Probably A Simple Problem...

[kitsune]

I've written the following code...The encrypt() method is supposed to read text from a file and encode it by shifting each letter three to the right. Decrypt() is supposed to decrypt in a similar way, by shifting each letter three to the left. The encrypt() method works just fine, and the decrypt() almost works...Instread of returning 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' when I enter the appropriate encoded string, it returns 'ABCDEFGHIJKLMNOPQRSTUVW>?@'

protected void encrypt(String ipfile, String opfile) throws IOException
  { 
    StringBuffer contents = new StringBuffer();
    StringBuffer newContents = new StringBuffer();
    BufferedReader input = null;
    Writer output = null;
    char letter, newLetter;
    int uni, answer;
      
      try 
      {
        input = new BufferedReader(new FileReader(ipfile) );
        String line = null;
        output = new BufferedWriter( new FileWriter(opfile) );
        
        while ((line = input.readLine()) != null)
        {
          contents.append(line);
          
          for (int i = 0; i < contents.length(); i++)
          {
            letter = contents.charAt(i);
            uni = (int)letter;
            
            if (uni == 32)
            {
              answer = 32;
            }
            else if (uni == 13)
            {
              answer = 13;
            }
            else
            {
              answer = (((uni - 65) + 3)%26) + 65;
            }
            newLetter = (char)answer;
            newContents.append(newLetter);
          }
          newContents.append(System.getProperty("line.separator"));
          contents.delete(0, contents.length());
        }
        String newContentString = newContents.toString();
        output.write(newContentString);
      }
    catch (IOException e)
    {
      System.out.println("I/O Error:  " + e.getMessage());
      System.exit(1);
    }
    finally 
    {
      if (output != null) output.close();
    }
  }
  
  protected void decrypt(String ipfile, String opfile) throws IOException
  {
    StringBuffer contents = new StringBuffer();
    StringBuffer newContents = new StringBuffer();
    BufferedReader input = null;
    Writer output = null;
    char letter, newLetter;
    int uni, answer;
      
      try 
      {
        input = new BufferedReader(new FileReader(ipfile) );
        String line = null;
        output = new BufferedWriter( new FileWriter(opfile) );
        
        while ((line = input.readLine()) != null)
        {
          contents.append(line);
          
          for (int i = 0; i < contents.length(); i++)
          {
            letter = contents.charAt(i);
            uni = (int)letter;
            
            if (uni == 32)
            {
              answer = 32;
            }
            else if (uni == 13)
            {
              answer = 13;
            }
            else
            {
              answer = (((uni - 65) - 3)%26) + 65;
            }
            newLetter = (char)answer;
            newContents.append(newLetter);
          }
          newContents.append(System.getProperty("line.separator"));
          contents.delete(0, contents.length());
        }
        String newContentString = newContents.toString();
        output.write(newContentString);
      }
    catch (IOException e)
    {
      System.out.println("I/O Error:  " + e.getMessage());
      System.exit(1);
    }
    finally 
    {
      if (output != null) output.close();
    }
  }

Can anybody see my mistake?

[sql_lall]

a tricky one this, gets lots of people in competitions...

the % thing, for negative numbers, gives negative results...
i.e. -3 % 7 = -3, -24 % 5 = -4...

so, when you get this line:
answer = (((uni - 65) - 3)%26) + 65;

and you are decrypting 'A' (=65), this happens:

answer = (-3 % 26) + 65 = -3 + 65 = 62
and, if you look at ASCII 62, 63 and 64: >?@ !!!!

so, just make it (((uni - 65) + 75)%26) + 65;

or even something bigger than 75, just that 75%26 = -3%26, This will assure you it will be positive!

Oh, btw, i have seen try...catch before, but never try...catch...finally?!?! what does this do?