A quick background

Enums introduced type safety to the use of constants, which were defined previously by using static, and final variables of a type such as int.

An example

Imagine limiting the sizes of a shirt to some predefined sizes (such as Small, Medium, and Large). The following code shows how you can do that with an enum (Size):

enum Size {SMALL, MEDIUM, LARGE}

Java's coding guidelines recommend using uppercase to define enum constants (such as SMALL). Multiple words in a constant can be separated by using an underscore.

The following code shows how you can use the Size enum in a class, Shirt, to restrict its sizes to constants defined in the Size enum:

class Shirt { 
    Size size;              // instance...