File handeling in java

Basic of file handeling

File handling capability made java a strong and robust programming language. One of the most frequently used task in programming is writing to and reading from a file. This function can achieve in java by include java’s io package. Classes related to input and output are present in the Java^TM language package java.io . Java technology uses "streams" as a general mechanism of handling data. Input streams act as a source of data. Output streams act as a destination of data. The file handeling capability can achive in java by two types

1)Byte Stream

2)Character Stream.

 You are probably aware that Java is one of the most popular programming languages that people use. It is so popular because it can be used so widely from application software to web applications and capable to read data from various device and application such as from Network socket,another file, other embaded device and also in XML  format.

To write anything to a file first of all we need a file name we want to use. The file name is a simple string like like this:

•  String fileName = "myfile.txt";

If you want to write in a file which is located elsewhere you need to define the
complete file name and path in your fileName variable:

•  String fileName = "c:\\path\\test.txt";

However if you define a path in your file name then you have to take care the path separator. On windows system the '\' is used and you need to backslash it so you need to write '\\', in Unix,Linux systems
the separator is a simple slash '/'.

Opening a file

To open a file for writing use the FileWriter class and create an instance from it.
The file name is passed in the constructor like this.New operator is used for allocating memory to object.

•  FileWriter writer = new FileWriter(fileName);

This code opens the file in overwrite mode if file already exhists. If you want to append to the file then
you need to use an other constructor like this:   

• FileWriter writer = new FileWriter(fileName,true);

Besides this the constructor can throw an IOException so we put all of the code inside
a try-catch block.

The syntax is simple

Try{

FileWriter writer = new FileWriter(fileName,true);

}

Catch(Exception ex)

{

 System.out.println(“Exception accur”);

}

Write to a text file

  public void writeFile() {

      String fileName = "c:\\test.txt";

   

      try {

          FileWriter writer = new FileWriter(fileName,true);

          writer.write("Test text.");

          writer.close();

      } catch (IOException e) {

          e.printStackTrace();

      }

  }

This program showing simple file write operation.

Reading the file content

File reading operation can achive by two types 1)Data input stream 2)File input stream

 File reading using data input stream

import java.io.*;

class FileRead {
  public static void main(String args[]) {
    try {
      FileInputStream fstream = new FileInputStream("C:/myfile.txt");
      DataInputStream in = new DataInputStream(fstream);
      BufferedReader br = new BufferedReader(new InputStreamReader(in));
      String str;
      while ((str = br.readLine()) != null) {
        System.out.println(str);
      }
      in.close();
    } catch (Exception e) {
      System.err.println(e);
    }
  }
}

This program  use DataInputStream for reading text file line by line with a BufferedReader. 

DataInputStream-This class read binary primitive data types in a machine-independent way. 

BufferedReader-This class read the text from a file line by line with it's readLine() method.

File reading using file input stream.

import java.io.*;

public class FileRead {
  public static void main(String[] args) {
    File file = new File("C:/file.txt");
    int ch;
    StringBuffer strContent = new StringBuffer("");
    FileInputStream fin = null;
    try {
      fin = new FileInputStream(file);
      while ((ch = fin.read()) != -1)
        strContent.append((char) ch);
      fin.close();
    } catch (Exception e) {
      System.out.println(e);
    }
    System.out.println(strContent.toString());
  }
}

FileInputStream- This class reads bytes from the given file name.

read()-The read() method of FileInputStream class reads a byte or array of bytes from the file. It returns -1 when the end-of-file has been reached.

StringBuffer- This class is used to store character strings that will be changed.

append()-The append() of StringBuffer class appends or adds the string representation of the char argument to this sequence.

Copy one file into another file

import java.io.*;    //Package import

public class CopyFile {
  private static void copyfile() {
    try {
      File f1 = new File("C:/file.txt");     //Source file
      File f2 = new File("C:/new.txt");      //Destination file  
      InputStream in = new FileInputStream(f1);
      OutputStream out = new FileOutputStream(f2, true);
      byte[] buf = new byte[1024];
      int len;
      while ((len = in.read(buf)) > 0) {
        out.write(buf, 0, len);
      }
      in.close();
      out.close();
      System.out.println("File copied.");
    } catch (Exception ex) {
      System.out.println(ex);
    }
  }

  public static void main(String[] args) {
    copyfile();
  }
}

Program to count words of a file

import java.io.*;

import java.util.*;

public class FileCountwords {

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

BufferedReader br = new BufferedReader(new FileReader("C:/file.txt"));

                String line = "", str = "";

                int count = 0;

                while ((line = br.readLine()) != null) {

                        str += line + " ";

                }

                StringTokenizer st = new StringTokenizer(str);

                while (st.hasMoreTokens()) {

                        String s = st.nextToken();

                        count++;

                }

                System.out.println("File has " + count + " words.");

        }

}

It will print the number of words into a file.