I think Ajay already understands what characters you can use in a variable name.
In Java source code, you can use Unicode escape sequences that look like \uNNNN where NNNN is the four-digit hexadecimal code of the character. These escape sequences are processed by the Java compiler as the very first step in the compilation process. Because of this, you can use an escape sequence in the place of a normal character.
Character 0061 is the lower case a. So
char \u0061 = 'a';
is exactly the same as:
char a = 'a';
You could even go further. Do you understand what the following line of code is?
\u0063\u0068\u0061\u0072\u0020\u0061\u0020\u003d\u0020\u0027\u0061\u0027\u003b [ February 28, 2008: Message edited by: Jesper Young ]