[Chapter 11] PushbackReader

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| Chapter 11 The java.io Package |

# PushbackReader

## Name

PushbackReader

## Synopsis

Class Name:

  `java.io.PushbackReader`

Superclass:

  `java.io.FilterReader`

Immediate Subclasses:

  None

Interfaces Implemented:

  None

Availability:

  New as of JDK 1.1

## Description

The `PushbackReader` class represents a character stream that allows data to be pushed back into the stream. In other words, after data has been read from a `PushbackReader`, it can be pushed back into the stream so that it can be reread. This functionality is useful for implementing things like parsers that need to read data and then return it to the input stream. `PushbackReader` is the character-oriented equivalent of `PushbackInputStream`.

## Class Summary

    public class java.io.PushbackReader extends java.io.FilterReader {
      // Constructors
      public PushbackReader(Reader in);
      public PushbackReader(Reader in, int size);
      // Instance Methods
      public void close();
      public boolean markSupported();
      public int read();
      public int read(char[] cbuf, int off, int len);
      public boolean ready();
      public void unread(int c);
      public void unread(char[] cbuf);
      public void unread(char[] cbuf, int off, int len);
    }

## Constructors

### PushbackReader

#### public PushbackReader(Reader in)

Parameters

  `in`

    The reader to wrap.

Description

  This constructor creates a `PushbackReader` that reads from the given `Reader`, using a pushback buffer with the default size of one byte.

#### public PushbackReader(Reader in, int size)

Parameters

  `in`

    The reader to wrap.

  `size`

    The size of the pushback buffer.

Description

  This constructor creates a `PushbackReader` that reads from the given `Reader`, using a pushback buffer of the given size.

## Instance Methods

### close

#### public void close() throws IOException

Throws

  `IOException`

    If any kind of I/O error occurs.

Overrides

  `FilterReader.close()`

Description

  This method closes the reader and releases the system resources that are associated with it.

### markSupported

#### public boolean markSupported()

Returns

  The `boolean` value `false`.

Overrides

  `FilterReader.markSupported()`

Description

  This method returns `false` to indicate that this class does not support `mark()` and `reset()`.

### read

#### public int read() throws IOException

Returns

  The next character of data or `-1` if the end of the stream is encountered.

Throws

  `IOException`

    If any kind of I/O error occurs.

Overrides

  `FilterReader.read()`

Description

  This method reads a character of data. If there is any data in the pushback buffer, the method returns the next character in the pushback buffer. Otherwise, it calls the `read()` method of the underlying stream. The method blocks until the character is read, the end of the stream is encountered, or an exception is thrown.

#### public int read(char[] cbuf, int off, int len) throws IOException

Parameters

  `cbuf`

    An array of characters to be filled from the stream.

  `off`

    An offset into the array.

  `len`

    The number of characters to read.

Returns

  The actual number of characters read or `-1` if the end of the stream is encountered immediately.

Throws

  `IOException`

    If any kind of I/O error occurs.

Overrides

  `FilterReader.read(char[], int, int)`

Description

  This method copies characters from the stream into the given array `cbuf`, starting at index `off` and continuing for `len` characters. If the array can be populated solely from the pushback buffer, the method returns immediately. Otherwise, the `read(char[],` `int,` `int)` method of the underlying stream is called to make up the difference. The method blocks until some data is available.

### ready

#### public boolean ready() throws IOException

Returns

  A `boolean` value that indicates whether the stream is ready to be read.

Throws

  `IOException`

    If the stream is closed.

Overrides

  `FilterReader.ready()`

Description

  If there is data in the pushback buffer, or if the underlying stream is ready, this method returns `true`. The underlying stream is ready if the next `read()` is guaranteed not to block.

### unread

#### public void unread(int c) throws IOException

Parameters

  `c`

    The value to push back.

Throws

  `IOException`

    If the pushback buffer is full.

Description

  This method puts the given character into the pushback buffer.

#### public void unread(char[] cbuf) throws IOException

Parameters

  `cbuf`

    An array of characters to push back.

Throws

  `IOException`

    If the pushback buffer is full.

Description

  This method puts all of the characters in the given array into the pushback buffer.

#### public void unread(char[] cbuf, int off, int len) throws IOException

Parameters

  `cbuf`

    An array of characters to push back.

  `off`

    An offset into the array.

  `len`

    The number of characters to push back.

Throws

  `IOException`

    If the pushback buffer is full.

Description

  This method puts `len` characters from the given array, starting at offset `off`, into the pushback buffer.

## Inherited Methods

Method | Inherited From | Method | Inherited From
--- | --- | --- | ---
`clone()` | `Object` | `equals(Object)` | `Object`
`finalize()` | `Object` | `getClass()` | `Object`
`hashCode()` | `Object` | `mark(int)` | `FilterReader`
`notify()` | `Object` | `notifyAll()` | `Object`
`read(char[])` | `FilterReader` | `reset()` | `FilterReader`
`skip(long)` | `FilterReader` | `toString()` | `Object`
`wait()` | `Object` | `wait(long)` | `Object`
`wait(long, int)` | `Object` |  |

## See Also

`FilterReader`, `IOException`, `Reader`

PushbackInputStream |  | RandomAccessFile