PushbackInputStream (Java Platform SE 8 )
Keywords: java.io.PushbackInputStream class
Date: 2026-07-22

compact1, compact2, compact3
java.io

## Class PushbackInputStream

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

- All Implemented Interfaces:
    Closeable, AutoCloseable

      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

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `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

    Constructors
    Constructor and Description
    `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

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `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.
    `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.
    `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)` 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

      `read`

    - ### 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 <= 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 this input stream has been closed by invoking its `close()` method, or 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 `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.
      Overrides:
        `read` in class `FilterInputStream`
      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)`

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

    - #### 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, or this input stream has been closed by invoking its `close()` method.
      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, or this input stream has been closed by invoking its `close()` method.
      Since:
        JDK1.1

    - #### available

          public int available()
                        throws IOException

      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`.

      Overrides:
        `available` in class `FilterInputStream`
      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()`

    - #### 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 the stream does not support seek, or the stream has been closed by invoking its `close()` method, or 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()`

    - #### mark

          public void mark(int readlimit)

      Marks the current position in this input stream.

      The `mark` method of `PushbackInputStream` does nothing.

      Overrides:
        `mark` in class `FilterInputStream`
      Parameters:
        `readlimit` - the maximum limit of bytes that can be read before the mark position becomes invalid.
      See Also:
        `InputStream.reset()`

    - #### reset

          public void reset()
                     throws IOException

      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`.

      Overrides:
        `reset` in class `FilterInputStream`
      Throws:
        `IOException` - if this method is invoked.
      See Also:
        `InputStream.mark(int)`, `IOException`

    - #### close

          public void close()
                     throws IOException

      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.
      Specified by:
        `close` in interface `Closeable`
      Specified by:
        `close` in interface `AutoCloseable`
      Overrides:
        `close` in class `FilterInputStream`
      Throws:
        `IOException` - if an I/O error occurs.
      See Also:
        `FilterInputStream.in`

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