java.io

Class PushbackInputStream

public class PushbackInputStream extends FilterInputStream

This subclass of FilterInputStream 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 byte, but this can be overridden by the creator of the stream.

Field Summary
protected byte[]	buf This is the buffer that is used to store the pushed back data
protected int	pos This is the position in the buffer from which the next byte will be read.

Constructor Summary
	PushbackInputStream(InputStream in) This method initializes a `PushbackInputStream` to read from the specified subordinate `InputStream` with a default pushback buffer size of 1.
	PushbackInputStream(InputStream in, int size) This method initializes a `PushbackInputStream` to read from the specified subordinate `InputStream` with the specified buffer size

Method Summary
int	available() This method returns the number of bytes that can be read from this stream before a read can block.
void	close() This method closes the stream and releases any associated resources.
boolean	markSupported() This method returns `false` to indicate that it does not support mark/reset functionality.
int	read() This method reads an unsigned byte from the input stream and returns it as an int in the range of 0-255.
int	read(byte[] b, int off, int len) This method read bytes from a stream and stores them into a caller supplied buffer.
void	reset() This method always throws an IOException in this class because mark/reset functionality is not supported.
long	skip(long n) This method skips the specified number of bytes in the stream.
void	unread(int b) This method pushes a single byte of data into the pushback buffer.
void	unread(byte[] b) This method pushes all of the bytes in the passed byte array into the pushback bfer.
void	unread(byte[] b, int off, int len) This method pushed back bytes from the passed in array into the pushback buffer.

Field Detail

buf

protected byte[] buf

This is the buffer that is used to store the pushed back data

pos

protected int pos

This is the position in the buffer from which the next byte 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

Constructor Detail

PushbackInputStream

public PushbackInputStream(InputStream in)

This method initializes a PushbackInputStream to read from the specified subordinate InputStream with a default pushback buffer size of 1.

Parameters: in The subordinate stream to read from

PushbackInputStream

public PushbackInputStream(InputStream in, int size)

This method initializes a PushbackInputStream to read from the specified subordinate InputStream with the specified buffer size

Parameters: in The subordinate InputStream to read from size The pushback buffer size to use

Method Detail

available

public int available()

This method returns the number of bytes that can be read from this stream before a read can block. A return of 0 indicates that blocking might (or might not) occur on the very next read attempt.

This method will return the number of bytes available from the pushback buffer plus the number of bytes available from the underlying stream.

Returns: The number of bytes that can be read before blocking could occur

Throws: IOException If an error occurs

close

public void close()

This method closes the stream and releases any associated resources.

Throws: IOException If an error occurs.

markSupported

public boolean markSupported()

This method returns false to indicate that it does not support mark/reset functionality.

Returns: This method returns false to indicate that this class does not support mark/reset functionality

read

public int read()

This method reads an unsigned byte from the input stream and returns it as an int in the range of 0-255. This method also will return -1 if the end of the stream has been reached. The byte returned will be read from the pushback buffer, unless the buffer is empty, in which case the byte will be read from the underlying stream.

This method will block until the byte can be read.

Returns: The byte read or -1 if end of stream

Throws: IOException If an error occurs

read

public int read(byte[] b, int off, int len)

This method read bytes 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 bytes. This method can return before reading the number of bytes requested. The actual number of bytes 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 bytes from the pushback buffer in order to satisfy the read request. If the pushback buffer cannot provide all of the bytes requested, the remaining bytes are read from the underlying stream.

Parameters: b The array into which the bytes read should be stored off The offset into the array to start storing bytes len The requested number of bytes to read

Returns: The actual number of bytes read, or -1 if end of stream.

Throws: IOException If an error occurs.

reset

public void reset()

This method always throws an IOException in this class because mark/reset functionality is not supported.

Throws: IOException Always thrown for this class

skip

public long skip(long n)

This method skips the specified number of bytes in the stream. It returns the actual number of bytes skipped, which may be less than the requested amount.

This method first discards bytes from the buffer, then calls the skip method on the underlying InputStream to skip additional bytes if necessary.

Parameters: n The requested number of bytes to skip

Returns: The actual number of bytes skipped.

Throws: IOException If an error occurs

Since: 1.2

unread

public void unread(int b)

This method pushes a single byte of data into the pushback buffer. The byte pushed back is the one that will be returned as the first byte 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.

Parameters: b The byte to be pushed back, passed as an int

Throws: IOException If the pushback buffer is full.

unread

public void unread(byte[] b)

This method pushes all of the bytes in the passed byte array into the pushback bfer. These bytes are pushed in reverse order so that the next byte read from the stream after this operation will be b[0] followed by b[1], etc.

If the pushback buffer cannot hold all of the requested bytes, an exception is thrown.

Parameters: b The byte array to be pushed back

Throws: IOException If the pushback buffer is full

unread

public void unread(byte[] b, int off, int len)

This method pushed back bytes from the passed in array into the pushback buffer. The bytes from b[offset] to b[offset + len] are pushed in reverse order so that the next byte read from the stream after this operation will be b[offset] followed by b[offset + 1], etc.

If the pushback buffer cannot hold all of the requested bytes, an exception is thrown.

Parameters: b The byte array to be pushed back off The index into the array where the bytes to be push start len The number of bytes to be pushed.

Throws: IOException If the pushback buffer is full