Top 15 Java String Coding Interview Questions (With Answers) Part -1

Top 15 Java String Coding Interview Questions (With Answers)

1. Reverse a String

public class ReverseString {
    public static void main(String[] args) {
        String input = "InterviewYatra";
        String reversed = "";
        for (int i = input.length() - 1; i >= 0; i--) {
            reversed += input.charAt(i);
        }
        System.out.println("Reversed: " + reversed);
    }
}

2. Check if a String is a Palindrome

public class PalindromeCheck {
    public static boolean isPalindrome(String str) {
        int i = 0, j = str.length() - 1;
        while (i < j) {
            if (str.charAt(i++) != str.charAt(j--)) return false;
        }
        return true;
    }

    public static void main(String[] args) {
        System.out.println(isPalindrome("madam")); // true
    }
}

3. Count Vowels and Consonants

public class VowelConsonantCount {
    public static void main(String[] args) {
        String str = "InterviewYatra";
        int vowels = 0, consonants = 0;

        for (char ch : str.toLowerCase().toCharArray()) {
            if (Character.isLetter(ch)) {
                if ("aeiou".indexOf(ch) != -1) vowels++;
                else consonants++;
            }
        }
        System.out.println("Vowels: " + vowels + ", Consonants: " + consonants);
    }
}

4. Remove Duplicate Characters

public class RemoveDuplicates {
    public static void main(String[] args) {
        String input = "interview";
        String result = "";
        for (int i = 0; i < input.length(); i++) {
            if (!result.contains(String.valueOf(input.charAt(i)))) {
                result += input.charAt(i);
            }
        }
        System.out.println("Unique: " + result); // intervw
    }
}

5. Check if Two Strings Are Anagrams

import java.util.Arrays;

public class AnagramCheck {
    public static boolean isAnagram(String s1, String s2) {
        char[] a = s1.toCharArray();
        char[] b = s2.toCharArray();
        Arrays.sort(a);
        Arrays.sort(b);
        return Arrays.equals(a, b);
    }
}

6. Character Frequency Count

import java.util.HashMap;

public class CharacterFrequency {
    public static void main(String[] args) {
        String str = "developer";
        HashMap<Character, Integer> map = new HashMap<>();
        for (char c : str.toCharArray()) {
            map.put(c, map.getOrDefault(c, 0) + 1);
        }
        System.out.println(map);
    }
}

7. First Non-Repeating Character

public class FirstUniqueChar {
    public static void main(String[] args) {
        String str = "aabbcde";
        for (char c : str.toCharArray()) {
            if (str.indexOf(c) == str.lastIndexOf(c)) {
                System.out.println("First Non-Repeating: " + c);
                break;
            }
        }
    }
}

8. Replace Spaces with Hyphens

public class ReplaceSpaces {
    public static void main(String[] args) {
        String sentence = "Java Developer";
        System.out.println(sentence.replace(" ", "-"));
    }
}

9. Convert to Title Case

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

10. String Contains Only Digits

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

11. Print All Substrings

public class Substrings {
    public static void main(String[] args) {
        String str = "abc";
        for (int i = 0; i < str.length(); i++) {
            for (int j = i + 1; j <= str.length(); j++) {
                System.out.println(str.substring(i, j));
            }
        }
    }
}

12. String Rotation Check

public class RotationCheck {
    public static void main(String[] args) {
        String s1 = "abcd";
        String s2 = "cdab";
        System.out.println((s1.length() == s2.length()) && ((s1 + s1).contains(s2)));
    }
}

13. Find Longest Word

public class LongestWord {
    public static void main(String[] args) {
        String sentence = "Java backend developer interview questions";
        String[] words = sentence.split(" ");
        String longest = "";
        for (String word : words) {
            if (word.length() > longest.length()) longest = word;
        }
        System.out.println("Longest Word: " + longest);
    }
}

14. Compare Strings Without equals()

public class CompareStrings {
    public static void main(String[] args) {
        String s1 = "Interview";
        String s2 = "Interview";
        boolean same = (s1.compareTo(s2) == 0);
        System.out.println("Same: " + same);
    }
}

15. Reverse Words in Sentence

public class ReverseWords {
    public static void main(String[] args) {
        String sentence = "Java is great";
        String[] words = sentence.split(" ");
        String reversed = "";
        for (int i = words.length - 1; i >= 0; i--) {
            reversed += words[i] + " ";
        }
        System.out.println("Reversed Words: " + reversed.trim());
    }
}