Java 1.5 Tiger A Developers Notebook [Electronic resources] نسخه متنی

3.11 Extending an Enum

It's often easy to define a hierarchy of enumerations. In this scenario, one
enum represents a base type of allowed values. Subclasses of that enum
would add additional values to the base enum, perhaps specialized to a
certain task.

3.11.1 How do I do that?

Here's another one of those pesky, "You don't" labs. Tiger does not allow
extension of an enum. For example, consider Example 3-12, a simple
extension of the Grade enum defined back in Example 3-1.

Example 3-12. Extending the Grade enum

package com.oreilly.tiger.ch03;
public enum CollegeGrade extends Grade { DROP_PASSING, DROP_FAILING }

In theory, this would take the values Grade.A, Grade.B, and so forth, and
add to them two new values, CollegeGrade.DROP_PASSING and
CollegeGrade.DROP_FAILING. However, you'll get compilation errors if
you try this:

compile-ch03-errors:
[echo] Compiling all Java files...
[javac] Compiling 13 source files to classes
[javac] src\ch03\CollegeGrade.java:3: '{' expected
[javac] public enum CollegeGrade extends Grade {DROP_PASSING, DROP_
FAILING}
[javac]                          ^
[javac] src\ch03\CollegeGrade.java:3: <identifier> expected
[javac] public enum CollegeGrade extends Grade {DROP_PASSING, DROP_
FAILING}
[javac]
^
[javac] 2 errors

NOTE

Use the Ant
target "compilech03-
errors" to try
and compile the
CollegeGrade.
java source file.

3.11.2 What about...

...using a class to extend an enum, instead of another enum? That
doesn't work either. There's just no getting around this limitation, at least
that I've been able to find..

NOTE

If you find a
sneaky way to
extend an enum,
let us know! We'll
add it to the
next edition.