JAVA: Streams

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Lesson 8

Streams

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.getInputStream()));

---- "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");

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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

input.

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

FileReader.

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

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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

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// 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){

System.out.println(line);

line = br.readLine();

}

// closing streams

br.close();

fr.close();

}catch(IOException ioex){

System.out.println(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.*;

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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

pw.println(s1);

// writing second string to file

pw.println(s2);

// flushing stream

pw.flush();

// closing streams

pw.close();

fw.close();

}catch(IOException ioex){

System.out.println(ioex);

}

} // end main

} // end class

After executing the program, check the output.txt file. Two lines will be written there.

Reference

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

author.

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