Java Implicit conversion char to int?

Question

I was given an interview question to output the most frequent occurrence of a character.

Given this string "aaxxaabbaa". The character 'a' is the most frequent.

The following code was something that I found searching on the internet. Note: I implemented it with 2 loops which is in-efficient (different topic)

public static char findMostUsedChar(String str){
    char maxchar = ' ';
    int maxcnt = 0;
    // if you are confident that your input will be only ascii, then this array can be size 128.
    // Create a character counter
    **int[] charcnt = new int[Character.MAX_VALUE + 1];**
    for(int i = 0; i < str.length()-1; i++){
        char **ch** = str.charAt(i);
        // increment this character's cnt and compare it to our max.
        if (**charcnt[ch]**++ >= maxcnt) {
            maxcnt = charcnt[ch];
            maxchar = ch;
        }
    }
    return maxchar;
}

They declared an int array, found the character at a specific index (ie 'a') then used as an index. After tracing the code out on the debugger in eclipse, I still don't understand how using a character to represent an int index works without explicitly casting it or using charVal.getNumericValue()?? Even most of the SO char to int topics explicitly cast.

Thanks in advance.

Answer 1

Array access expressions undergo implicit unary numeric promotion, which will widen an expression to int .

The index expression undergoes unary numeric promotion (§5.6.1).

The char datatype is widened to int via its Unicode value, eg 'A' -> 65 .