Prior to the introduction of enums, developers often used public, static, and final variables to define constants. The following is an example:

class Size { 
    public final static int SMALL = 1; 
    public final static int MEDIUM = 2; 
    public final static int LARGE = 3; 
}

The major drawback of using public, static, final, and int variables is type safety; any int value could be assigned to a variable of the type int, instead of the Size.SMALL, Size.MEDIUM, or Size.LARGE constants.

Java 5 introduced enums, an addition to the language construct, to enable developers to define an enumeration of constants. Here's a quick example:

enum Size {SMALL, MEDIUM, LARGE} 
class SmallTShirt { 
    Size size = Size.SMALL; 
    //..other code 
} 

With a variable of the type Size, an assignment is limited to the constants defined in Size. An enum is a perfect example of how language can simplify the implementation of a model, at the cost of certain constraints. Enums limit the extensibility to interfaces. Other than that, enums are full-fledged classes. As a developer, you can add states and behaviors to them. Another benefit is that an enum can also switch constructs, which was previously limited to primitives and a String class.