https://openjdk.java.net/ 
/*
 * Copyright (c) 1994, 2019, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

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} 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} is equal to
     * {@code buf.length}; when the buffer is full, {@code pos} 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}
     * with a pushback buffer of the specified {@code size},
     * and saves its argument, the input stream
     * {@code in}, 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.
     * @throws 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}
     * with a 1-byte pushback buffer, and saves its argument, the input stream
     * {@code in}, 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} in the range
     * {@code 0} to {@code 255}. If no byte is available
     * because the end of the stream has been reached, the value
     * {@code -1} 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} method of its underlying
     * input stream and returns whatever value that method returns.
     *
     * @return     the next byte of data, or {@code -1} if the end of the
     *             stream has been reached.
     * @throws     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} 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} bytes have been read then it
     * reads from the underlying input stream. If {@code len} is not zero, the method
     * blocks until at least 1 byte of input is available; otherwise, no
     * bytes are read and {@code 0} is returned.
     *
     * @param      b     the buffer into which the data is read.
     * @param      off   the start offset in the destination array {@code b}
     * @param      len   the maximum number of bytes read.
     * @return     the total number of bytes read into the buffer, or
     *             {@code -1} if there is no more data because the end of
     *             the stream has been reached.
     * @throws     NullPointerException If {@code b} is {@code null}.
     * @throws     IndexOutOfBoundsException If {@code off} is negative,
     *             {@code len} is negative, or {@code len} is greater than
     *             {@code b.length - off}
     * @throws     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}.
     *
     * @param      b   the {@code 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 {@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:
  • NullPointerException – If b is null.
  • 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]}, the byte after that will
     * have the value {@code b[off+1]}, 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.
     * @throws    NullPointerException If {@code b} is {@code null}.
     * @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 {@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:
  • NullPointerException – If b is null.
  • 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]}, the byte after that will have the
     * value {@code b[1]}, and so forth.
     *
     * @param     b the byte array to push back
     * @throws    NullPointerException If {@code b} is {@code null}.
     * @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 {@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.
     * @throws     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} bytes of data from this
     * input stream. The {@code skip} method may, for a variety of
     * reasons, end up skipping over some smaller number of bytes,
     * possibly zero.  If {@code n} is negative, no bytes are skipped.
     *
     * <p> The {@code skip} method of {@code PushbackInputStream}
     * first skips over the bytes in the pushback buffer, if any.  It then
     * calls the {@code skip} 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} and
     * {@code reset} methods, which it does not.
     *
     * @return   {@code false}, since this class does not support the
     *           {@code mark} and {@code reset} 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} method of {@code PushbackInputStream}
     * 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:
See Also:
/**
     * Repositions this stream to the position at the time the
     * {@code mark} method was last called on this input stream.
     *
     * <p> The method {@code reset} for class
     * {@code PushbackInputStream} does nothing except throw an
     * {@code IOException}.
     *
     * @throws  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.
     *
     * @throws     IOException  if an I/O error occurs.
     */
    public synchronized void close() throws IOException {
        if (in == null)
            return;
        in.close();
        in = null;
        buf = null;
    }
}