Java: Why String.compareIgnoreCase() uses both Character.toUpperCase() and Character.toLowerCase()? [duplicate]

Question

The compareToIgnoreCase method of String Class is implemented using the method in the snippet below(jdk1.8.0_45).

i. Why are both Character.toUpperCase(char) and Character.toLowerCase(char) used for comparison? Wouldn't either of them suffice the purpose of comparison?

ii. Why was s1.toLowerCase().compare(s2.toLowerCase()) not used to implement compareToIgnoreCase? - I understand the same logic can be implemented in different ways. But, still I would like to know if there are specific reasons to choose one over the other.

    public int compare(String s1, String s2) {
        int n1 = s1.length();
        int n2 = s2.length();
        int min = Math.min(n1, n2);
        for (int i = 0; i < min; i++) {
            char c1 = s1.charAt(i);
            char c2 = s2.charAt(i);
            if (c1 != c2) {
                c1 = Character.toUpperCase(c1);
                c2 = Character.toUpperCase(c2);
                if (c1 != c2) {
                    c1 = Character.toLowerCase(c1);
                    c2 = Character.toLowerCase(c2);
                    if (c1 != c2) {
                        // No overflow because of numeric promotion
                        return c1 - c2;
                    }
                }
            }
        }
        return n1 - n2;
    }

Answer 1

Here's an example using Turkish i's:

System.out.println(Character.toUpperCase('i') == Character.toUpperCase('İ'));
System.out.println(Character.toLowerCase('i') == Character.toLowerCase('İ'));

The first line prints false; the second true. Ideone demo.