PushbackInputStream adds
+ * functionality to another input stream, namely
+ * the ability to "push back" or "unread"
+ * one byte. 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 JDK1.0
+ */
+public
+class PushbackInputStream extends FilterInputStream {
+ /**
+ * The pushback buffer.
+ * @since JDK1.1
+ */
+ 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 JDK1.1
+ */
+ 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 PushbackInputStream
+ * with a pushback buffer of the specified size,
+ * and saves its argument, the input stream
+ * in, for later use. Initially,
+ * there is no pushed-back byte (the field
+ * pushBack is initialized to
+ * -1).
+ *
+ * @param in the input stream from which bytes will be read.
+ * @param size the size of the pushback buffer.
+ * @exception IllegalArgumentException if size is <= 0
+ * @since JDK1.1
+ */
+ 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
+ * and saves its argument, the input stream
+ * in, for later use. Initially,
+ * there is no pushed-back byte (the field
+ * pushBack is initialized to
+ * -1).
+ *
+ * @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.
+ *
+ * @return the next byte of data, or -1 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.
+ *
+ * @param b the buffer into which the data is read.
+ * @param off the start offset in the destination array b
+ * @param len the maximum number of bytes read.
+ * @return 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.
+ * @exception NullPointerException If b is null.
+ * @exception IndexOutOfBoundsException If off is negative,
+ * len is negative, or len is greater than
+ * b.length - off
+ * @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;
+ }
+ 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.
+ *
+ * @param b the int 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.
+ *
+ * @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 JDK1.1
+ */
+ 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;
+ 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.
+ *
+ * @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 JDK1.1
+ */
+ 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 {@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.
+ *
+ * @param n {@inheritDoc}
+ * @return {@inheritDoc}
+ * @exception IOException if the stream does not support seek,
+ * or the 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#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.
+ *
+ * @return false, since this class does not support the
+ * mark and 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.
+ *
+ * @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.
+ *
+ * @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;
+ }
+ static void arraycopy(byte[] value, int srcBegin, byte[] dst, int dstBegin, int count) {
+ while (count-- > 0) {
+ dst[dstBegin++] = value[srcBegin++];
+ }
+ }
+}