CAESAR JAVA DEC

/**
 * Created by MOHIT on 07-11-2016.
 */
import java.io.File;
import java.io.IOException;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;
import java.util.Random;
import java.io.FileWriter;

public class ceasarDecipher {

    public static void main(String arg[]){
        Scanner sc = new Scanner(System.in);

        //System.out.print("Enter the String to be decrypted : ");
        //String inString = sc.nextLine();

        System.out.print("\nEnter the Shift value : ");
        int shift_value = sc.nextInt();

        // String finalString = decrypt_(inString,cipher);
        decrypt_file(shift_value);
        // System.out.print("\nThe decrypted String is : " + finalString);
    }

    static String decrypt(String original_msg, int shift_value){

        String allChars = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ !\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~";
        int all_chars_length = allChars.length();
        String decrypted_msg = "";
        for(int i=0; i< original_msg.length(); i++){
            Character letter = original_msg.charAt(i);
            if(allChars.indexOf(letter) >= 0) {
                int position = allChars.indexOf(letter);
                position -= shift_value;
                if(position < 0){
                    position = all_chars_length + position;
                }
                position %= all_chars_length;
                Character e_letter = allChars.charAt(position);
                decrypted_msg += e_letter;

            }else{
                decrypted_msg += letter;
            }
        }


        return decrypted_msg;
    }

    static void decrypt_file(int shift_value){
        FileInputStream fin;
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter the path of the file : ");
        String file_name = sc.nextLine();
        String fullFile = "";

        try {
            fin = new FileInputStream(file_name);
            Scanner fp = new Scanner(fin);
            int size = 0;

            while (fp.hasNextLine()) {
                String line = fp.nextLine();
                fullFile += line + "\n";
                size += line.length();
            }
            System.out.println("File read successfully!!");
            fullFile = decrypt(fullFile, shift_value);
            System.out.println("Data decrypted successfully!!");
            System.out.println(fullFile);
        }catch (Exception e){
            e.printStackTrace();
        }


        try {

            File newTextFile = new File("C:\\Users\\MOHIT\\Desktop\\decrypted_file.txt");
            FileWriter f1 = new FileWriter(newTextFile);
            f1.write(fullFile);
            f1.close();
            System.out.println("decrypted file stored on Desktop");
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }


}

/*
Output for decrypting a file

Enter the Shift value : 3
Enter File name :F:/python_3.4/sample.txt
File read successufully!!
File data decrypted successfully!!
decrypted file stored on Desktop!!


 */