Add @CheckReturnValue to detect common mistake through static analysis

A common mistake among new to ParSeq is to treat Task as if it’s creation caused is execution e.g.:

Task<RelevanceItems> getRelevanceItemsTask = _relevanceBackendClient.get(keys)
  .map(relevanceItems -> {
    if (Sets.difference(userItems, relevanceItems).size() > 0) {
      // Doesn’t actually execute since this is a Task but isn’t connected to parent task
      _userItemsClient.update(userId, userItems.addAll(relevanceItems));
    }
    return _userItems;
  });

Using jsr305 @CheckReturnValue on methods that return Task would allow detecting this mistake through static analysis.

[jodzga]

One possibility is to create a custom FindBugs rule. It should not be too hard: refactor rule that validates @CheckReturnValue annotation with a twist that it applies to all methods (annotation is not necessary) which return Task type. This could be a tiny contrib project.

[linjer]

If this turns out to be high effort, I want to note that findbugs community is moving towards the spotbugs project.

Some alternatives could include

• Adding this as a feature in spotbugs instead
• Use Google ErrorProne