When should I use the dollar symbol ($) in a variable name?

Question

The dollar symbol ($) is a valid character to name a variable, e.g. String superSecretFormula$;, but when we\'re talking about naming conventions, when should I

Answer 1

Theoretically you can use dollar sign in variable names, but it's strongly discouraged because that symbol is used internally by the compiler(e.g. inner or anonymous classes's name).

Please refer to two related questions on the stackoverflow: What is the meaning of $ in a variable name? and Java class name containing dollar sign fails to compile if an inner class is present