Custom Sorting in way that A comes before a and B comes before b

Question

I have a List of colors like this:

Pink, Blue, Red, blue, Grey, green, purple, black ...etc

List<String> listOfColors =  Arrays.asList("Pink", "Blue", "Red", "blue", "Grey", "green", "purple", "black");

There are some intermediate operation like filtering some fruit colors, now I am left with filtered results where I want them to be sorted in order:

Blue, black, blue, Grey, green, Pink, purple, Red

I have tried :

List<String> collect = listOfColors.stream().sorted(String::compareToIgnoreCase)
        .collect(Collectors.toList());

It does not work as expected.

The output is the following:

black, Blue, blue, green, Grey, Pink, purple, Red

I want the following:

Blue, black, blue, Grey, green, Pink, purple, Red

Answer 1

You can use RuleBasedCollator to define your own Rules.

Example of custom rule:

String rules = "< c,C < b,B";

The above rule is decoded as that both uppercase and lowercase C's are to appear before both uppercase and lowercase B's when comparing strings.

String customRules = "<A<a<B<b<C<c<D<d<E<e<F<f<G<g<H<h<I<i<J<j<K<k<L<l<M<m<N<n<O<o<P<p<Q<q<R<r<S<s<T<t<U<u<V<v<X<x<Y<y<Z<z";
RuleBasedCollator myRuleBasedCollator = new RuleBasedCollator(customRules);
Collections.sort(listOfColors,myRuleBasedCollator);
System.out.println(listOfColors);

Output:

[Blue, black, blue, Grey, green, Pink, purple, Red]

---

Edit: Instead of writing the customRules by hand, you can use the below to code to generate it.

String a = IntStream.range('a', 'z' + 1).mapToObj(c -> Character.toString((char) c))
        .flatMap(ch -> Stream
            .of("<", ch.toUpperCase(), "<", ch)).collect(Collectors.joining(""));