It is an internal class without a name, only an object is created for it. Anonymous inner classes are useful when creating object instances with certain "additional features", such as overloading methods of a class or interface, without actually subclassing the class.

Anonymous inner classes are useful when writing implementation classes for the listener interface in graphical programming.

Anonymous inner class are mainly created in two ways:

• Class (may be abstract or concrete)
• Interface

In Java, an anonymous inner class is a type of class that is declared and instantiated at the same time, without having a name. It is a class that is defined inside another class, method, or even as an argument to a method call, and does not have an explicit class declaration.

Anonymous inner classes are useful when you need to define a class that will be used only once and is not needed anywhere else in your code. They are often used in event handling and other situations where you need to provide a callback function or interface implementation.

To create an anonymous inner class, you define the class and instantiate it in one step, using the syntax: 

new SomeClass() {
    // class definition
};

Here, SomeClass is the class that the anonymous inner class extends or implements, and the class definition is enclosed in curly braces.

Anonymous inner classes can also access the variables and methods of the enclosing class, which can be useful for event handling and other purposes. However, because they do not have a name, anonymous inner classes cannot be reused or referenced elsewhere in your code, which can make your code harder to read and maintain.