Is it possible to create Java enums based on input file?

Question

I'm using Java 6.

Suppose I have a file availableFruits.txt

APPLE
ORANGE
BANANA

Suppose I want an enum FruitType that contains values listed in availableFruits.txt, will I be able to do this?

Answer 1

You can't populate an enum type at execution time, no - at least, not without something like BCEL, or by calling the Java compiler.

You can write code to create a Java source file, of course, and build that when you build your app, if you don't need it to be changed afterwards.

Otherwise, I'd just create a wrapper class which is able to take a set of known values and reuse them. Exactly what you need to do will depend on how you wanted to use the enum, of course.

Answer 2

Well the point of an Enum is to use it at compile time. If you don't know at compile time what values your Enum has then it's not an Enum it's a collection.

If you do know and you just want to create a class file base on the values in the text file then yes it's possible by reading the txt then generating the source code.

Answer 3

I expect it's possible, by writing your own ClassLoader subclass, creating the bytecode for the enum in a byte array, and using defineClass. Hard, maybe, but possible. I expect once you know the byte sequence for an enum, it's not that hard to custom-generate it from the info in the JVM spec.

Now, whether it's a good idea...well, I suspect only in a very small number of edge cases. (I can't think of one; I mean, having created it, you'd have to generate code to use it, right?) Otherwise, you're probably better off with a Map or similar.