public class

PushbackInputStream

extends FilterInputStream
java.lang.Object
   ↳ java.io.InputStream
     ↳ java.io.FilterInputStream
       ↳ java.io.PushbackInputStream

Class Overview

Wraps an existing InputStream and adds functionality to "push back" bytes that have been read, so that they can be read again. Parsers may find this useful. The number of bytes which may be pushed back can be specified during construction. If the buffer of pushed back bytes is empty, bytes are read from the underlying input stream.

Summary

Fields
protected byte[] buf The buffer that contains pushed-back bytes.
protected int pos The current position within buf.
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Inherited Fields
From class java.io.FilterInputStream
Public Constructors
PushbackInputStream(InputStream in)
Constructs a new PushbackInputStream with the specified input stream as source.
PushbackInputStream(InputStream in, int size)
Constructs a new PushbackInputStream with in as source input stream.
Public Methods
int available()
Returns an estimated number of bytes that can be read or skipped without blocking for more input.
void close()
Closes this stream.
void mark(int readlimit)
Marks the current position in this stream.
boolean markSupported()
Indicates whether this stream supports the mark(int) and reset() methods.
int read()
Reads a single byte from this stream and returns it as an integer in the range from 0 to 255.
int read(byte[] buffer, int offset, int length)
Reads at most length bytes from this stream and stores them in the byte array buffer starting at offset.
void reset()
Resets this stream to the last marked position.
long skip(long count)
Skips count bytes in this stream.
void unread(byte[] buffer, int offset, int length)
Pushes a subset of the bytes in buffer back to this stream.
void unread(byte[] buffer)
Pushes all the bytes in buffer back to this stream.
void unread(int oneByte)
Pushes the specified byte oneByte back to this stream.
[Expand]
Inherited Methods
From class java.io.FilterInputStream
From class java.io.InputStream
From class java.lang.Object
From interface java.io.Closeable

Fields

protected byte[] buf

Since: API Level 1

The buffer that contains pushed-back bytes.

protected int pos

Since: API Level 1

The current position within buf. A value equal to buf.length indicates that no bytes are available. A value of 0 indicates that the buffer is full.

Public Constructors

public PushbackInputStream (InputStream in)

Since: API Level 1

Constructs a new PushbackInputStream with the specified input stream as source. The size of the pushback buffer is set to the default value of 1 byte.

Warning: passing a null source creates an invalid PushbackInputStream. All read operations on such a stream will fail.

Parameters
in the source input stream.

public PushbackInputStream (InputStream in, int size)

Since: API Level 1

Constructs a new PushbackInputStream with in as source input stream. The size of the pushback buffer is set to size.

Warning: passing a null source creates an invalid PushbackInputStream. All read operations on such a stream will fail.

Parameters
in the source input stream.
size the size of the pushback buffer.
Throws
IllegalArgumentException if size is negative.

Public Methods

public int available ()

Since: API Level 1

Returns an estimated number of bytes that can be read or skipped without blocking for more input.

Note that this method provides such a weak guarantee that it is not very useful in practice.

Firstly, the guarantee is "without blocking for more input" rather than "without blocking": a read may still block waiting for I/O to complete — the guarantee is merely that it won't have to wait indefinitely for data to be written. The result of this method should not be used as a license to do I/O on a thread that shouldn't be blocked.

Secondly, the result is a conservative estimate and may be significantly smaller than the actual number of bytes available. In particular, an implementation that always returns 0 would be correct. In general, callers should only use this method if they'd be satisfied with treating the result as a boolean yes or no answer to the question "is there definitely data ready?".

Thirdly, the fact that a given number of bytes is "available" does not guarantee that a read or skip will actually read or skip that many bytes: they may read or skip fewer.

It is particularly important to realize that you must not use this method to size a container and assume that you can read the entirety of the stream without needing to resize the container. Such callers should probably write everything they read to a ByteArrayOutputStream and convert that to a byte array. Alternatively, if you're reading from a file, length() returns the current length of the file (though assuming the file's length can't change may be incorrect, reading a file is inherently racy).

The default implementation of this method in InputStream always returns 0. Subclasses should override this method if they are able to indicate the number of bytes available.

