Case-specific Sorting of Strings

class Solution {
    //Function to perform case-specific sorting of strings.
    public static String caseSort(String str) {
        int[] freqLowerCase = new int[26];
        int[] freqUpperCase = new int[26];
        StringBuilder res = new StringBuilder();
        
        for(int i = 0; i < str.length(); i++) {
            if(str.charAt(i) >= 'a' && str.charAt(i) <= 'z') 
                freqLowerCase[str.charAt(i) - 'a']++;
            else 
                freqUpperCase[str.charAt(i) - 'A']++;
        }
        
        int indexi = 0, indexj = 0;
        
        for(int i = 0; i < str.length(); i++) {
            if(str.charAt(i) >= 'a' && str.charAt(i) <= 'z')  {
                char x = 'a';
                while(freqLowerCase[indexi] == 0) {
                    indexi++;
                }
                res.append(x += indexi);
                freqLowerCase[indexi]--;
            }
            else{
                char x = 'A';
                while(freqUpperCase[indexj] == 0) {
                    indexj++;
                } 
                res.append(x += indexj);
                freqUpperCase[indexj]--;
            }
        }
        
        return res.toString();
    }
}