关于数组越界问题

package countletter;

import java.util.Scanner;

public class Countletter {
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        String str = in.nextLine();
        str=str.toLowerCase();
        int[] a = new int[26];
        for(int i = 0;i < str.length();i++) {
            char c = str.charAt(i);
            int index = c - 'a';
            a[index] = a[index] + 1;
        }
        for(int i = 0;i < a.length;i++) {
            if(a[i]!=0) {
                System.out.println("("+(char)(i+'A')+")"+"Num="+a[i]);
            }
        }
    }
}