Returns
  • the estimated number of bytes available
Throws
IOException

public void close ()

Since: API Level 1

Closes this stream. This implementation closes the source stream and releases the pushback buffer.

Throws
IOException if an error occurs while closing this stream.

public void mark (int readlimit)

Since: API Level 1

Marks the current position in this stream. Setting a mark is not supported in this class; this implementation does nothing.

Parameters
readlimit the number of bytes that can be read from this stream before the mark is invalidated; this parameter is ignored.

public boolean markSupported ()

Since: API Level 1

Indicates whether this stream supports the mark(int) and reset() methods. PushbackInputStream does not support them, so it returns false.

Returns
  • always false.
See Also

public int read ()

Since: API Level 1

Reads a single byte from this stream and returns it as an integer in the range from 0 to 255. If the pushback buffer does not contain any available bytes then a byte from the source input stream is returned. Blocks until one byte has been read, the end of the source stream is detected or an exception is thrown.

Returns
  • the byte read or -1 if the end of the source stream has been reached.
Throws
IOException if this stream is closed or an I/O error occurs while reading from this stream.

public int read (byte[] buffer, int offset, int length)

Since: API Level 1

Reads at most length bytes from this stream and stores them in the byte array buffer starting at offset. Bytes are read from the pushback buffer first, then from the source stream if more bytes are required. Blocks until count bytes have been read, the end of the source stream is detected or an exception is thrown.

Parameters
buffer the array in which to store the bytes read from this stream.
offset the initial position in buffer to store the bytes read from this stream.
length the maximum number of bytes to store in buffer.
Returns
  • the number of bytes read or -1 if the end of the source stream has been reached.
Throws
IndexOutOfBoundsException if offset < 0 or length < 0, or if offset + length is greater than the length of buffer.
IOException if this stream is closed or another I/O error occurs while reading from this stream.
NullPointerException if buffer is null.

public void reset ()

Since: API Level 1

Resets this stream to the last marked position. Resetting the stream is not supported in this class; this implementation always throws an IOException.

Throws
IOException if this method is called.

public long skip (long count)

Since: API Level 1

Skips count bytes in this stream. This implementation skips bytes in the pushback buffer first and then in the source stream if necessary.

Parameters
count the number of bytes to skip.
Returns
  • the number of bytes actually skipped.
Throws
IOException if this stream is closed or another I/O error occurs.

public void unread (byte[] buffer, int offset, int length)

Since: API Level 1

Pushes a subset of the bytes in buffer back to this stream. The subset is defined by the start position offset within buffer and the number of bytes specified by length. The bytes are pushed back in such a way that the next byte read from this stream is buffer[offset], then buffer[1] and so on.

If this stream's internal pushback buffer cannot store the selected subset of buffer, an IOException is thrown. Parts of buffer may have already been copied to the pushback buffer when the exception is thrown.

Parameters
buffer the buffer containing the bytes to push back to this stream.
offset the index of the first byte in buffer to push back.
length the number of bytes to push back.
Throws
IndexOutOfBoundsException if offset < 0 or length < 0, or if offset + length is greater than the length of buffer.
IOException if the free space in the internal pushback buffer is not sufficient to store the selected contents of buffer.

public void unread (byte[] buffer)

Since: API Level 1

Pushes all the bytes in buffer back to this stream. The bytes are pushed back in such a way that the next byte read from this stream is buffer[0], then buffer[1] and so on.

If this stream's internal pushback buffer cannot store the entire contents of buffer, an IOException is thrown. Parts of buffer may have already been copied to the pushback buffer when the exception is thrown.

Parameters
buffer the buffer containing the bytes to push back to this stream.
Throws
IOException if the free space in the internal pushback buffer is not sufficient to store the contents of buffer.

public void unread (int oneByte)

Since: API Level 1

Pushes the specified byte oneByte back to this stream. Only the least significant byte of the integer oneByte is pushed back. This is done in such a way that the next byte read from this stream is (byte) oneByte.

If this stream's internal pushback buffer cannot store the byte, an IOException is thrown.

Parameters
oneByte the byte to push back to this stream.
Throws
IOException if this stream is closed or the internal pushback buffer is full.