Identifiers: A name in java program is called identifier, which can be used for identification purposes. It can be a method name, variable name, class name, or label name . Rules for defining java identifiers: The only allowed characters in java identifiers are a - z, A - Z, 0 - 9, $, and _. If we are using other characters, we will get a compile-time error. Identifiers can't start with digits. Java identifiers are case sensitive. Of course, java is treated as case sensitive programming language. There is no limit for java identifiers but it is not recommended to take too lengthy identifiers. We can't use reserved words as identifiers. All predefined java class name and interface name we can use as identifiers. e.g. ...