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Web Design & Development ­ CS506
Lesson 8
I/O libraries often use the abstraction of a stream, which represents any data source or sink as an
object capable of producing or receiving pieces of data.
The Java library classes for I/O are divided by input and output. You need to import java.io package to
use streams. There is no need to learn all the streams just do it on the need basis.
The concept of "streams"
It is an abstraction of a data source/sink
We need abstraction because there are lots of different devices (files, consoles, network,
memory, etc.).  We need to talk to the devices in different ways
(sequential, random access, by lines, etc.)  Streams make the task easy by acting in the same
way for every device. Though inside handling of devices may be quite different, yet on the
surface everything is similar. You might read from a file, the keyboard, memory or network
connection, different devices may require specialization of the basic stream, but you can
treat them all as just "streams". When you read from a network, you do nothing different
than when you read from a local file or from user's typing
//Reading from console
BufferedReader stdin = new BufferedReader(new InputStreamReader(
System.in ));
-------- ( your console)
// Reading from file
BufferedReader br=new BufferedReader(new FileReader("input.txt"));
//Reading from network
BufferedReader br = new BufferedReader(new InputStreamReader
---- "s" is the socket
So you can consider stream as a data path. Data can flow through this path in one direction
between specified terminal points (your program and file, console, socket etc.)
Stream classification based on Functionality
Based on functionality streams can be categorized as Node Stream and Filter Stream. Node
Streams are those which connect directly with the data source/sick and provide basic
functionality to read/write data from that source/sink
FileReader fr = new FileReader("input.txt");
Web Design & Development ­ CS506
You can see that FileReader is taking a data/source "input.txt" as its argument and hence it is a
node stream.
FilterStreams sit on top of a node stream or chain with other filter stream and provide some
additional functionality e.g. compression, security etc. FilterStreams take other stream as their
BufferedReader bt = new BufferedReader(fr);
BufferedReader makes the IO efficient (enhances the functionality) by buffering the input before
delivering. And as you can see that BufferedReader is sitting on top of a node stream which is
Stream classification based on data
Two type of classes exists.
Classes which contain the word stream in their name are byte oriented and are here since JDK1.0. These
streams can be used to read/write data in the form of bytes. Hence classes with the word stream in their
name are byte-oriented in nature. Examples of byte oriented streams are FileInputStream,
ObjectOutputStream etc.
Classes which contain the word Reader/Writer are character oriented and read and write data in the form of
characters. Readers and Writers came with JDK1.1. Examples of Reader/Writers are FileReader,
PrintWriter etc
Web Design & Development ­ CS506
Example Code 8.1: Reading from File
The ReadFileEx.java reads text file line by line and prints them on console. Before we move on to the
code, first create a text file (input.txt) using notepad and write following text lines inside it.
Hello World
Pakistan is our homeland
Web Design and Development
// File ReadFileEx.java
import java.io.*;
public class ReadFileEx {
public static void main (String args[ ]) {
FileReader fr = null;
BufferedReader br = null;
try {
// attaching node stream with data source
fr = new FileReader("input.txt");
// attatching filter stream over node stream
br = new BufferedReader(fr);
// reading first line from file
String line = br.readLine();
// printing and reading remaining lines
while (line != null){
line = br.readLine();
// closing streams
}catch(IOException ioex){
} // end main
} // end class
Example Code 8.2: Writing to File
The WriteFileEx.java writes the strings into the text file named "output.txt". If
"output.txt" file does not exist, the java will create it for you.
// File WriteFileEx.java
import java.io.*;
Web Design & Development ­ CS506
public class WriteFileEx {
public static void main (String args[ ]) {
FileWriter fw = null;
PrintWriter pw = null;
try {
// attaching node stream with data source
// if file does not exist, it automatically creates it
fw = new FileWriter ("output.txt");
// attatching filter stream over node stream
pw = new PrintWriter(fw);
String s1 = "Hello World";
String s2 = "Web Design and Development";
// writing first string to file
// writing second string to file
// flushing stream
// closing streams
}catch(IOException ioex){
} // end main
} // end class
After executing the program, check the output.txt file. Two lines will be written there.
Example code, their explanations and corresponding figures for this handout are taken from the book
JAVA A Lab Course by Umair Javed. This material is available just for the use of VU students of the
course Web Design and Development and not for any other commercial purpose without the consent of