why is that?

Wow I wrote that comment three years ago, surprised to see a reply now. I learned to write it like that from my high school CS teacher, but generally speaking string addition with "+" is bad practice since it is space inefficient (Strings are immutable, so it creates a new string every time you add). It seems to be fine in this case, however, Character.toString(c) may be considered best practice. String.valueOf(c) should likely work as well.

Can I ask? Where did you get to learn how to use it like this? Cant seem to find a book that teaches methods.

Yes, I believe that c + "" would also result in a String and the code should run just fine. As for why I wrote it as "" + c is that it is more explicit in showing that I am appending a character onto a string. I guess it is also better practice because that was how I was taught to do it. :)