Classes are usually first defined and then instantiated using the new operator. Anonymous classes combine the process of definition and instantiation into a single step. Anonymous classes are defined at the location they are instantiated, using additional syntax with the new operator. As these classes do not have a name, an instance of the class can only be created together with the definition.

An anonymous class can be defined and instantiated in contexts where a reference can be used (i.e., as expressions that evaluate to a reference denoting an object). Anonymous classes are typically used for creating objects on the fly in contexts such as the value in a return statement, an argument in a method call, or in initialization of variables. Anonymous classes are heavily used to implement event listeners in GUI-based applications.