I have a situation where I have three requirements: Lazy initialization - don\'t

Question

I have a situation where I have three requirements:

Lazy initialization - don't create the collection until asked for it
Caching - keep the collection in memory on the object
Reinitialization - be able to reinitialize the collection when desired, instead of simply getting the existing results.
This is simply an optimization inside a single class - it is not loading anything from a database and ideally I'd like just a good method design pattern for this, not a multiple-class design.

Usually for lazy initialization I'd have this:

Collection getCollection() {
    if (collection != null) {
         // generate and set collection
    }
    return collection;
}
But now I'm having trouble deciding on the best way to provide for reinitialization of a fresh collection and getting that collection. A fresh boolean parameter would work, but adding a parameter to a getter doesn't seem to feel right (maybe that's the Java in me talking

Explanation / Answer

If you're limiting yourself to a single method call then I can't honestly imagine a parameterless solution that distinguishes between retrieving the cached value and a reinitialized value.

Every cache I've seen uses one of the following patterns:

An Invalidate or Expire method that flags the value for reinitialization on the next lookup;

A boolean or enumeration parameter, such as the one you've ruled out;

A separate reinitialization method, i.e. getNewCollection.

I do think it must be the Java in you talking, because in other languages I'm very much accustomed to passing parameters in cache lookups - in some cases one of the parameters may even be an anonymous method or function pointer telling the cache how to get the value.

When designing a cache based on deferred initialization, you'll almost certainly also want to have method overloads that take parameters for priority and/or expiration, since there's no longer any Put or Store method to hold them. So, I think the notion of a parameterless lazy-loaded lookup method with optional refresh is pretty much out the window.

If you're uncomfortable with it being a getter, then just give it a different name like Load or Lookup.

P.S. I realize you may not actually be designing a cache here, but the principles still apply; you don't want to have something that acts like a cache internally but doesn't provide cache semantics. That would just lead to headaches.

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