FilterReader provides the ability to
* unread data from a stream. It maintains an internal buffer of unread
* data that is supplied to the next read operation. This is conceptually
* similar to mark/reset functionality, except that in this case the
* position to reset the stream to does not need to be known in advance.
*
* The default pushback buffer size one char, but this can be overridden
* by the creator of the stream.
*
* @author Aaron M. Renn (arenn@urbanophile.com)
* @author Warren Levy (warrenl@cygnus.com)
*/
public class PushbackReader extends FilterReader
{
/**
* This is the default buffer size
*/
private static final int DEFAULT_BUFFER_SIZE = 1;
/**
* This is the buffer that is used to store the pushed back data
*/
private char[] buf;
/**
* This is the position in the buffer from which the next char will be
* read. Bytes are stored in reverse order in the buffer, starting from
* buf[buf.length - 1] to buf[0]. Thus when
* pos is 0 the buffer is full and buf.length when
* it is empty
*/
private int pos;
/**
* This method initializes a PushbackReader to read from the
* specified subordinate Reader with a default pushback buffer
* size of 1.
*
* @param in The subordinate stream to read from
*/
public PushbackReader(Reader in)
{
this(in, DEFAULT_BUFFER_SIZE);
}
/**
* This method initializes a PushbackReader to read from the
* specified subordinate Reader with the specified buffer
* size
*
* @param in The subordinate Reader to read from
* @param bufsize The pushback buffer size to use
*/
public PushbackReader(Reader in, int bufsize)
{
super(in);
if (bufsize < 0)
throw new IllegalArgumentException("buffer size must be positive");
buf = new char[bufsize];
pos = bufsize;
}
/**
* This method closes the stream and frees any associated resources.
*
* @exception IOException If an error occurs.
*/
public void close() throws IOException
{
synchronized (lock)
{
buf = null;
super.close();
}
}
/**
* This method throws an exception when called since this class does
* not support mark/reset.
*
* @param read_limit Not used.
*
* @exception IOException Always thrown to indicate mark/reset not supported.
*/
public void mark(int read_limit) throws IOException
{
throw new IOException("mark not supported in this class");
}
/**
* This method returns false to indicate that it does not support
* mark/reset functionality.
*
* @return This method returns false to indicate that this
* class does not support mark/reset functionality
*
*/
public boolean markSupported()
{
return(false);
}
/**
* This method always throws an IOException in this class because
* mark/reset functionality is not supported.
*
* @exception IOException Always thrown for this class
*/
public void reset() throws IOException
{
throw new IOException("reset not supported in this class");
}
/**
* This method determines whether or not this stream is ready to be read.
* If it returns false to indicate that the stream is not
* ready, any attempt to read from the stream could (but is not
* guaranteed to) block.
*
* This stream is ready to read if there are either chars waiting to be
* read in the pushback buffer or if the underlying stream is ready to
* be read.
*
* @return true if this stream is ready to be read,
* false otherwise
*
* @exception IOException If an error occurs
*/
public boolean ready() throws IOException
{
synchronized (lock)
{
if (buf == null)
throw new IOException ("stream closed");
if (((buf.length - pos) > 0) || super.ready())
return(true);
else
return(false);
}
}
// Don't delete this method just because the spec says it shouldn't be there!
// See the CVS log for details.
/**
* This method skips the specified number of chars in the stream. It
* returns the actual number of chars skipped, which may be less than the
* requested amount.
*
* This method first discards chars from the buffer, then calls the
* skip method on the underlying Reader to
* skip additional chars if necessary.
*
* @param num_chars The requested number of chars to skip
*
* @return The actual number of chars skipped.
*
* @exception IOException If an error occurs
*/
public long skip(long num_chars) throws IOException
{
synchronized (lock)
{
if (num_chars <= 0)
return(0);
if ((buf.length - pos) >= num_chars)
{
pos += num_chars;
return(num_chars);
}
int chars_discarded = buf.length - pos;
pos = buf.length;
long chars_skipped = in.skip(num_chars - chars_discarded);
return(chars_discarded + chars_skipped);
}
}
/**
* This method reads an unsigned char from the input stream and returns it
* as an int in the range of 0-65535. This method also will return -1 if
* the end of the stream has been reached. The char returned will be read
* from the pushback buffer, unless the buffer is empty, in which case
* the char will be read from the underlying stream.
