[configuration] Enum constants as keys

[Christof May]

Hi all,

I'm not sure if this issue has been discussed before (couldn't find 
anything on the mail list thou...), but what do you guys think of using 
type-safe enum constants as keys instead of plain String values?

I assume there is a general understanding here that using enum constants 
instead of strings is the "right thing" to do, but obviously there are 
also important reasons not do so (legacy code, interface changes, 
pre-Java1.5 stuff etc...). But I guess the most important one is that 
Java enums never have been designed to work in a generic form (namely: 
no abstract enums and/or enum inheritance). So there is no way to put an 
enum placeholder in a library, and provide the concrete enum values in 
the implementing application. An issue which I and other people already 
have bemoaned (see 
http://java.dzone.com/articles/java-should-have-extended for example), 
but it is nevertheless a given fact we have to live with in the 
foreseeable future... :(

Having said that, I just see two ways of using enum constants for 
fetching config values. For one just using a lame 
config.getWhatever(MyEnum.key.name()) everywhere. It would be a start, 
but well.. not really what I was searching for...

The other solution I see would be to mark the enums with a marker 
interface, and take that as the key placeholder, such as:

public interface Configurable {
    public String name();
}

public interface Configuration {
boolean getBoolean(Configurable key);
    (other methods follow here...)
}

In the application you would define the keys in an enum such as that:

public enum MyKeys implements Configurable {
    FOO,
    BAR,
    ...;
}

Then you could access the config values in real type-safe way:

boolean myValue = config.getBoolean(MyKeys.BAR);

Another advantage would be that enum constants can be easily enriched 
with meta-data (via a custom annotation), for example:

public enum MyKeys implements Configurable {
    @ConfigData(
        defaultValue="foo",
        type=String.class,
        mandatory=true,
        pattern="[a-z]{1,4}",
        reload=false)
    FOO,
    ...;
}

The possibilities here are endless (see also my pet project at 
www.soplets.org exploring more in-depth the meta-data aspects of 
annotations), but for a beginning having just enum constants alone would 
be a good start in my view...

What do you think about that proposal, does that make any sense? Any 
other options I have overlooked so far? Looking forward hearing your 
opinions...

regards,

Chris May