Reshma Bidikar

In this blog post, I will be demonstrating how you can determine the number of occurrences of each character in a String. Consider the following code snippet:

package learnjava.strings;

import java.util.HashMap; import java.util.Map;

public class CountCharactersDemo {

public static void main(String[] args) { 
    String str = "Hello World"; 
    //map stores each character and its count 
    Map<Character,Integer> characterCountMap = new HashMap<Character,Integer>(); 
    char[] charsInStr = str.toCharArray(); 
    for(char c:charsInStr){ 
        if(characterCountMap.containsKey(c)){ 
            //if the character is already in the map, just increment its count 
            int count = characterCountMap.get(c); 
            count++; 
            characterCountMap.put(c, count); 
        } 
        else { 
            //if the character is not in the map, add it to the map 
            characterCountMap.put(c, 1); 
        }
    } 
    for(Character c:characterCountMap.keySet()){ 
        System.out.println("Character "+c+" occurs "+characterCountMap.get(c)); 
    }

}

}

The characterCountMap defines a HashMap that stores each character as a key and its count as a value. A for loop then iterates through the characters in the input String. It checks if the character is already in the map. If so, it just increments the count. If the character is not in the map, it adds it to the Map. So when you run this program, you will get the following output:

Character occurs 1
Character r occurs 1
Character d occurs 1
Character e occurs 1
Character W occurs 1
Character H occurs 1
Character l occurs 3
Character o occurs 2