String comparator as a lambda

Question

I am writing a Comparator for my TreeMap. It looks like this:

private static final Comparator<String> CASE_INSENSITIVE_COMPARATOR = 
new Comparator<String>() {
    @Override
    public int compare(String o1, String o2) {
        int cmp = o1.toLowerCase().compareTo(o2.toLowerCase());
        if (cmp != 0) return cmp;

        return o1.compareTo(o2);
    }
};

My IDE hints me that it could be written in a lambda. I came up with this idea:

private static final Comparator<String> CASE_INSENSITIVE_COMPARATOR_LAMBDA = 
(s1, s2) -> 
s1.toLowerCase().compareTo(s2.toLowerCase()) == 0 ? 
s1.compareTo(s2) : s1.toLowerCase().compareTo(s2.toLowerCase());

But then I am duplicating the first call s1.toLowerCase().compareTo(s2.toLowerCase(). This is doing two String manipulation and one comparison more than the not lambda expression.

Answer 1

You can also chain the Comparators using thenComparing

CASE_INSENSITIVE_COMPARATOR = Comparator.comparing((Function<String, String>) String::toLowerCase)
                                        .thenComparing(o -> o);

Official Java docs on thenComparing.

Answer 2

I suggest a modification of @Ankint Soni solution with less structural diversity:

CASE_INSENSITIVE_COMPARATOR = Comparator.<String, String>comparing(o -> o.toLowerCase()).thenComparing(o -> o);

Or a slightly different flavor, more expressive:

CASE_INSENSITIVE_COMPARATOR = Comparator.<String, String>comparing(o -> o.toLowerCase()).thenComparing(Comparator.naturalOrder());

Answer 3

Even in the lambda, the RHS can be a code block with braces.

private static final Comparator<String> CASE_INSENSITIVE_COMPARATOR_LAMBDA = 
(s1, s2) -> {
    int cmp = o1.toLowerCase().compareTo(o2.toLowerCase());
    if (cmp != 0) return cmp;

    return o1.compareTo(o2);
};