Multi criteria sorting of a list of objects with Guava Ordering

Question

I have a class WHICH CANNOT implement comparable, but needs to be sorted based on 2 fields. How can I achieve this with Guava?

Let\'s say the class is:

c         


        

Answer 1

Java 8 provides methods on Comparator to concisely specify chained comparators. Together with the newly-introduced List.sort, you can do:

lotsOfX.sort(
    Comparator.comparingInt(x -> stringValueSortFunction.apply(x.stringValue))
        .thenComparing(x -> x.dateValue, Comparator.reverseOrder()));

This mutates the list, of course -- make a copy first if you want to leave the original list unchanged, or wrap the comparator in an Ordering and use immutableSortedCopy if you want an immutable copy.

Answer 2

I suspect you want Ordering.compound. You could do it all in one statement, but I'd use:

Ordering<X> primary = Ordering.natural().onResultOf(stringValueSortFunction);
Ordering<X> secondary = Ordering.natural()
                              .onResultOf(dateValueSortFunction)
                              .reverse();
Ordering<X> compound = primary.compound(secondary);

List<X> sortedList = compound.immutableSortedCopy(lotsOfX);

Answer 3

A less functional, but arguably cleaner, solution:

new Ordering<X>() {
  public int compare(X x1, X x2) {
    return ComparisonChain.start()
      .compare(x1.stringValue, x2.stringValue)
      .compare(x2.dateValue, x1.dateValue) // flipped for reverse order
      .result();
  }
}.immutableSortedCopy(listOfXs);