The only nonalphabetic characters allowed to start a name are $ and _(underscore). : identifier « Java Source And Data Type « SCJP

1.6.9.The only nonalphabetic characters allowed to start a name are $ and _(underscore).

public class MainClass {

  public static void main(String[] argv) {

    int _i;

    int $i;

    System.out.println();

  }

}

1.6.identifier
	1.6.1.	Java Identifiers
	1.6.2.	Java names always considers case.
	1.6.3.	legal identifiers:
	1.6.4.	Java Keywords and Reserved Words
	1.6.5.	An identifier is a word used by a programmer to name a variable, method, class, or label.
	1.6.6.	Keywords and reserved words may not be used as identifiers.
	1.6.7.	An identifier must begin with a letter, a dollar sign ($), or an underscore (_)
	1.6.8.	Identifier's Subsequent characters may be letters, dollar signs, underscores, or digits.
	1.6.9.	The only nonalphabetic characters allowed to start a name are $ and _(underscore).
	1.6.10.	Embedded keywords are ok
	1.6.11.	Illegal identifier: starts with a digit
	1.6.12.	Illegal Identifier: bad 1st char
	1.6.13.	The following are invalid Java identifiers:
	1.6.14.	A simple identifier followed by an expression

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