طلب صغير لو سمحتم استفسار في لغة الجافا

يمكنك فك التشفير بالرجوع إلى الوراء بواسطة مقدار الإزاحة

/* 
 * To change this template, choose Tools | Templates 
 * and open the template in the editor. 
 */ 

/** 
 * 
 * @author Administrator 
 */ 
import javax.swing.*; 

public class Cipher { 

	private String inputString; 
	private int intOffset; 

	public Cipher() { 

    	inputString = null; 
    	intOffset = 0; 

	} 

	public static void main(String[] args) { 


    	Cipher c2 = new Cipher(); 
    	String tmp = c2.encrypt(); 
    	System.out.println(tmp); //System.out.println( decrypt( tmp ) ); 
    	tmp = c2.decrypt();
    	System.out.println(tmp);
    	System.exit(0); 

	} 

	public String encrypt() { 

    	inputString = ((String) JOptionPane.showInputDialog("Enter the message you wish to encrypt")).toLowerCase().trim(); 
    	System.out.println(inputString); 
    	intOffset = Integer.parseInt(JOptionPane.showInputDialog("Enter Key")); 
    	String ALPHABET = "abcdefghijklmnopqrstuvwxyz"; 


    	StringBuffer sb = new StringBuffer(); 

    	int holder = inputString.length(); 

    	for (int i = 0; i < holder; i++) { 

        	String tmp = "" + inputString.charAt(i); 
        	int offset = ALPHABET.indexOf(tmp); 
        	offset += intOffset; 
        	if (offset > 25) { 
            	int newOffset = 0; 
            	newOffset = offset % 25; 
            	sb.append(ALPHABET.charAt(newOffset)); 
        	} else { 
            	sb.append(ALPHABET.charAt(offset)); 
        	} 
    	}//forloop 
    	return sb.toString(); 
	} 
   public String decrypt() { 

    	inputString = ((String) JOptionPane.showInputDialog("Enter the message you wish to decrypt")).toLowerCase().trim(); 
    	System.out.println(inputString); 
    	intOffset = Integer.parseInt(JOptionPane.showInputDialog("Enter Key")); 
    	String ALPHABET = "abcdefghijklmnopqrstuvwxyz"; 


    	StringBuffer sb = new StringBuffer(); 

    	int holder = inputString.length(); 

    	for (int i = 0; i < holder; i++) { 

        	String tmp = "" + inputString.charAt(i); 
        	int offset = ALPHABET.indexOf(tmp); 
        	offset -= intOffset; 
        	if (offset < 0) { 
            	int newOffset = 25; 
            	newOffset = offset % 25; 
            	sb.append(ALPHABET.charAt(newOffset)); 
        	} else { 
            	sb.append(ALPHABET.charAt(offset)); 
        	} 
    	}//forloop 
    	return sb.toString(); 
	} 
}//class

اخوي من وين بالزبط؟

يا ريت توضح

وشكراااااااااااا