title: Add characters to an empty String after it is initialized
body: <p>Just curious as to how I would add a letter or character to a pre-existing <code>string</code>.</p>
  
  <p>Here is what I'm trying to do, </p>
  
  <p>I'm taking a key and its cipher text and subtracting their values (stored in arraylist) to find the plaintext value (I get an int, and then use that to find the String value in the arraylist) and then continue to do that for each letter in the two strings(Cipher and Key).</p>
  
  <p>Is this possible, or would I have to use a <code>substring</code> or something along those lines.</p>
  
  <p>Any suggestions on how to do this, or the answer to my initial question is greatly appreciated!</p>
  
  <p>Thanks,</p>