强制类实现实现另一个接口的两个接口之一

Question

I am not really sure about Java Annotations, but I think they can solve my problem.

I have an java interface "Target". This is an empty interface, so I can give that implementation into an "TargetHolder", which is simply a list of Targets.

Now I only have 2 Types of Targets. Type "Alpha" and type "Beta".

Type "Alpha" has no functionality in common with Type "Beta".

Easiest way would be to just extend "Beta" and "Alpha" from "Target". But with this solution it is possible for a programmer to create a class that extends "Target" only, which must not be possible.

1. Can I solve that with annotations?
2. How?

Answer 1

In theory you might be able to implement the checks (at compile time) using an annotation processor. The problem is that javac will only run an annotation processor on a source file if it finds the right kind annotation in the source.

"After scanning the source files and classes on the command line to determine what annotations are present, the compiler queries the processors to determine what annotations they process. When a match is found, the processor will be invoked."

(Javac manual)

But it seems like you want an annotation on an interface to constrain all classes that implement that interface. That means checking all such classes ... but I can't see how you could trigger the running of an annotation processor on a class that has no relevant annotations.

That leaves you with a couple of options:

• Implement the checking as (say) a PMD rule.
• Write a tool to find the relevant interfaces at runtime, retrieve their annotations, then trawl for all classes that implement the annotated interfaces.

---

My advice would be to put this into the "too hard" basket. It is probably going to take more time to implement this than you would save in picking up related coding errors earlier. (I'm thinking that the case that you are trying to avoid will be picked up when someone tries to use class. So, you (or your client) should find your (their) incorrect class in testing ...)

Answer 2

How about?

1. Create a package just for this work. Let us call it target.
2. Put Target.java in package target - package private.
3. Put Alpha.java in package target - public
4. Put Beta.java in package target - public
5. Compile, jar, and seal package target.

Answer 3

Using Tool like JArchitect allows to enforce design rules. In your case you can use the following cqlinq query:

warnif count > 0 from t in Types where t.Implement ("Target")
&& (!t.Implement("Alpha")|| !t.Implement("Beta"))
select  t