Java Program to Count The Frequency of Each Character in a String

In this post we’ll see a Java program to count the frequency of each character in a string. Here two ways of counting the number of times each character appears in a String are given.

1. Using HashMap where character is the key and count is the value. For every character you need to verify if the key already exists in the HashMap. If it does, increase the count otherwise add it to the map as a new key with count 1. See example.
2. Another way is using char array where you will have to take the char at the first index of the array and iterate the whole array to check if that char is found again, if yes increment the count. At the end replace() method of the String class is used to remove all the occurrences of that character. See example.

Java program Using HashMap to count the frequency of each character

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

public class CountChars {
  public static void main(String[] args) {
    CountChars.countCharactersMap("This is a test line");
  }
	
  public static void countCharactersMap(String str){
    Map<Character, Integer> charMap = new HashMap<Character, Integer>();
    for(int i = 0; i < str.length(); i++){
      char c = str.charAt(i);
      // For space or tab skip the process
      if(((c == ' ') || (c == '\t'))){
        continue;
      }
      // if character is already there in the map
      // increment the count
      if(charMap.containsKey(c)){
        charMap.put(c, charMap.get(c) + 1);
      }else{
        // If new character put that character in the map
        // with the value as 1
        charMap.put(c, 1);
      }
    }
    // Displaying the map values
    Set<Map.Entry<Character, Integer>> numSet = charMap.entrySet();
    for(Map.Entry<Character, Integer> m : numSet){
      System.out.println(m.getKey() + " - " + m.getValue());
    }
  }
}

Output

a - 1
s - 3
T - 1
t - 2
e - 2
h - 1
i - 3
l - 1
n - 1

Using char array and String replace() method

Here is a Java program to count the frequency of each character in a String using char array and String replace() method .

public class CountChars {
  public static void main(String[] args) {
    CountChars.countCharacters("This is a test line");
  }
	
  public static void countCharacters(String str){
    char[] strArr;
    do{
      strArr = str.toCharArray();
      char ch = strArr[0];
      int count = 1;
      for(int i = 1; i < strArr.length; i++){
        if(ch == strArr[i]){
          count++;
        }
      }
      // don't display for space or tab
      if(((ch != ' ') && (ch != '\t'))){
        System.out.println(ch + " - " + count);
      }
      // replace all occurrence of the character 
      // which is already counted
      str = str.replace(""+ch, "");
      // If string is of length 0 then come
      // out of the loop
      if(str.length() == 0) {
        break;
      }
    }while(strArr.length > 1);	
  }
}

Output

T - 1
h - 1
i - 3
s - 3
a - 1
t - 2
e - 2
l - 1
n - 1

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