Sort a list alphabetically with a specific value at head (if present)

Question

I'm looking for the most readable definition for a comparator that satisfies the following test case:

@Test
public void testComparator() {
    List<String> toSort = Lists.newArrayList("a", "b", "c", "d", "e", "f");
    Collections.shuffle(toSort);
    Comparator<String> theSolution = ???; 
    Collections.sort(toSort, theSolution);
    System.out.println(toSort); // Prints [c, a, b, d, e, f]
}

I've tried a comparator using Guava's Ordering defined like this:

Ordering.explicit("c").thenComparing(Ordering.natural());

However, explicit throws an exception for items not enumerated. So that solution fails. Any suggestions?

Answer 1

You could explicitly write a comparison function, e.g.

Comparator<String> theSolution = Comparator.comparing(a -> a.equals("c") ? "" : a);
// treat "c" as the same as the empty string "" when sorting which will be ranked first.

Answer 2

It's unclear from your example if you want the members that are not "c" to remain in their current order or be sorted into alphabetical order. If you want to sort "c" to the front and leave the remainder then you could use:

Comparator.comparing("c"::equals).reversed();

Reversing the comparator is required because the natural ordering for a boolean is false first.

Sorting the remaining items in alphabetical order can be achieved by:

Comparator.comparing("c"::equals).reversed()
    .thenComparing(Object::toString, String::compareTo);