Класс Java.io.PushbackReader в Java

java.io.PushbackReader - это класс считывателя потока символов, который позволяет помещать символы обратно в поток.

Декларация:

открытый класс PushbackReader
   расширяет FilterReader

Конструкторы:

• PushbackReader (Reader push): создает новый Pushback Reader - «push» с символьным буфером Pushback.
• PushbackReader (Reader push, int size): Создает новое средство чтения Pushback с буфером Pushback определенного размера.

Методы класса PushbackReader:

• 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(" We have used skip() method in 1 : ");             System.out.print(" Use 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(" Is 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

  This article is contributed by Mohit Gupta . If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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