The map function counts the number of occurrences of characters

stem ：

  Gets the number of occurrences of each character on the custom text
Tips ： Traverse every character of the text ; The characters and the number of occurrences are saved in Map in ; take Map Write data to file
Included ：aabbccddwab    ----->a3b3c2d2w1

analysis :

• We can divide the results into several parts : a(5),b(4),c(3),d(2),e(1)
• Each part can be regarded as : Characters and the number of times corresponding to characters
• Data like this , We can go through HashMap Collection to store , Keys are characters , The value is the number of times the character appears

Be careful : key yes character , The type should be Character; value Characters appear frequency , The type should be Integer

Ideas :

• Enter a string on the keyboard
• establish HashMap aggregate , The key is Character, The value is Integer
• Traversal string , Get every character
• Take every character you get as a key to HashMap Find the corresponding value in the set , Look at the return value

        If the return value is null: Indicates that the character is in HashMap There is no such thing as , Just use this character as a key ,1 Store as a value
        If the return value is not null: Indicates that the character is in HashMap Exists in collection , Add this value to 1, Then re store the character and the corresponding value

• Traverse HashMap aggregate , Get health and value , Splice as required
• Output splicing results
   

  Abominable ！ It's a mistake , Null pointer exception NullPointerException

We found the reason , It turned out that there was no else sentence （ I should have missed it when I was writing notes , I thought I wrote , But I didn't ：D）

After the change

Complete code ：

package com.B.IOStream_14.IODemo;

import java.util.Scanner;
import java.util.Set;
import java.util.TreeMap;

//2.  Gets the number of occurrences of each character on the custom text 
// Tips ： Traverse every character of the text ; The characters and the number of occurrences are saved in Map in ; take Map Write data to file 
// Included ：aabbccddwab    ----->a3b3c2d2w1
public class A2Map {
    private static Object Character;

    public static void main(String[] args) {
//         Keyboard input string 
        Scanner sc = new Scanner(System.in);
        System.out.println(" Please enter the string ：");
        String line = sc.nextLine();

//        establish TreeMap aggregate ( Orderly , If it is HashMap It's out of order ), The key is Character, The value is Integer
        TreeMap<Character, Integer> map = new TreeMap<Character, Integer>();

// Traversal string 
        for (int i = 0; i < line.length(); i++) {
            char key = line.charAt(i); // Use character as key 
            // Take every character you get as a key to TreeMap Find the corresponding value in the set , Look at the return value 
            Integer value = map.get(key);

            if (value == null) {
                // If the return value is null： Indicates that the character is in TreeMap There is no such thing as , Just use this character as a key ,1 Store as a value 
                map.put(key, 1);
            }else {
                // If the return value is not null： Indicates that the character is in TreeMap Exists in collection , Add this value to 1, Then re store the character and the corresponding value 
                value++;
                map.put(key, value);
            }
        }
// Traverse TreeMap aggregate , And we get the bond and the value , Splice as required 
        StringBuilder sb = new StringBuilder();
//        Set Make sure that the keys are not repeated 
        Set<Character> KeySet = map.keySet();
//         iterator 
        for (Character key : KeySet) {
            Integer value = map.get(key);
            sb.append(key).append(value);
        }
        String result = sb.toString();
//         Output 
        System.out.println(result);
    }
}

result ：

 Please enter the string ：
wsgdgagdgdg
a1d3g5s1w1

 Process ended , Exit code  0