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Calculating the Checksum of the file/CRC32

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A Checksum is used for error checking while transferring a file. Data flow across the network in the form of packets. So, checksum is a computed value that is dependent on the contents of a file.

Calculating the Checksum of the file/CRC32

     

A Checksum is used for error checking while transferring a file. Data flow across the network in the form of packets. So, checksum is a computed value that is dependent on the contents of  a file. For each packet the computed checksum will  be different. This computed value is transmitted with the packet when it is transmitted. The receiving system checks the checksum and on the basis of checksum it receives and rejects the packet. It is mainly used where it becomes necessary to check the packets before accepting it. 

CRC:  CRC means Cyclic Redundancy Check. It is a error checking technique used to check the accuracy of data while transmitting it to the other end. Errors are checked while the data is transferring. CRC performs some type of calculation before transmitting the data and send the result to the other end. The other end repeats the same operation before accepting the data. If both the devices get the result same, it means the transmission is error free. 

To make a program on Checksum first of all make a class named CheckSumCRC32. Inside the class declare one method checkSum(String file) which will take a value of a file which has to pass at run time. Now make a object of FileInputStream, CheckedInputStream and CRC32 and pass the instance of FileInputStream, CRC32 into the constructor of CheckedInputStream class. To calculate the size of the file call the method length() of File class. We have define a array of type byte, the size of the array is 100, i.e. the size of each packet. The Checksum for each packet will be generated randomly by the CheckedInputStream class. It returns the long data type. Now define a main method inside which we will call checkSum() method which will give us the checksum, size of the file and name of the file.
To achieve the desired result we have used the following classes and methods.

FileInputStream: It is a class of java.io package. It extends InputStream class. It is used for reading in byte form.

CheckedInputStream: It is a input stream that keeps the checksum. Its constructor use two parameter, first is InputStream and second is Checksum.

CRC32: It is a class of java.util.zip package. It implements Checksum. This class is used to calculate the CRC-32 of the stream.

length(): It is a method of File class. It returns the length of the file.

read(): It reads a byte.

getChecksum(): It returns the Checksum.

getValue(): It is a method of Checksum interface. It returns the checksum value.

The code of the program is given below.

import java.io.*;
import java.util.zip.CheckedInputStream;
import java.util.zip.CRC32;

public class ChecksumCRC32 {
  public void checkSum(String file){
  try{
  FileInputStream fis = null;
  CheckedInputStream cis = null;
  CRC32 crc = null;
  long sizeOfFile = 0;
  try{
  fis = new FileInputStream(file);  
  crc = new CRC32();
  cis = new CheckedInputStream(fis, crc);
  sizeOfFile = new File(file).length();
 }
  catch (Exception e){
  System.out.println("File  Not found ");
  System.exit(1);
  }
  byte[] buffer = new byte[100];
  while(cis.read(buffer)>=0){
  long checksum = cis.getChecksum().getValue();
  System.out.println(checksum + " " + sizeOfFile + " " + file);
  }
  }
  catch(IOException e){
  System.out.println("the exception has been thrown" + e);
  System.exit(1);
  }
  }
  public static void main(String[] args) {
  ChecksumCRC32 crc = new ChecksumCRC32();
  if(args.length!= 1){
  System.out.println("Please enter the valid file name : " );
  }
  else{
  crc.checkSum(args[0]);
  }
  }
}

The output of the program is given below.

In the output we can see that first we have created a file javalearner.txt in the java folder and pass some information in it. We can see that for each packet a checksum is calculated. The size of each packet is 100 bytes. It can be changed. The output will show 100 checksum value as 10 packets are generated. In the output firstly we are displaying the checksum of the packet, secondly the size of the file and lastly the name of the file. 

C:\java>java ChecksumCRC32  javalearner.txt
566064419   958  javalearner.txt
3839930655   958  javalearner.txt
3892475745   958  javalearner.txt
3357861592   958  javalearner.txt
4227549807   958  javalearner.txt
459090179   958  javalearner.txt
3273102972   958  javalearner.txt
1119235310   958  javalearner.txt
2705113445   958  javalearner.txt
695211703   958  javalearner.txt

Download this program.

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Posted on: December 19, 2008

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