identifier in Java

Java identifiers are composed of Unicode characters, numbers, currency symbols, and connecting characters (like underscores). 
* Identifiers must start with a letter, a currency character ($), or a connecting character such as the underscore (_). Identifiers cannot start with a number!
* After the first character, identifiers can contain any combination of letters, currency characters, connecting characters, or numbers.
* In practice, there is no limit to the number of characters an identifier can contain.
* You can't use a Java keyword as an identifier. 
* Identifiers in Java are case-sensitive; foo and FOO are two different identifiers.
```
class Main {
    public static void main(String[] args) {
        //some legal identifiers:
        int _a;
        int $c;
        int ______2_w;
        int _$;
        int this_is_a_very_detailed_name_for_an_identifier;
        // some illegal identifiers:
        int :b;
        int -d;
        int e#;
        int .f;
        int 7g;
    }
}
```
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