*
* This method will block until the char can be read.
*
* @return The char read or -1 if end of stream
*
* @exception IOException If an error occurs
*/
public int read() throws IOException
{
synchronized (lock)
{
if (buf == null)
throw new IOException("stream closed");
if (pos == buf.length)
return(super.read());
++pos;
return((buf[pos - 1] & 0xFFFF));
}
}
/**
* This method read chars from a stream and stores them into a caller
* supplied buffer. It starts storing the data at index offset
* into
* the buffer and attempts to read len chars. This method can
* return before reading the number of chars requested. The actual number
* of chars read is returned as an int. A -1 is returned to indicate the
* end of the stream.
*
* This method will block until some data can be read. *
* This method first reads chars from the pushback buffer in order to * satisfy the read request. If the pushback buffer cannot provide all * of the chars requested, the remaining chars are read from the * underlying stream. * * @param buffer The array into which the chars read should be stored * @param offset The offset into the array to start storing chars * @param length The requested number of chars to read * * @return The actual number of chars read, or -1 if end of stream. * * @exception IOException If an error occurs. */ public synchronized int read(char[] buffer, int offset, int length) throws IOException { synchronized (lock) { if (buf == null) throw new IOException("stream closed"); if (offset < 0 || length < 0 || offset + length > buffer.length) throw new ArrayIndexOutOfBoundsException(); int numBytes = Math.min(buf.length - pos, length); if (numBytes > 0) { System.arraycopy (buf, pos, buffer, offset, numBytes); pos += numBytes; return numBytes; } return super.read(buffer, offset, length); } } /** * This method pushes a single char of data into the pushback buffer. * The char pushed back is the one that will be returned as the first char * of the next read. *
* If the pushback buffer is full, this method throws an exception. *
* The argument to this method is an int. Only the low eight
* bits of this value are pushed back.
*
* @param b The char to be pushed back, passed as an int
*
* @exception IOException If the pushback buffer is full.
*/
public void unread(int b) throws IOException
{
synchronized (lock)
{
if (buf == null)
throw new IOException("stream closed");
if (pos == 0)
throw new IOException("Pushback buffer is full");
--pos;
buf[pos] = (char)(b & 0xFFFF);
}
}
/**
* This method pushes all of the chars in the passed char array into
* the pushback buffer. These chars are pushed in reverse order so that
* the next char read from the stream after this operation will be
* buf[0] followed by buf[1], etc.
*
* If the pushback buffer cannot hold all of the requested chars, an
* exception is thrown.
*
* @param buf The char array to be pushed back
*
* @exception IOException If the pushback buffer is full
*/
public synchronized void unread(char[] buf) throws IOException
{
unread(buf, 0, buf.length);
}
/**
* This method pushed back chars from the passed in array into the pushback
* buffer. The chars from buf[offset] to
* buf[offset + len]
* are pushed in reverse order so that the next char read from the stream
* after this operation will be buf[offset] followed by
* buf[offset + 1], etc.
*
* If the pushback buffer cannot hold all of the requested chars, an * exception is thrown. * * @param buffer The char array to be pushed back * @param offset The index into the array where the chars to be push start * @param length The number of chars to be pushed. * * @exception IOException If the pushback buffer is full */ public synchronized void unread(char[] buffer, int offset, int length) throws IOException { synchronized (lock) { if (buf == null) throw new IOException("stream closed"); if (pos < length) throw new IOException("Pushback buffer is full"); // Note the order that these chars are being added is the opposite // of what would be done if they were added to the buffer one at a time. // See the Java Class Libraries book p. 1397. System.arraycopy(buffer, offset, buf, pos - length, length); // Don't put this into the arraycopy above, an exception might be thrown // and in that case we don't want to modify pos. pos -= length; } } }