PushbackInputStream example program in Java

By: Sam Chen Emailed: 1765 times Printed: 2515 times    

Latest comments
By: rohit kumar - how this program is work
By: Kirti - Hi..thx for the hadoop in
By: Spijker - I have altered the code a
By: ali mohammed - why we use the java in ne
By: ali mohammed - why we use the java in ne
By: mizhelle - when I exported the data
By: raul - no output as well, i'm ge
By: Rajesh - thanx very much...
By: Suindu De - Suppose we are executing

One of the novel uses of buffering is the implementation of pushback. Pushback is used on an input stream to allow a byte to be read and then returned (that is, "pushed back") to the stream. The PushbackInputStream class implements this idea. It provides a mechanism to "peek" at what is coming from an input stream without disrupting it. PushbackInputStream has the following constructors:

PushbackInputStream(InputStream inputStream)
PushbackInputStream(InputStream inputStream, int numBytes)

The first form creates a stream object that allows one byte to be returned to the input stream. The second form creates a stream that has a pushback buffer that is numBytes long. This allows multiple bytes to be returned to the input stream.

Beyond the familiar methods of InputStream, PushbackInputStream provides unread(), shown here:

void unread(int ch)
void unread(byte buffer[ ])
void unread(byte buffer, int offset, int numChars)

The first form pushes back the low-order byte of ch. This will be the next byte returned by a subsequent call to read( ). The second form returns the bytes in buffer. The third form pushes back numChars bytes beginning at offset from buffer. An IOException will be thrown if there is an attempt to return a byte when the pushback buffer is full. Java 2 makes a small change to PushbackInputStream: it now implements the skip( ) method. 

Here is an example that shows how a programming language parser might use a PushbackInputStream and unread( ) to deal with the difference between the = = operator for comparison and the = operator for assignment:

// Demonstrate unread().
import java.io.*;
class PushbackInputStreamDemo {
public static void main(String args[]) throws IOException {
String s = "if (a == 4) a = 0;\\n";
byte buf[] = s.getBytes();
ByteArrayInputStream in = new ByteArrayInputStream(buf);
PushbackInputStream f = new PushbackInputStream(in);
int c;
while ((c = f.read()) != -1) {
switch(c) {
case '=':
if ((c = f.read()) == '=')
else {
System.out.print((char) c);

Here is the output for this example. Notice that = = was replaced by ".eq." and = was replaced by "<-".

if (a .eq. 4) a <- 0;

Caution: PushbackInputStream has the side effect of invalidating the mark( ) or reset( ) methods of the InputStream used to create it. Use markSupported( ) to check any stream on which you are going to use mark( )/reset( ).

This tutorial is an extract from the "The Complete Reference Part 2 by Herbert Schildt".

Java Home | All Java Tutorials | Latest Java Tutorials

Sponsored Links

If this tutorial doesn't answer your question, or you have a specific question, just ask an expert here. Post your question to get a direct answer.

Bookmark and Share


1. View Comment


View Tutorial          By: ayyannan at 2011-01-21 21:27:35

Your name (required):

Your email(required, will not be shown to the public):

Your sites URL (optional):

Your comments:

More Tutorials by Sam Chen
Extract filename from full path using perl
Encrypting Passwords in Tomcat using Servlets
malloc, calloc - Storage Management - in C
Character Pointers and Functions in C
Declarations in C
strcpy() and strncpy() sample program in C++
Sample program to draw a rectangle in J2ME
Design Patterns for Properties in a Java Bean
PushbackInputStream example program in Java
FileInputStream - sample program in Java

More Tutorials in Java
Update contents of a file within a jar file
Tomcat and httpd configured in port 8080 and 80
Java File
Java String
Count number of vowels, consonants and digits in a String in Java
Reverse a number in Java
Student marks calculation program in Java
Handling Fractions in Java
Calculate gross salary in Java
Calculate average sale of the week in Java
Vector in Java - Sample Program
MultiLevel Inheritance sample in Java
Multiple Inheritance sample in Java
Java program using Method Overriding
Java program to check if user input is an even number

More Latest News
Most Viewed Articles (in Java )
A Serialization Example in Java
Using One-Dimensional Arrays in Java
Method Overriding in Java
indexOf( ) and lastIndexOf( ) in Java
The java Buzzwords
Write to a file in Java - Sample Program
XML and Java - Parsing XML using Java Tutorial
Student marks calculation program in Java
Handling Fractions in Java
Calculate average sale of the week in Java
How to create an array and method in JSP
How to use set, get basic and nested properties for Spring framework
How to access instance from an inner class and accessing outer class variables in java
Converting a number into its equalant value in words in Java
What is Java?
Most Emailed Articles (in Java)
Converting a number into its equalant value in words in Java
java.lang.reflect package
Disadvantages of using Native methods in Java
What is UCS? What is ISO 10646?
Generating Your Key Pair example using keytool in Java
Operator Precedence in Java
PushbackReader sample program in Java
concat(), replace(), and trim() Strings in Java
instanceof sample program in Java
Use of - new - in Java
The Benefits of OOP
Increment and Decrement Operator
Characters in java
The continuing Revolution of java
Why java is important to the Internet