You cannot put bold tags inside code tags; they do not work and I removed them. You had marked line 19.
That is quite easy. If you look here, you find that a char isn’t a letter, but a number between 0 and 0xffff (65535) inclusive. If it is a number, you can do arithmetic with it.
Where did you find that code? It has some interesting features, like the inappropriately‑named get method, the excessive use of static members, and the unusual close() call.
Please add this line after line 11 and before line 12:-
…and see what happens
The System.exit(0) call is probably completely unnecessary, and System.exit can be harmful in a multi‑threaded application, causing corrupted data in certain circumstances.