Top 15 Java String Coding Interview Questions – Part 2

1. Reverse a String Using Recursion

public class ReverseRecursively {
    public static String reverse(String str) {
        if (str.isEmpty()) return str;
        return reverse(str.substring(1)) + str.charAt(0);
    }

    public static void main(String[] args) {
        System.out.println(reverse("Interview"));
    }
}

2. Find All Permutations of a String

public class StringPermutations {
    public static void permute(String str, String ans) {
        if (str.length() == 0) {
            System.out.println(ans);
            return;
        }
        for (int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            String ros = str.substring(0, i) + str.substring(i + 1);
            permute(ros, ans + ch);
        }
    }

    public static void main(String[] args) {
        permute("abc", "");
    }
}

3. Count Words in a String

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

4. Check if String is Rotation of Another

public class StringRotation {
    public static void main(String[] args) {
        String s1 = "ABCD";
        String s2 = "CDAB";
        boolean result = (s1.length() == s2.length()) && ((s1 + s1).contains(s2));
        System.out.println("Is Rotation: " + result);
    }
}

5. Count the Number of Alphabets, Digits and Special Characters

public class CountAllTypes {
    public static void main(String[] args) {
        String str = "Java123@#";
        int letters = 0, digits = 0, special = 0;

        for (char c : str.toCharArray()) {
            if (Character.isLetter(c)) letters++;
            else if (Character.isDigit(c)) digits++;
            else special++;
        }

        System.out.println("Letters: " + letters);
        System.out.println("Digits: " + digits);
        System.out.println("Special: " + special);
    }
}

6. Find the Most Frequent Character

import java.util.HashMap;

public class MostFrequentChar {
    public static void main(String[] args) {
        String str = "success";
        HashMap<Character, Integer> map = new HashMap<>();

        for (char c : str.toCharArray()) {
            map.put(c, map.getOrDefault(c, 0) + 1);
        }

        char maxChar = ' ';
        int max = 0;
        for (char c : map.keySet()) {
            if (map.get(c) > max) {
                max = map.get(c);
                maxChar = c;
            }
        }

        System.out.println("Most Frequent: " + maxChar);
    }
}

7. Convert Char Array to String

public class CharArrayToString {
    public static void main(String[] args) {
        char[] arr = {'J', 'a', 'v', 'a'};
        String str = new String(arr);
        System.out.println(str);
    }
}

8. Remove All Whitespaces From String

public class RemoveSpaces {
    public static void main(String[] args) {
        String str = "  Java  Developer ";
        System.out.println(str.replaceAll("\\s", ""));
    }
}

9. Replace Each Vowel with '*' in String

public class ReplaceVowels {
    public static void main(String[] args) {
        String str = "interview";
        str = str.replaceAll("[aeiouAEIOU]", "*");
        System.out.println(str);
    }
}

10. Capitalize First Letter of Each Word

public class CapitalizeWords {
    public static void main(String[] args) {
        String[] words = "java interview questions".split(" ");
        StringBuilder sb = new StringBuilder();
        for (String w : words) {
            sb.append(Character.toUpperCase(w.charAt(0)))
              .append(w.substring(1)).append(" ");
        }
        System.out.println(sb.toString().trim());
    }
}

11. Remove All Digits from String

public class RemoveDigits {
    public static void main(String[] args) {
        String str = "Java123Code456";
        System.out.println(str.replaceAll("\\d", ""));
    }
}

12. Toggle Case of Each Character

public class ToggleCase {
    public static void main(String[] args) {
        String str = "Java123";
        StringBuilder sb = new StringBuilder();
        for (char c : str.toCharArray()) {
            if (Character.isUpperCase(c))
                sb.append(Character.toLowerCase(c));
            else if (Character.isLowerCase(c))
                sb.append(Character.toUpperCase(c));
            else
                sb.append(c);
        }
        System.out.println(sb.toString());
    }
}

13. Check if a String is Numeric

public class IsNumeric {
    public static void main(String[] args) {
        String input = "12345";
        boolean result = input.matches("\\\\d+");
        System.out.println("Is Numeric: " + result);
    }
}

14. Count Occurrence of a Given Word

public class WordOccurrence {
    public static void main(String[] args) {
        String str = "Java is awesome. Java is powerful.";
        String word = "Java";
        int count = str.split(word, -1).length - 1;
        System.out.println("Count: " + count);
    }
}

15. Find the Longest Palindromic Substring

public class LongestPalindrome {
    public static void main(String[] args) {
        String str = "abaxyzzyxf";
        String result = "";
        for (int i = 0; i < str.length(); i++) {
            for (int j = i + 1; j <= str.length(); j++) {
                String sub = str.substring(i, j);
                if (isPalindrome(sub) && sub.length() > result.length()) {
                    result = sub;
                }
            }
        }
        System.out.println("Longest Palindrome: " + result);
    }

    public static boolean isPalindrome(String s) {
        return s.equals(new StringBuilder(s).reverse().toString());
    }
}

---

Conclusion

Yeh 15 naye string coding questions interview ke liye important aur unique level ke hain. Inhe bar-bar practice karo taaki logic dimaag mein fit ho jaye.

Don’t forget to check Part-1: Java String Coding Questions – Part 1

For more real interview prep, stay connected with InterviewYatra.com