Java Program to Count the Occurrences of Each Character

In Java, counting the occurrences of each character in a string is a fundamental operation that can be done in different ways. This process involves identifying the frequency of each character in the input string and displaying it in a readable format.

Example: Input/output to count the occurrences of each character.

Input: s = "Geeks"
Output: G = 1, e = 2, k = 1, s=1.

Input: s = "Hello"
Output: H = 1, e= 1, l= 2, o= 1.

Naive Approach

This is the basic approach where, we manually compare each character of the string using nested loops and track the occurrences.

Algorithm:

• First, convert the string into an array of characters.
• Then, create a Boolean array of the same size as the input string with the characters.
• Iterate each character of the string one by one using a loop.
  • If the character is counted, then make the boolean array at the same index as true ( The character is already counted ). Increment the count by 1.
  • If the character is not counted then use another loop and compare it with the other characters of the string.
• After finishing the comparison, print the character occurrence count one by one.

Example: Iterative approach to counting the occurrences of each character in a String using nested loops.

// Java program to count character occurrences 
// using the Naive Approach
class Geeks{
  
    public static void main(String[] args) {
      
        String s = "Program";
        char[] ch = s.toCharArray();
        boolean[] b = new boolean[s.length()];

        System.out.println("Character Occurrences:");
        for (int i = 0; i < ch.length; i++) {
            if (b[i]) continue;     // Skip already counted characters
            int c = 1;
            for (int j = i + 1; j < ch.length; j++) {
                if (ch[i] == ch[j]) {
                    c++;
                    b[j] = true;     // Mark character as counted
                }
            }
            System.out.println(ch[i] + " : " + c);
        }
    }
}

Character Occurrences:
P : 1
r : 2
o : 1
g : 1
a : 1
m : 1

Explanation: The above example uses a "b" Boolean array to track counted characters. Then, nested loops iterate over the character array to calculate occurrences.

Other Methods to Count the Occurrences of Each Character

1. Using Counter Array

This method uses an integer array to store the frequency of characters based on their ASCII values.

Algorithm:

• First, create and integer array of size 256, which is used to store the ASCII values of the character.
• Now iterate through each character of the string one by one.
  • Convert the character to it's equivalent ASCII value.
  • Now we use the ASCII value as index and we increament the count at that index if we found the character i.e for a = 97, arr[97] = arr[97]+1.
• After counting the frequency, print their count.

Example:

// Java program to count character occurrences 
// using Counter Array
class Geeks
{
  
    public static void main(String[] args) {
      
        String s = "Program";
        int[] c = new int[256]; // Array for ASCII characters

        // Increment count for each character
        for (char ch : s.toCharArray()) {
            c[ch]++;
        }

        System.out.println("Character Occurrences:");

        // Print characters with non-zero counts
        for (int i = 0; i < c.length; i++) {
            if (c[i] > 0) {
                System.out.println((char) i + " : " + c[i]);
            }
        }
    }
}

Character Occurrences:
P : 1
a : 1
g : 1
m : 1
o : 1
r : 2

Explanation: In the above example, the ASCII value of each character acts as the index in the counter array. The program prints only non-zero counts by representing the character frequencies. This is efficient for ASCII characters but not suitable for Unicode.

2. Using Java HashMap

HashMap is used to dynamically map each character to its count. This approach is suitable for both ASCII and Unicode characters.

Algorithm:

• Create a HashMap to store characters as key and value a their frequency.
• Iterate through each character of the string
  • If the Character exists, increament its frequency.
  • If it doesn't exist, add it to the Map with initial frequency as 1.
• Then, print the each character with its frequencies.

Example:

// Java program to count character occurrences 
// using HashMap
import java.util.HashMap;
import java.util.Map;

class Geeks
{
  
    public static void main(String[] args) {
      
        String s = "Programming";
        Map<Character, Integer> countMap = new HashMap<>();

        for (char c : s.toCharArray()) {
            countMap.put(c, countMap.getOrDefault(c, 0) + 1);           // Update count
        }

        System.out.println("Character Occurrences:");
        for (Map.Entry<Character, Integer> entry : countMap.entrySet()) {
            System.out.println(entry.getKey() + " : " + entry.getValue());
        }
    }
}

Character Occurrences:
P : 1
a : 1
r : 2
g : 2
i : 1
m : 2
n : 1
o : 1

Explanation: In the above example, the HashMap stores characters as keys and their frequencies as values. The getOrDefault() method simplifies updating the count for each character. This approach is efficient and works for both ASCII and Unicode strings.

3. Using Java 8 Streams

We can use Java 8, streams and collectors that provide a functional and concise way to count character occurrences.

Algorithm:

• Convert the String into a stream of characters using chars(), it returns IntStreasm of ASCII values.
• Use mapToobj() to convert the stream of integers to a stream of character object.
• Use Collectors.groupinBy() to group the characters and then use Collectors.counting() to count the occurrences of each character.
• Then,we store it in Map, where the key as is the character and frequency as its value.

Example:

// Java program to count character occurrences 
// using Java 8 Streams
import java.util.Map;
import java.util.stream.Collectors;

class Geeks
{
  
    public static void main(String[] args) {
      
        String s = "Programming";

        Map<Character, Long> countMap = s.chars()
                                             .mapToObj(c -> (char) c)
                                             .collect(Collectors.groupingBy(c -> c, Collectors.counting()));

        System.out.println("Character Occurrences:");
        countMap.forEach((k, v) -> System.out.println(k + " : " + v));
    }
}

Character Occurrences:
P : 1
a : 1
r : 2
g : 2
i : 1
m : 2
n : 1
o : 1

Explanation: In the above example, the chars() method creates a stream of character codes from the string. The groupingBy collector groups characters and counts their occurrences. This is a modern and concise solution but is the slightly slower due to stream overhead.

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