Okay, this is a little bizarre, but your code doesn't compile. Did you possible type it in from another source?

Anyway I fixed it up so it compiles, and here's my comments...

1. Hardcode the translations as part of this class instead of in a separate file. They aren't likely to ever change, and it makes it so all the logic is contained in one place. I can't really run this without the morse.txt file. I would use a HashMap<Character,String>, and add all the codes (e.g. map.add('c', "-.-.");). That way you don't have to loop through your lookup to find the translation -- you just do map.get(c).

2. "Character" is a bad variable name as it's the name of another fundamental Java class, and it's capitalized where your other fields aren't.

3. There's no need for the morseBuffer array -- just build the completed string as you go. Create a StringBuilder, then loop through your input string and call builder.append(map.get(sentence.charAt(index))) or builder.append(" "). And at the end just return builder.toString().

4. The processLine method is useless -- it could be replaced with value.substring(2).