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Android Reference

PushbackInputStream


java.io

Class PushbackInputStream

  • All Implemented Interfaces:
    Closeable, AutoCloseable


    public class PushbackInputStream
    extends FilterInputStream
    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.
    • Field Summary

      Fields
      Modifier and Type Field and Description
      protected byte[] buf
      The buffer that contains pushed-back bytes.
      protected int pos
      The current position within buf.
    • Method Summary

      Methods
      Modifier and Type Method and Description
      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 byteCount)
      Skips byteCount bytes in this stream.
      void unread(byte[] buffer)
      Pushes all the bytes in buffer back to this stream.
      void unread(byte[] buffer, int offset, int length)
      Pushes a subset of the bytes in buffer back to this stream.
      void unread(int oneByte)
      Pushes the specified byte oneByte back to this stream.
    • Field Detail

      • buf

        protected byte[] buf
        The buffer that contains pushed-back bytes.
      • pos

        protected int pos
        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.
    • Constructor Detail

      • PushbackInputStream

        public PushbackInputStream(InputStream in)
        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.
      • PushbackInputStream

        public PushbackInputStream(InputStream in,
                           int size)
        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.
    • Method Detail

      • available

        public int available()
                      throws IOException
        Description copied from class: InputStream
        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, 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.

        Overrides:
        available in class FilterInputStream
        Returns:
        the estimated number of bytes available
        Throws:
        IOException - if this stream is closed or an error occurs
      • markSupported

        public boolean markSupported()
        Indicates whether this stream supports the mark(int) and reset() methods. PushbackInputStream does not support them, so it returns false.
        Overrides:
        markSupported in class FilterInputStream
        Returns:
        always false.
        See Also:
        mark(int), reset()
      • read

        public int read()
                 throws IOException
        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.
        Overrides:
        read in class FilterInputStream
        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.
      • read

        public int read(byte[] buffer,
               int offset,
               int length)
                 throws IOException
        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.
        Overrides:
        read in class FilterInputStream
        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.
      • skip

        public long skip(long byteCount)
                  throws IOException
        Skips byteCount bytes in this stream. This implementation skips bytes in the pushback buffer first and then in the source stream if necessary.
        Overrides:
        skip in class FilterInputStream
        Parameters:
        byteCount - 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.
        See Also:
        FilterInputStream.mark(int), FilterInputStream.reset()
      • unread

        public void unread(byte[] buffer)
                    throws IOException
        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.
      • unread

        public void unread(byte[] buffer,
                  int offset,
                  int length)
                    throws IOException
        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.
      • unread

        public void unread(int oneByte)
                    throws IOException
        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.
      • mark

        public void mark(int readlimit)
        Marks the current position in this stream. Setting a mark is not supported in this class; this implementation does nothing.
        Overrides:
        mark in class FilterInputStream
        Parameters:
        readlimit - the number of bytes that can be read from this stream before the mark is invalidated; this parameter is ignored.
        See Also:
        FilterInputStream.markSupported(), FilterInputStream.reset()


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