Java.io.PushbackReader Class in Java

java.io.PushbackReader is a character-stream reader class that allows characters to be pushed back into the Stream.

Declaration:

public class PushbackReader
   extends FilterReader

Constructors :

• PushbackReader(Reader push) : Creates a new Pushback Reader – “push” with a character Pushback buffer.
• PushbackReader(Reader push, int size) : Creates a new Pushback Reader with a Pushback buffer of a particular size.

Methods of PushbackReader class :

• read() : java.io.PushbackReader.read() reads a single character.
  Syntax :

  public int read()
  Parameters :
  -----------
  Return : 
  reads single character from the Pushback buffer 
  else, -1 i.e. when end of file is reached.
  Exception :
  ->  IOException : If I/O error occurs.

• read(char[] carray, int offset, int maxlen) : java.io.PushbackReader.read(char[] carray, int offset, int maxlen) reads some portion of the character array
  Syntax :

  public int read(char[] carray, int offset, int maxlen)
  Parameters :
  carray : destination buffer to be read into
  offset : starting position of the carray
  maxlen : maximum no. of characters to be read
  Return : 
  reads some portion of the character array
  else, -1 i.e. when end of file is reached.
  Exception :
  ->  IOException : If I/O error occurs.

• close() : java.io.PushbackReader.close() closes the PushbackReader and releases system resources associated with this stream to Garbage Collector.
  Syntax :

  public void close()
  Parameters :
  ------
  Return : 
  void
  Exception :
  ->  IOException : If I/O error occurs.

• mark(int arg) : java.io.PushbackReader.mark(int arg) marks the current position of the PushbackReader. In case of PushbackReader, this method always throws an exception.
  Syntax :

  public void mark(int arg)
  Parameters :
  arg : integer specifying the read limit of the Stream
  Return : 
  void

• skip(long char) : java.io.PushbackReader.skip(long char) skips and discards ‘char’ no. of characters from PushbackReader data.
  Syntax :

  public long skip(long char)
  Parameters : 
  char : no. of bytes of PushbackReader data to skip.
  Return : 
  no. of bytes to be skipped
  Exception: 
  -> IOException : in case I/O error occurs

• reset() : java.io.PushbackReader.reset() is invoked by mark() method. It repositions the PushbackReader to the marked position. In case of PushbackReader, this method always throws an exception.
  Syntax :

  public void reset()
  Parameters :
  ----
  Return : 
  void
  Exception :
  ->  IOException : If I/O error occurs.
  

• markSupported() : java.io.PushbackReader.markSupported() Tells whether this stream supports the mark() operation, which it does not.
  Syntax :

  public boolean markSupported()
  Parameters :
  -------
  Return : 
  true if PushbackReader supports the mark() method  else,false

• ready() : java.io.PushbackReader.ready() tells whether the stream is ready to be read or not
  Syntax :

  public boolean ready()
  Parameters :
  -------
  Return : 
  true if the stream is ready to be read else,false

• unread(int char) : java.io.PushbackReader.unread(int char) pushes a single character to Pushback buffer. The next character to be read will have (char)char value, after this method returns.

  Syntax :

  public void unread(int char)
  Parameters :
  char : int value of the character to be pushed back
  Return : 
  void
  Exception :
  ->  IOException : If I/O error occurs or Pushback buffer is full.
  

• unread(char[] carray) : java.io.PushbackReader.unread(char[] carray) pushes an array of character to Pushback buffer. The array being pushed will have its index starting from 0.
  Syntax :

  public void unread(char[] carray)
  Parameters :
  carray : character array to be pushed back
  Return : 
  void
  Exception :
  ->  IOException : If I/O error occurs or Pushback buffer is full.
  

Java code explaining the PushbackReader method : read(char[] carray), close(), markSupported(), read(), mark(), ready(), skip(), unread()

// Java program illustrating the working of PushbackReader
// read(char[] carray), close(), markSupported()
// read(), mark(), ready(), skip(), unread()

import java.io.*;
public class NewClass
{
    public static void main(String[] args) throws IOException
    {
        try
        {
            // Initializing a StringReader and PushbackReader
            String s = "GeeksForGeeks";

            StringReader str_reader = new StringReader(s);
            PushbackReader geek_pushReader1 = new PushbackReader(str_reader);
            PushbackReader geek_pushReader2 = new PushbackReader(str_reader);

            // Use of ready() method :
            System.out.println("Is stream1 ready : " + geek_pushReader1.ready());
            System.out.println("Is stream2 ready : " + geek_pushReader2.ready());

            // Use of read() :
            System.out.println("\nWe have used skip() method in 1 : ");
            System.out.print("\nUse of read() in 1 : ");
            for (int i = 0; i < 6; i++)
            {
                char c = (char) geek_pushReader1.read();
                System.out.print(c);

                // Use of skip() :
                geek_pushReader1.skip(1);
            }
            System.out.println("");

            // USing read() :
            char[] carray = new char[20];
            System.out.println("Using read() in 2 : " + geek_pushReader2.read(carray));


            // USe of markSupported() :
   System.out.println("\nIs mark supported in 1  : " + geek_pushReader1.markSupported());

            geek_pushReader2.unread('F');

            // read the next char, which is the one we unread
            char c3 = (char) geek_pushReader2.read();
            System.out.println("USe of unread() : " + c3);

            // USe of mark() :
            geek_pushReader1.mark(5);

            // Use of close() :
            geek_pushReader1.close();

        }
        catch (IOException excpt)
        {
            System.out.println("mark not supported in 1");

        }
    }
}

Output :

Is stream1 ready : true
Is stream2 ready : true

We have used skip() method in 1 : 

Use of read() in 1 : GesoGe
Using read() in 2 : 1

Is mark supported in 1 : false
USe of unread() : F
mark not supported in 1