Java IO Tutorial

 

Java IO Tutorial

Hi Friends In, this tutorial i will going to explain about Java I/O. Basically Java I/O (Input and Output) is used to process the input and produce the output. Java uses the concept of a stream to make I/O operation fast. The java.io package contains all the classes required for input and output operations.

Java IO Overview

A good place to start learning about Java IO is the Java IO Overview tutorial. That tutorial gives you a quick overview of the central concepts in the Java IO API, and an overview of all the central classes in the Java IO API.

 

Java IO is an API that comes with Java which is targeted at reading and writing data (input and output). Most applications need to process some input and produce some output based on that input. For instance, read data from a file or over network, and write to a file or write a response back over the network.

 

The Java IO API is in the Java IO package (java.io). If you look at the Java IO classes in the java.io package the vast amount of choices can be rather confusing. What is the purpose of all these classes? Which one should you choose for a given task? How do you create your own classes to plugin? etc.

 

The purpose of this tutorial is to try to give you an overview of how all information related to Java I/O and the classes which are grouped in this package, and the purpose behind them, so you don't have to wonder whether you chose the right class, or whether a class already exists for your purpose.

 

The java.io package contains nearly every class you might ever need to perform input and output (I/O) in Java. All these streams represent an input source and an output destination. The stream in the java.io package supports many data such as primitives, object, localized characters, etc.

Java IO Class Overview Table

Here is a table listing most (if not all) Java IO classes divided by input, output, being byte based or character based, and any more specific purpose they may be addressing, like buffering, parsing etc.

Stream

A stream can be defined as a sequence of data. 

There are two kinds of Streams −

InPutStream − The InputStream is used to read data from a source.

OutPutStream − The OutputStream is used for writing data to a destination.

 

Byte Streams

Java byte streams are used to perform input and output of 8-bit bytes. Though there are many classes related to byte streams, but the most frequently used classes are, FileInputStream and FileOutputStream. Following is an example which makes use of these two classes to copy an input file into an output file −

Example

import java.io.*;
public class CopyFileTo {

   public static void main(String args[]) throws IOException {  
      FileInputStream in = null;
      FileOutputStream out = null;



      try {
         in = new FileInputStream("input.txt");
         out = new FileOutputStream("output.txt");
         
         int c;
         while ((c = in.read()) != -1) {
            out.write(c);
         }
      }finally {
         if (in != null) {
            in.close();
         }
         if (out != null) {
            out.close();
         }
      }
   }
}

 Now let's have a file input.txt with the following content −

This is test for copy file.

As a next step, compile the above program and execute it, which will result in creating output.txt file with the same content as we have in input.txt. So let's put the above code in CopyFile.java file and do the following −

$javac CopyFile.java
$java CopyFile

Character Streams

Java Byte streams are used to perform input and output of 8-bit bytes, whereas Java Character streams are used to perform input and output for 16-bit unicode. Though there are many classes related to character streams, but the most frequently used classes are, FileReader and FileWriter. Though internally FileReader uses FileInputStream and FileWriter uses FileOutputStream but here the major difference is that FileReader reads two bytes at a time and FileWriter writes two bytes at a time.

We can re-write the above example, which makes the use of these two classes to copy an input file (having unicode characters) into an output file −

Example

import java.io.*;
public class CopyFileTo {

   public static void main(String args[]) throws IOException {
      FileReader in = null;
      FileWriter out = null;

      try {
         in = new FileReader("input.txt");
         out = new FileWriter("output.txt");
         
         int c;
         while ((c = in.read()) != -1) {
            out.write(c);
         }
      }finally {
         if (in != null) {
            in.close();
         }
         if (out != null) {


            out.close();
         }
      }
   }
}

Now let's have a file input.txt with the following content -

This is test for copy file.

As a next step, compile the above program and execute it, which will result in creating output.txt file with the same content as we have in input.txt. So let's put the above code in CopyFile.java file and do the following −

$javac CopyFile.java
$java CopyFile

Reading and Writing File

As described earlier, a stream can be defined as a sequence of data. The InputStream is used to read data from a source and the OutputStream is used for writing data to a destination.

The two important streams are FileInputStream and FileOutputStream, which would be discussed in this tutorial.

FileInputStream

This stream is used for reading data from the files. Objects can be created using the keyword new and there are several types of constructors available.

Following constructor takes a file name as a string to create an input stream object to read the file –

InputStream f = new FileInputStream("C:/java/hello");

Following constructor takes a file object to create an input stream object to read the file. First we create a file object using File() method as follows File f = new File("C:/java/hello");

InputStream f = new FileInputStream(f);

Once you have InputStream object in hand, then there is a list of helper methods which can be used to read to stream or to do other operations on the stream.

There are other important input streams available, for more detail you can refer to the following links −

ByteArrayInputStream

DataInputStream

FileOutputStream

FileOutputStream is used to create a file and write data into it. The stream would create a file, if it doesn't already exist, before opening it for output.

 Here are two constructors which can be used to create a FileOutputStream object.

Following constructor takes a file name as a string to create an input stream object to write the file −

OutputStream f = new FileOutputStream("C:/java/hello")

Following constructor takes a file object to create an output stream object to write the file. First, we create a file object using File() method as follows −

File f = new File("C:/java/hello");

OutputStream f = new FileOutputStream(f);

Once you have OutputStream object in hand, then there is a list of helper methods, which can be used to write to stream or to do other operations on the stream.

There are other important output streams available, for more detail you can refer to the following links −

ByteArrayOutputStream

DataOutputStream

Example

Following is the example to demonstrate InputStream and OutputStream −

import java.io.*;
public class fileStream {



   public static void main(String args[]) {
   
      try {
         byte bWrite [] = {11,21,3,40,5};
         OutputStream os = new FileOutputStream("test1.txt");
         for(int x = 0; x < bWrite.length ; x++) {
            os.write( bWrite[x] );   // writes the bytes
         }
         os.close();
     
         InputStream is = new FileInputStream("test2.txt");
         int size = is.available();



         for(int i = 0; i < size; i++) {
            System.out.print((char)is.read() + "  ");
         }
         is.close();
      } catch (IOException e) {
         System.out.print("Exception");
      }	
   }
}

The above code would create file test.txt and would write given numbers in binary format. Same would be the output on the stdout screen.

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