统计字母出现频率

要求：输出某个英文文本文件中 26 字母出现的频率，由高到低排列，并显示字母出现的百分比，精确到小数点后面两位。
代码：
 1 import java.io.BufferedReader;
 2 import java.io.File;
 3 import java.io.FileReader;
 4 import java.io.IOException;
 5 import java.util.Scanner;
 6 
 7 public class tongJi {
 8     public static void main(String[] args) throws IOException {
 9         File file = new File("D:\Harry Potter\harry potter\Harry Potter and the Sorcerer's Stone.txt");
10         if (!file.exists()) {
11             System.out.println("文件不存在");
12             return;
13         }
14         BufferedReader buf = new BufferedReader(new FileReader(file));
15 
16         int[] num = new int[100];
17         char[] ying = new char[100];
18         String s = buf.readLine();
19         while (s != null) {
20             if (s.equals("exit")) {
21                 System.out.println("文件存在");
22                 System.exit(1);
23             } else {
24                 String[] lineWords = s.split(" ");
25                 for (int i = 0; i < lineWords.length; i++) {
26                     for (int j = 0; j < lineWords[i].length(); j++) {
27                         if (lineWords[i].charAt(j) >= 'A' && lineWords[i].charAt(j) <= 'Z')
28                             num[lineWords[i].charAt(j) - 'A' + 1]++;
29                         else if (lineWords[i].charAt(j) >= 'a' && lineWords[i].charAt(j) <= 'z')
30                             num[lineWords[i].charAt(j) - 'a' + 1 + 24]++;
31                     }
32                 }
33                 s = buf.readLine();
34                 
35             }
36         }
37         char a = 'A';
38         char b = 'a';
39         for (int i = 1; i <= 52; i++) {
40             if (i <= 26)
41                 ying[i] = a++;
42             else
43                 ying[i] = b++;
44         }
45 
46         
47         int sum = 0;
48         for (int i = 1; i <= 52; i++) {
49             sum += num[i];
50         }
51         
52         for (int i = 1; i <= 52; i++) {
53             for (int j = i + 1; j <= 52; j++) {
54                 if (num[i] < num[j]) {
55                     int t = num[i];
56                     num[i] = num[j];
57                     num[j] = t;
58 
59                     char m = ying[i];
60                     ying[i] = ying[j];
61                     ying[j] = m;
62                 }
63             }
64         }
65         System.out.println("字母总数："+sum);
66         System.out.println("按频率从大到小排列：");
67         for (int i = 1; i <= 52; i++) {
68             double ans = num[i] * 1.0 / sum * 100;
69             System.out.println(ying[i] + " frequency is " + String.format("%.2f", ans) + "%");
70             
71         }
72     }
73 }