Using a dollar sign ($) as part of an identifier is not recommended.
Section 3.8 of the Java Language Specification reserves the dollar sign ($) for identifiers that are used only in mechanically generated source code.
Example:
int un$afe;
[1] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 398
[2] J. Gosling, B. Joy, G. Steele, G. Bracha The Java Language Specification, Second Edition Addison-Wesley