1. Introduction

Finding the frequency of each character in a string is a common task in Java programming. This task is essential for various applications, such as data compression, cryptography, and text analysis. In this tutorial, we will write a Java program to find the frequency of each character in a given string using a HashMap.

2. Program Steps

1. Define a main method to serve as the entry point of the program.

2. Declare and initialize a String whose characters’ frequencies we want to find.

3. Create a HashMap to store each character of the string as a key and its frequency as the value.

4. Loop through each character of the string.

5. For each character, check if it is already in the HashMap. If it is, increment its frequency. If it’s not, add it to the HashMap with a frequency of 1.

6. Print the HashMap which contains the character frequencies.

3. Code Program

import java.util.HashMap;

public class CharacterFrequency {

    public static void main(String[] args) {

        // Step 2: Declare and initialize a String whose characters' frequencies we want to find
        String str = "programming";

        // Step 3: Create a HashMap to store each character of the string as a key and its frequency as the value
        HashMap<Character, Integer> charFrequency = new HashMap<>();

        // Step 4: Loop through each character of the string
        for (char c : str.toCharArray()) {

            // Step 5: Check if character is already in the HashMap. If yes, increment its frequency, if not, add it with frequency 1
            if (charFrequency.containsKey(c)) {
                charFrequency.put(c, charFrequency.get(c) + 1);
            } else {
                charFrequency.put(c, 1);
            }
        }

        // Step 6: Print the HashMap which contains the character frequencies
        System.out.println("Character frequencies: " + charFrequency);
    }
}

Output:

Character frequencies: {a=1, g=2, i=1, m=2, n=1, o=1, p=1, r=1}

4. Step By Step Explanation

– Step 1: The main method is defined, acting as the entry point of the program.

– Step 2: A String named str is declared and initialized with the value "programming".

– Step 3: A HashMap named charFrequency is created to store each character of the string as the key and its frequency as the value.

– Step 4: A for-each loop is used to loop through each character of the string.

– Step 5: Inside the loop, we check whether the character is already in the HashMap. If it is, its frequency is incremented. If it's not, it is added to the HashMap with a frequency of 1.

– Step 6: The HashMap charFrequency, which contains the frequencies of each character, is printed to the console.

This program demonstrates how to use a HashMap in Java to find the frequency of each character in a string efficiently.