Question

Why are special characters (except $, _) not allowed in Java variable names?

Answer 1

This is not the case – many special characters are actually valid for identifiers. It is defined in the JLS #3.8:

An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter.
[…]
A “Java letter” is a character for which the method Character.isJavaIdentifierStart(int) returns true.
A “Java letter-or-digit” is a character for which the method Character.isJavaIdentifierPart(int) returns true.

For example, this is a valid variable name:

String sçèêûá¢é£¥ = "bc";

You can see all the valid characters with this simple code:

public static void main(String args[]) {
    for (int i = 0; i < Character.MAX_VALUE; i++) {
        if (Character.isJavaIdentifierPart(i)) {
            System.out.println("i = " + i + ": " + (char) i);
        }
    }
}

ps: nice examples on @PeterLawrey’s blog