Example showing RxJava for synchronous operations with no subscribers.

eff_levels.java

// Example of calculating effective character level from a MUD.
// Single-classed character of level 20 would be level 20.
// Multi-classed character would gain an additional level for
// every 3 levels in their non-primary classes (primary class
// is highest level class).
// e.g. Mage: 30, Ranger: 5:, Thief: 9 # Effective Level: 34
public int effectiveLevel() {
  if (classes.size() == 1) {
    return classes.get(0).level;
  }

  List<Integer> levels = Observable.from(classes).map((charClass) -> charClass.level)
    .toList().toBlockingObservable().last();

  int highestLevel = Collections.max(levels);
  int effectiveLevels = Observable.from(levels)
    .filter((level) -> level != highestLevel)
    .map((level) -> level / 3)
    .reduce((seed, level) -> seed + level)
    .toBlockingObservable().last();

  return highestLevel + effectiveLevels;
}

Normally you'd use RxJava in conjunction with subscribers. Also, technically since I'm using Java 8 here (lambdas etc.), I could have used the new stream API to achieve a similar result, but I am kind of liking the RxJava API better.