Enum in Java. Advantages?

What are some advantages of making enum in Java similar to a class, rather than just a collection of constants as in C/C++?

You get free compile time checking of valid values. Using

public static int OPTION_ONE = 0;
public static int OPTION_TWO = 1;

does not ensure

void selectOption(int option) {
...
}

will only accept 0 or 1 as a parameter value. Using an enum, that is guaranteed. Moreover, this leads to more self documenting code, because you can use code completion to see all enum values.

Type safety is one reason.

Another, that I find more important, is that you can attach metadata to enum values in Java. For example, you could use an enum to define the set of legal operations for a webservice, and then attach metadata for the type of request and data class:

AddItem(HttpMethod.POST, ProductEntry.class),

Java 5 enums originated from a typesafe enum pattern from Joshua Bloch's Effective Java (the first edition) to avoid the pitfalls of enums in C/C++/C# (which are simply thinly-veiled int constants) and the use in Java of final static int constants.

Primarily int constants and int enums aren't typesafe. You can pass in any int value. In C/C++ you can do this:

enum A { one, two, three };
enum B { beef, chicken, pork } b = beef;

void func(A a) { ... }
func((A)b);

Unfortunately the typesafe enum pattern from Effective Java had a lot of boilerplate, not all of it obvious. The most notable is you had to override the private method readResolve to stop Java creating new instances on deserialization, which would break simple reference checking (ie using the == operator instead of equals()).

So Java 5 enums offer these advantages over ints:

• Type safety;
• Java 5 enums can have behaviour and implement interfaces;
• Java 5 enums have some extremely lightweight data structures like EnumSet and EnumMap.

Java 5 enums over these advantages over just using classes:

• Less error-prone boilerplate (private constructor, readResolve() etc);
• Semantic correctness. You see something is an enum and you know it's just representing a value. You see a class and you're not sure. Maybe there's a static factory method somewhere, etc. Java 5 enums much more clearly indicate intent.

Enums are already a class in Java.

If you're asking why this is better, I'd say that better type safety and the ability to add other attributes besides a mere ordinal value would come to mind.