学以致用——Java源码——统计用户输入的一句话中各个字符出现的次数（字频统计）（Counting Letters）

不错，半小时之内解题成功！

源码如下：

//Java how to program, 10th edition
//Exercise. 16.14: CharCount.java
//(Counting Letters) Modify the program of Fig. 16.18 to count the number of occurrences
//	of each letter rather than of each word. For example, the string \"HELLO THERE\" contains two Hs, three
//	Es, two Ls, one O, one T and one R. Display the results.
import java.util.Map;
import java.util.HashMap;
import java.util.Set;
import java.util.TreeSet;
import java.util.Scanner;

public class CharCount
{
public static void main(String[] args)
{
   // create HashMap to store String keys and Integer values
   Map<String, Integer> myMap = new HashMap<>(); 

   createMap(myMap); // create map  d on user input
   displayMap(myMap); // display map content
} // end main

// create map from user input
private static void createMap(Map<String, Integer> map) 
{
   Scanner scanner = new Scanner(System.in); // create scanner
   System.out.println(\"Enter a string:\"); // prompt for user input
   String input = scanner.nextLine();

   // tokenize the input（将输入的字符串转换为字符数组）
   char[] tokens = input.toCharArray();
            
   // processing input text 
   for (char token : tokens) 
   {
                     
      // if the map contains the word （将字符数组中的字符转换为字符串，因为java.util.Map.put(String key, Integer value)中要求字符串数据类型）
      if (map.containsKey(String.valueOf(token))) // is word in map
      {
         int count = map.get(String.valueOf(token)); // get current count
         map.put(String.valueOf(token), count + 1); // increment count
      } 
      else 
         map.put(String.valueOf(token), 1); // add new char with a count of 1 to map
   } 
   
   scanner.close();
} 

// display map content
private static void displayMap(Map<String, Integer> map) 
{     
   Set<String> keys = map.keySet(); // get keys

   // sort keys
   TreeSet<String> sortedKeys = new TreeSet<>(keys);

   System.out.printf(\"%nMap contains:%nKey\\t\\tValue%n\");

   // generate output for each key in map
   for (String key : sortedKeys)
      System.out.printf(\"%-10s%10s%n\", key, map.get(key));
   
   System.out.printf(
      \"%nsize: %d%nisEmpty: %b%n\", map.size(), map.isEmpty());
} 
} // end class

运行结果：

Enter a string:
Hello Pandeng, enjoy yourself in Java World!

Map contains:
Key		Value
                   6
!                  1
,                  1
H                  1
J                  1
P                  1
W                  1
a                  3
d                  2
e                  4
f                  1
g                  1
i                  1
j                  1
l                  4
n                  4
o                  4
r                  2
s                  1
u                  1
v                  1
y                  2

size: 22
isEmpty: false

Enter a string:
天行健，君子当自强不息；地势坤，君子以厚德载物

Map contains:
Key		Value
不                  1
以                  1
健                  1
势                  1
厚                  1
君                  2
地                  1
坤                  1
天                  1
子                  2
强                  1
当                  1
德                  1
息                  1
物                  1
自                  1
行                  1
载                  1
，                  2
；                  1

size: 20
isEmpty: false