Files and I/O

This section contains the detail about the Files and I/O in java.

Files and I/O

The every class which you need to perform input and output (I/O) is contained by java.io package.

A sequence of data can be defined as a stream. To read data from a source we used InputStream and to write data to destination, we need OutputStream.

Reading Console Input

Java input console is accomplished by reading from System.in. To obtain a character-based stream that is attached to the console, you wrap System.in in a BufferedReader object, to create a character stream. Here is most common syntax to obtain BufferedReader :

BufferedReader br = new BufferedReader(new 
                      InputStreamReader(System.in));

Once BufferedReader is obtained, we can use read( ) method to reach a character or readLine( ) method to read a string from the console.

Example of Reading Characters

import java.io.*;

class BRRead {
   public static void main(String args[]) throws IOException
   {
      char c;
      // Create a BufferedReader using System.in
      BufferedReader br = new BufferedReader(new 
                         InputStreamReader(System.in));
      System.out.println("Enter characters, 'q' to quit.");
      // read characters
      do {
         c = (char) br.read();
         System.out.println(c);
      } while(c != 'q');
   }
}

Output :

C:\Program Files\Java\jdk1.6.0_18\bin>javac BRRead.java C:\Program Files\Java\jdk1.6.0_18\bin>java BRRead Enter characters, 'q' to quit. 123abcq 1 2 3 a b c q

Example of Reading Strings

import java.io.*;
class BRReadLines {
   public static void main(String args[]) throws IOException
   {
      // Create a BufferedReader using System.in
      BufferedReader br = new BufferedReader(new
                              InputStreamReader(System.in));
      String str;
      System.out.println("Enter lines of text.");
      System.out.println("Enter 'end' to quit.");
      do {
         str = br.readLine();
         System.out.println(str);
      } while(!str.equals("end"));
   }
}

Output :

C:\Program Files\Java\jdk1.6.0_18\bin>javac BRReadLines.java C:\Program Files\Java\jdk1.6.0_18\bin>java BRReadLines Enter lines of text. Enter 'end' to quit. This is line one This is line one This is line two This is line two end end

Writing Console Output

Using print( ) and println( ), you can easily write to console.The PrintStream class has definition of these method. These objects are referenced by "System.out". Even though System.out is a byte stream, using it for simple program output is still acceptable.

Because PrintStream is an output stream derived from OutputStream, it also implements the low-level method write( ). Thus, write( ) can be used to write to the console.

Example :

import java.io.*;


class WriteDemo {
   public static void main(String args[]) {
      int b; 
      b = 'A';
      System.out.write(b);
      System.out.write('\n');
   }
}

Output :

C:\Program Files\Java\jdk1.6.0_18\bin>javac BRReadLines.java C:\Program Files\Java\jdk1.6.0_18\bin>java BRReadLines A

Reading & Writing File

A sequence of data can be defined as stream. To read data from Source, we use InputStream. To write data to target we use OutputStream.

Here is a hierarchy of classes to deal with Input and Output streams :

FileInputStream

FileInputStream is used to read data from the files. Objects can be created using the keyword new.

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");

But before this you need to create a file object using "File()" method as follows :

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

FileOutputStream

For creating a file and writing data to it , we use FileoutputStream. 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");
FileOutputStream fop = new FileOutputStream(f);

Example :

The below code would create file test.txt and would write given numbers in binary format :

import java.io.*;

public class fileStreamDemo {

public static void main(String[] args) throws IOException {

File f = new File("C:/textfile1.txt");
FileOutputStream fop = new FileOutputStream(f);

if (f.exists()) {
String str = "This data is written through the 
program";
fop.write(str.getBytes());

fop.flush();
fop.close();
System.out.println("The data has been written");
}

else
System.out.println("This file is not exist");

try {
InputStream is = new 
FileInputStream("C:/textfile1.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");
}

}

}

Output :

C:\Program Files\Java\jdk1.6.0_18\bin>javac fileStreamDemo.java C:\Program Files\Java\jdk1.6.0_18\bin>java fileStreamDemo The data has been written This data is written through the program

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