Added checks in encode() and decode() so they ignore everything that is not a letter. These methods also do not currently work. They end up returning the input string instead of actually encoding or decoding.
Changed setKey() so that the input will always be uppercase.
Made changes to encode() and decode(). They now call private methods encodeChar() and decodeChar() respectively to get the encoded letter. I also changed the if statements in both to use Character.isLetter()
Tests for encode() and decode().