java.io
Class PushbackInputStream

java.lang.Object
  |
  +--java.io.InputStream
        |
        +--java.io.FilterInputStream
              |
              +--java.io.PushbackInputStream

public class PushbackInputStream

extends FilterInputStream

A 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.

Since:

JDK1.0

Field Summary

protected byte[]	buf The pushback buffer.
protected int	pos The position within the pushback buffer from which the next byte will be read.

Fields inherited from class java.io.FilterInputStream

in

Constructor Summary

PushbackInputStream(InputStream in)
Creates a PushbackInputStream and saves its argument, the input stream in, for later use.

PushbackInputStream(InputStream in, int size)
Creates a PushbackInputStream with a pushback buffer of the specified size, and saves its argument, the input stream in, for later use.

Method Summary

int	available() Returns the number of bytes that can be read from this input stream without blocking.
void	close() Closes this input stream and releases any system resources associated with the stream.
boolean	markSupported() Tests if this input stream supports the mark and reset methods, which it does not.
int	read() Reads the next byte of data from this input stream.
int	read(byte[] b, int off, int len) Reads up to len bytes of data from this input stream into an array of bytes.
long	skip(long n) Skips over and discards n bytes of data from this input stream.
void	unread(byte[] b) Pushes back an array of bytes by copying it to the front of the pushback buffer.
void	unread(byte[] b, int off, int len) Pushes back a portion of an array of bytes by copying it to the front of the pushback buffer.
void	unread(int b) Pushes back a byte by copying it to the front of the pushback buffer.

Methods inherited from class java.io.FilterInputStream

mark, read, reset

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

buf

protected byte[] buf

The pushback buffer.

Since:

JDK1.1

pos

protected int pos

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

Constructor Detail

PushbackInputStream

public PushbackInputStream(InputStream in,
                           int size)

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).

Parameters:

in - the input stream from which bytes will be read.

size - the size of the pushback buffer.

Throws:

IllegalArgumentException - if size is <= 0

Since:

JDK1.1

PushbackInputStream

public PushbackInputStream(InputStream in)

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).

Parameters:

in - the input stream from which bytes will be read.

Method Detail

read

public int read()
         throws IOException

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.

Overrides:

read in class FilterInputStream

Returns:

the next byte of data, or -1 if the end of the stream has been reached.

Throws:

IOException - if an I/O error occurs.

See Also:

InputStream.read()

read

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

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 than len bytes have been read then it reads from the underlying input stream. This method blocks until at least 1 byte of input is available.

Overrides:

read in class FilterInputStream

Parameters:

b - the buffer into which the data is read.

off - the start offset of the data.

len - the maximum number of bytes read.

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.

Throws:

IOException - if an I/O error occurs.

See Also:

InputStream.read(byte[], int, int)

unread

public void unread(int b)
            throws IOException

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.

Parameters:

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.

unread

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

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.

Parameters:

b - the byte array to push back.

off - the start offset of the data.

len - the number of bytes to push back.

Throws:

IOException - If there is not enough room in the pushback buffer for the specified number of bytes.

Since:

JDK1.1

unread

public void unread(byte[] b)
            throws IOException

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.

Parameters:

b - the byte array to push back

Throws:

IOException - If there is not enough room in the pushback buffer for the specified number of bytes.

Since:

JDK1.1

available

public int available()
              throws IOException

Returns the number of bytes that can be read from this input stream without blocking. This method calls the available method of the underlying input stream; it returns that value plus the number of bytes that have been pushed back.

Overrides:

available in class FilterInputStream

Returns:

the number of bytes that can be read from the input stream without blocking.

Throws:

IOException - if an I/O error occurs.

See Also:

FilterInputStream.in, InputStream.available()

skip

public long skip(long n)
          throws IOException

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.

Overrides:

skip in class FilterInputStream

Parameters:

n - the number of bytes to be skipped.

Returns:

the actual number of bytes skipped.

Throws:

IOException - if an I/O error occurs.

Since:

1.2

See Also:

FilterInputStream.in, InputStream.skip(long n)

markSupported

public boolean markSupported()

Tests if this input stream supports the mark and reset methods, which it does not.

Overrides:

markSupported in class FilterInputStream

Returns:

false, since this class does not support the mark and reset methods.

See Also:

InputStream.mark(int), InputStream.reset()

close

public void close()
           throws IOException

Closes this input stream and releases any system resources associated with the stream.

Overrides:

close in class FilterInputStream

Throws:

IOException - if an I/O error occurs.