Java tutorial
/* * Copyright (c) 1994, 2016, 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 <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 */ protected byte[] buf; /** * 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 */ private void ensureOpen() throws IOException { if (in == null) throw new IOException("Stream closed"); } /** * 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 <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 <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 <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 * <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 <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 <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. * * <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 <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 <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. * * <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 * <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. * * @exception IOException if an I/O error occurs. */ public synchronized void close() throws IOException { if (in == null) return; in.close(); in = null; buf = null; } }