java - How to sort a character by number of occurrences in a String using a Map?

Question

I wrote code that counts frequency of every character in a given string and displays it:

Map<Character, Integer> occurrences = new HashMap<>();
char[] chars = str2.toCharArray();
for (char character : chars) {
    Integer integer = occurrences.get(character);
    occurrences.put(character, integer);
    if (integer == null) {
        occurrences.put(character, 1);
    } else {
        occurrences.put(character, integer + 1);
    }
}
System.out.println(occurrences);

Now I want to modify my code, so it shows the characters ordered by their frequency. Starting with the character, that is most frequently repeated, then second most frequently, then third and so on.

For example the string Java should be displayed as character-frequency in following order: a=2, j=1, v=1.

Answer 1

Consider using a TreeMap ( https://docs.oracle.com/javase/7/docs/api/java/util/TreeMap.html )

TreeMap is a map implementation that keeps its entries sorted according to the natural ordering of its keys.

You can refer to the below code for reference.

import java.util.Map;
import java.util.TreeMap; 

class Test { 
    static void characterCount(String inputString) 
    { 
        TreeMap<Character, Integer> charCountMap = new TreeMap<Character, Integer>(); 
        char[] strArray = inputString.toCharArray(); 
        for (char c : strArray) { 
            if (charCountMap.containsKey(c)) { 
                charCountMap.put(c, charCountMap.get(c) + 1); 
            } 
            else { 
                charCountMap.put(c, 1); 
            } 
        } 
        for (Map.Entry entry : charCountMap.entrySet()) { 
            System.out.println(entry.getKey() + "=" + entry.getValue()); 
        } 
    } 

    public static void main(String[] args) 
    { 
        String str = "welcometostackoverflow"; 
        characterCount(str); 
    } 
} 

Output :

a=1
c=2
e=3
f=1
k=1
l=2
m=1
o=4
r=1
s=1
t=2
v=1
w=2