What exactly does ++array[s.charAt(i) - 'A'] do?

Question

for (int i = 0; i < s.length(); ++i) 
    {
        if (s.charAt(i) >= \'A\' && s.charAt(i) <= \'Z\') 
        {
            ++array[s.charAt(i) - \         


        

Answer 1

array seems to be a "counter per capital letter". By subtracting character 'A' from an arbitrary character in a string, you get the letter's index in the array:

'A' - 'A' == 0
'B' - 'A' == 1
'C' - 'A' == 2

To understand this, you should understand, that Java treats char the same as (unsigned) short. Hence, you can make calculations with char