the stream has been closed, further read(), unread(),
     * available(), reset(), or skip() invocations will throw an IOException.
     * Closing a previously closed stream has no effect.
     *
     * @exception  IOException  if an I/O error occurs.
     */
    public synchronized void close() throws IOException {
        if (in == null)
            return;
        in.close();
        in = null;
        buf = null;
    }
}