Common String Programs

☝🏽 Count the Number of Vowels in a String

This program counts how many vowels (a, e, i, o, u) are present in a given string. It first converts the string to lowercase, then iterates through each character and checks if it’s a vowel.

Input: Google

Expected Output: 3

public class VowelCount {
    public static void main(String[] args) {
        String s = "Hello";
        int count = 0;
        s = s.toLowerCase();

        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o' || ch == 'u') {
                count++;
            }
        }

        System.out.println("Vowel count: " + count);
    }
}

Vowel count: 2

⚖️ Check if a String is a Palindrome

This program checks if a given string reads the same forward and backward. It reverses the string and compares it with the original.

Input: madam

Expected Output: Palindrome

public class PalindromeCheck {
    public static void main(String[] args) {
        String s = "madam";
        String reversed = "";

        for (int i = s.length() - 1; i >= 0; i--) {
            reversed += s.charAt(i);
        }

        if (s.equals(reversed)) {
            System.out.println("Palindrome");
        } else {
            System.out.println("Not a Palindrome");
        }
    }
}

Palindrome

🔁 Reverse a String

This program reverses the characters of a given string manually using a loop.

Input: Java

Expected Output: avaJ

public class ReverseString {
    public static void main(String[] args) {
        String s = "Java";
        String reversed = "";

        for (int i = s.length() - 1; i >= 0; i--) {
            reversed += s.charAt(i);
        }

        System.out.println("Reversed String: " + reversed);
    }
}

Reversed String: avaJ

🔤 Count the Number of Words in a Sentence

This program splits a sentence based on whitespace and counts the number of words.

Input: Java is awesome

Expected Output: 3

public class WordCount {
    public static void main(String[] args) {
        String s = "Java is awesome";
        String[] words = s.trim().split("\\\\s+");
        System.out.println("Word count: " + words.length);
    }
}

Word count: 3

✔️ Check if Two Strings Are Anagrams

This program checks whether two strings are anagrams by sorting their characters and comparing them.

Input: listen, silent

Expected Output: Anagrams

import java.util.Arrays;

public class AnagramCheck {
    public static void main(String[] args) {
        String s1 = "listen";
        String s2 = "silent";

        char[] a1 = s1.toCharArray();
        char[] a2 = s2.toCharArray();

        Arrays.sort(a1);
        Arrays.sort(a2);

        if (Arrays.equals(a1, a2)) {
            System.out.println("Anagrams");
        } else {
            System.out.println("Not Anagrams");
        }
    }
}

Anagrams

␣ Remove All Whitespaces from a String

This program removes all whitespace characters (spaces, tabs, newlines) from a string using regex.

Input: " Java Programming "

Expected Output: JavaProgramming

public class RemoveSpaces {
    public static void main(String[] args) {
        String s = "  Java   Programming  ";
        s = s.replaceAll("\\\\s+", "");
        System.out.println("Without spaces: " + s);
    }
}

Without spaces: JavaProgramming

🧮 Find the Frequency of a Character in a String

This program counts how many times a specific character appears in a given string.

Input: programming, character: g

Expected Output: 2

public class CharFrequency {
    public static void main(String[] args) {
        String s = "programming";
        char target = 'g';
        int count = 0;

        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == target) {
                count++;
            }
        }

        System.out.println("Frequency of '" + target + "': " + count);
    }
} 

Frequency of 'g': 2