Question

Assuming I have

final Iterable<String> unsorted = asList("FOO", "BAR", "PREFA", "ZOO", "PREFZ", "PREFOO");

What can I do to transform this unsorted list into this:

[PREFZ, PREFA, BAR, FOO, PREFOO, ZOO]

(a list which begin with known values that must appears first (here “PREFA” and “PREFZ”) and the rest is alphabetically sorted)

I think there are some usefull classes in guava that can make the job (Ordering, Predicates…), but I have not yet found a solution…

Answer 1

You specifically mentioned guava; along with Sylvain M’s answer, here’s another way (more as an academic exercise and demonstration of guava’s flexibility than anything else)

// List is not efficient here; for large problems, something like SkipList 
// is more suitable
private static final List<String> KNOWN_INDEXES = asList("PREFZ", "PREFA");

private static final Function<Object, Integer> POSITION_IN_KNOWN_INDEXES 
    = new Function<Object, Integer>() {
  public Integer apply(Object in) {
     int index = KNOWN_INDEXES.indexOf(in);
     return index == -1 ? null : index;
  }     
};


...


List<String> values = asList("FOO", "BAR", "PREFA", "ZOO", "PREFZ", "PREFOO");

Collections.sort(values,
  Ordering.natural().nullsLast().onResultOf(POSITION_IN_KNOWN_INDEXES).compound(Ordering.natural())
);

So, in other words, sort on natural order of the Integer returned by List.indexOf(), then break ties with natural order of the object itself.

Messy, perhaps, but fun.