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Java Fundamental Classes Reference

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Reader

Name

Reader

Synopsis

Class Name:

java.io.Reader

Superclass:

java.lang.Object

Immediate Subclasses:

java.io.BufferedReader, java.io.CharArrayReader,

java.io.FilterReader, java.io.InputStreamReader,

java.io.PipedReader, java.io.StringReader

Interfaces Implemented:

None

Availability:

New as of JDK 1.1

Description

The Reader class is an abstract class that is the superclass of all classes that represent input character streams. Reader defines the basic input methods that all character streams provide. A similar hierarchy of classes, based around InputStream, deals with byte streams instead of character streams.

Reader is designed so that read() and read(char[]) both call read(char[], int, int). Thus, a subclass can simply override read(char[], int, int), and all of the read methods will work. Note that this is different from the design of InputStream, where the read() method is the catch-all method. The design of Reader is cleaner and more efficient.

Reader also defines a mechanism for marking a position in the stream and returning to it later, via the mark() and reset() methods. Another method, markSupported(), tells whether or not this mark-and-reset functionality is available in a particular subclass.

Class Summary

public abstract class java.io.Reader extends java.lang.Object {
  // Variables
  protected Object lock;
  // Constructors
  protected Reader();
  protected Reader(Object lock);
  // Instance Methods
  public abstract void close();
  public void mark(int readAheadLimit);
  public boolean markSupported();
  public int read();
  public int read(char[] cbuf);
  public abstract int read(char[] cbuf, int off, int len);
  public boolean ready();
  public void reset();
  public long skip(long n) throws IOException;
}

Variables

lock

protected Object lock

Description

The object used to synchronize operations on this Reader object. For efficiency's sake, a particular implementation of a character stream can choose to synchronize its operations on something other than instances of itself. Thus, any subclass should synchronize on the lock object, instead of using a synchronized method or the this object.

Constructors

Reader

protected Reader()

Description

This constructor creates a Reader that synchronizes on the Reader itself, or in other words, on the this object.

protected Reader(Object lock)

Parameters

lock

The object to use for synchronization.

Description

This constructor creates a Reader that synchronizes on the given object.

Instance Methods

close

public abstract void close() throws IOException

Throws

IOException

If any kind of I/O error occurs.

Description

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

A subclass of Reader must implement this method.

mark

public void mark(int readheadLimit) throws IOException

Parameters

readAheadLimit

The maximum number of characters that can be read before the saved position becomes invalid.

Throws

IOException

If any kind of I/O error occurs.

Description

This method tells this Reader object to remember its current position, so that the position can be restored by a call to the reset() method. The Reader can read readAheadLimit characters beyond the marked position before the mark becomes invalid.

The implementation of the mark() method in Reader simply throws an exception to indicate that the mark-and-reset functionality is not implemented. A subclass must override the method to provide the functionality.

markSupported

public boolean markSupported()

Returns

true if this reader supports mark() and reset(); false otherwise.

Description

This method returns a boolean value that indicates whether or not this object supports mark-and-reset functionality.

The markSupported() method in Reader always returns false. A subclass that implements the mark-and-reset functionality should override the method to return true.

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.

Description

This method reads the next character of input. The character is returned as an integer in the range 0x0000 to 0xFFFF. The method blocks until the character is read, the end of stream is encountered, or an exception is thrown.

The implementation of this method in Reader reads the character by calling read(cb, 0, 1), where cb is a character array, and returning cb[0]. Although it is not strictly necessary, a subclass that wants to provide efficient single-character reads should override this method.

public int read(char[] cbuf) throws IOException

Parameters

cbuf

An array of characters to be filled from the stream.

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.

Description

This method reads characters of input to fill the given array by calling read(cbuf, 0, cbuf.length). The method blocks until some data is available.

A subclass does not usually need to override this method, as it can override read(char[], int, int) and have read(char[]) work automatically.

 public abstract 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.

Description

This method reads up to len characters of input into the given array starting at index off. The method blocks until some data is available.

A subclass of Reader must implement this method.

ready

public boolean ready() throws IOException

Returns

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

Throws

IOException

If any kind of I/O error occurs.

Description

This method returns true if the next read() is guaranteed to not block.

The implementation of the ready() method in Reader always returns false. A subclass should override this method as appropriate.

reset

public void reset() throws IOException

Throws

IOException

If there was no previous call to the mark() method or the saved position has been invalidated.

Description

This method restores the position of the stream to the position that was saved by a previous call to mark().

The implementation of the reset() method in Reader throws an exception to indicate that mark-and-reset functionality is not supported by default. A subclass must override the method to provide the functionality.

skip

public long skip(long n) throws IOException

Parameters

n

The number of characters to skip.

Returns

The actual number of characters skipped.

Throws

IOException

If any kind of I/O error occurs.

Description

This method skips n characters of input. In other words, it moves the position of the stream forward by n characters.

The implementation of the skip() method in Reader simply calls read(cb, 0, n) where cb is a character array that is at least n bytes long. A subclass may want to override this method to implement a more efficient skipping algorithm.

Inherited Methods

Method

Inherited From

Method

Inherited From

clone()

Object

equals(Object)

Object

finalize()

Object

getClass()

Object

hashCode()

Object

notify()

Object

notifyAll()

Object

toString()

Object

wait()

Object

wait(long)

Object

wait(long, int)

Object

   

See Also

BufferedReader, CharArrayReader, FilterReader, InputStreamReader, IOException, PipedReader, StringReader


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