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khmarbaise
khmarbaise

Reputation: 97467

Why not extending from Enum<E extends Enum<E>>

I stumbled over the following problem that i can't extend and implement from this class which is defined in Java 1.5 (java.lang package)

public abstract class Enum<E extends Enum<E>> {...
}..

The problem i have is to create my own enum type which has different ordinal values. I don't want to implement it by using different name of ordinal like getCode() etc. So i thought i could go the way to extend the above class.

public final class XYZ extends Enum<XYZ> { //Does not work.
  //
  A("A", 1),
  B("B", 7);
  .
}

I know that i can do the following:

public enum NEWEnum {
   A(1),
   B(7);

   private int code;
   private NEWEnum(int code) {
     this.code = code;
   }
   public int getCode() {
     return this.code;
   }
}

I would prefer to have the usual namings in Enum's like ordinal() and name() instead.

Upvotes: 4

Views: 1214

Answers (5)

Anderson
Anderson

Reputation: 2748

For example,if you implement it explicitly, you don't have to init the property of "name" and "ordinal" in your constructor. It may cause NPE later.

Upvotes: 0

Thomas
Thomas

Reputation: 88727

Enums have a special meaning in the language core and special uses. Thus you can use them in switch statements, which isn't possible with normal classes.

Upvotes: 0

Philipp Wendler
Philipp Wendler

Reputation: 11433

The Enum class is special in Java, as it is used to implement the Enum language feature. Thus it is not just a class like any other class in Java, but more special (like String, for which the + operator is overloaded, or like the boxed classes). For these features to work, special requirements are needed for enums (like, I guess, consecutive ordinals). Thus the Java Language Specification explicitly forbids manually inheriting from the Enum class.

Upvotes: 1

josefx
josefx

Reputation: 15656

Enum gets special treatment by the compiler and classes like EnumSet make assumptions based on this treatment. The behavior of Enum methods like ordinal() and name() is strictly defined and implementing them yourself to replicate this behavior makes no sense. Also the syntax to declare enum values is only valid in an enum and wont compile in a class.

It is important to remember that you should never use ordinal() yourself, so defining it to suit your problem makes no sense.

Upvotes: 4

Joeri Hendrickx
Joeri Hendrickx

Reputation: 17445

You can't extend Enum like that. It's built in to the compiler, or the runtime (not sure which).

But it looks like you're trying to solve the wrong issue; the ordinal of an enum value is not supposed to have any functional meaning. As soon as you give it one, it should have a different name than 'Ordinal'. Your second code snippet is far superior to your first one.

In general, relying on ordinal for anything is bad practice; it probably never should've been exposed in the first place. The only thing you can rely on is the name, and any value you assign yourself.

If it's that important to you to name your field 'ordinal', just use the typesafe enum pattern (item 21), which Enum is just an implementation of.

Upvotes: 5

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