How to find the index of the first unique character of the given string in Java?

Question

Can someone please explain me the following program from Line 11 to Line 17.

import java.util.*;
public  class  Solution {
    public static void main(String[] args) {
        String s = "wresource";
        System.out.println("Original String: "+s);
        System.out.println("Index of the First unique character of the given string is at : "+first_Uniq_Char(s));
    }

    public static int first_Uniq_Char(String s) {
         int[] freq = new int[256];
        for (char c : s.toCharArray()) {   //Line 11
            freq[c - 'a']++;
        }
        for (int i = 0; i < s.length(); i++) {
            if (freq[s.charAt(i) - 'a'] == 1) return i;
        }
        return -1;  //Line17
    }
 }

Output:

Original String: wresource
Index of the First unique character of the given string is at : 0

.

Answer 1

freq[] is an array which holds each element frequency ie

freq[w] =1

freq[r] =2

freq[e] =2

freq[s] =1 , etc

freq[c - 'a']++; this line converts your each character to ASCII code

Answer 2

As we know array index is of type integer only, we cannot have a[c] ( c is a character).

So what we have done is we have converted c into ASCII ie freq[c-'a'] means 99 - 97 = 3 ( a in ASCII is 97 and C is 99) freq[3] = 3 is same as freq[c]