PushbackInputStream

AI-generated Key Takeaways

• PushbackInputStream allows "unreading" or pushing back bytes into the input stream for later processing.

• It extends FilterInputStream and uses an internal buffer to store pushed-back bytes.

• Key methods include read(), unread(), and available(), which handle reading, pushing back, and checking available bytes, respectively.

• PushbackInputStream does not support marking and will throw an IOException if mark(), reset(), or markSupported() are called.

• It's useful for scenarios like parsing data streams where you might need to examine a byte without fully consuming it from the stream.

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.

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.

Inherited Field Summary

From class java.io.FilterInputStream

protected InputStream

in

The input stream to be filtered.

Public Constructor Summary

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

Public Method Summary

int	available() 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.
synchronized void	close() Closes this input stream and releases any system resources associated with the stream.
synchronized void	mark(int readlimit) Marks the current position in this input 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.
synchronized void	reset() Repositions this stream to the position at the time the mark method was last called on this input stream.
long	skip(long n) Skips over and discards n bytes of data from this input stream.
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.
void	unread(byte[] b) Pushes back an array of bytes by copying it to the front of the pushback buffer.

Inherited Method Summary

From class java.io.FilterInputStream

int	available() Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next caller of a method for this input stream.
void	close() Closes this input stream and releases any system resources associated with the stream.
synchronized void	mark(int readlimit) Marks the current position in this input stream.
boolean	markSupported() Tests if this input stream supports the mark and reset methods.
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.
int	read(byte[] b) Reads up to byte.length bytes of data from this input stream into an array of bytes.
synchronized void	reset() Repositions this stream to the position at the time the mark method was last called on this input stream.
long	skip(long n) Skips over and discards n bytes of data from the input stream.

From class java.io.InputStream

int	available() 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.
void	close() Closes this input stream and releases any system resources associated with the stream.
synchronized void	mark(int readlimit) Marks the current position in this input stream.
boolean	markSupported() Tests if this input stream supports the mark and reset methods.
abstract int	read() Reads the next byte of data from the input stream.
int	read(byte[] b, int off, int len) Reads up to len bytes of data from the input stream into an array of bytes.
int	read(byte[] b) Reads some number of bytes from the input stream and stores them into the buffer array b.
synchronized void	reset() Repositions this stream to the position at the time the mark method was last called on this input stream.
long	skip(long n) Skips over and discards n bytes of data from this input stream.

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.io.Closeable

abstract void

close() Closes this stream and releases any system resources associated with it.

From interface java.lang.AutoCloseable

abstract void

close() Closes this resource, relinquishing any underlying resources.

Fields

protected byte[] buf

The pushback buffer.

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.

Public Constructors

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 <= 0

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.

Public Methods

public int available ()

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 available.

Returns

• the number of bytes that can be read (or skipped over) from the input stream without blocking.

Throws

IOException	if this input stream has been closed by invoking its close() method, or an I/O error occurs.

See Also

• FilterInputStream.in
• InputStream.available()

public synchronized void close ()

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.

Throws

IOException	if an I/O error occurs.

public synchronized void mark (int readlimit)

Marks the current position in this input stream.

The mark method of PushbackInputStream does nothing.

Parameters

readlimit	the maximum limit of bytes that can be read before the mark position becomes invalid.

See Also

• InputStream.reset()

public boolean markSupported ()

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

Returns

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

See Also

• InputStream.mark(int)
• InputStream.reset()

public int read ()

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.

Returns

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

Throws

IOException	if this input stream has been closed by invoking its close() method, or an I/O error occurs.

See Also

• InputStream.read()

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

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.

Parameters

b	the buffer into which the data is read.
off	the start offset in the destination array b
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

NullPointerException	If b is null.
IndexOutOfBoundsException	If off is negative, len is negative, or len is greater than b.length - off
IOException	if this input stream has been closed by invoking its close() method, or an I/O error occurs.

See Also

• InputStream.read(byte[], int, int)

public synchronized void reset ()

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.

Throws

IOException	if this method is invoked.

See Also

• InputStream.mark(int)
• IOException

public long skip (long n)

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.

Parameters

n

Returns

• the actual number of bytes skipped.

Throws

IOException	if the stream does not support seek, or the stream has been closed by invoking its close() method, or an I/O error occurs.

See Also

• FilterInputStream.in
• InputStream.skip(long)

public 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. 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, or this input stream has been closed by invoking its close() method.

public void unread (int b)

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, or this input stream has been closed by invoking its close() method.

public void unread (byte[] b)

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, or this input stream has been closed by invoking its close() method.