从函数生成常量表达式

Question

For my Google Wave robot, on the onDocumentChanged event I want to apply a filter as follows:

@Capability(filter = FILTER)
@Override
public void onDocumentChanged(DocumentChangedEvent event) {
    ...
}

I want the filter to be generated the first time the robot is run, which I'm trying to do as follows:

private static final String FILTER = generateFilter();

private static final String generateFilter(){
    ...
}

However, it complains FILTER isn't a constant expression when used within @Capability.

generateFilter() will return the same string every time it is called, I'm only using it to create the string so that when I make changes, I don't need to worry about updating the filter.

Now I could be going about this all wrong, so wondered if anyone knew what I'm doing wrong, or knew a better way in which I could generate a constant expression from the function.

Answer 1

I'm unfamiliar with Google Wave, but a static initializer might be acceptable, as shown here and outlined below.

private static final String FILTER;
static { FILTER = "..."; }

Addendum: On closer scrutiny, this approach is not possible, as an annotation value must be (among other things) a constant expression .

Answer 2

The compiler needs the contant value in the annotation at compile time and your initialization will happen i think at the application initialization time.

You could probably do it like this:

private static final String FILTER = "YOUR STRING";

private static final String generateFilter() {
   return FILTER;
}

That way if you need to change it and not worry you will go the the method and from there to the constant value :).