Top 20 Java Coding Interview Questions with Real Solutions

1. Reverse a String

This question tests string manipulation concepts in Java.


public String reverse(String input) {
    return new StringBuilder(input).reverse().toString();
}

Time Complexity: O(n)

2. Check if a String is Palindrome

Palindrome questions are commonly asked to test loops and condition handling.


public 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;
}

Time Complexity: O(n)

3. Find Duplicates in Array

This problem checks understanding of HashSet and duplicate detection.


Set<Integer> set = new HashSet<>();

for (int num : arr) {
    if (!set.add(num)) {
        System.out.println("Duplicate: " + num);
    }
}

Time Complexity: O(n)

4. Check if Two Strings Are Anagrams


public boolean isAnagram(String s1, String s2) {

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

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

    return Arrays.equals(a1, a2);
}

5. Frequency of Characters in String

HashMap is commonly used to count character frequency.


Map<Character, Integer> map = new HashMap<>();

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

6. First Non-Repeating Character


Map<Character, Integer> freq = new LinkedHashMap<>();

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

for (Map.Entry<Character, Integer> entry : freq.entrySet()) {

    if (entry.getValue() == 1) {
        System.out.println(entry.getKey());
        break;
    }
}

7. Remove Duplicates from List


List<String> distinct = list.stream()
    .distinct()
    .collect(Collectors.toList());

8. FizzBuzz


for (int i = 1; i <= 100; i++) {

    if (i % 15 == 0) {
        System.out.println("FizzBuzz");
    }
    else if (i % 3 == 0) {
        System.out.println("Fizz");
    }
    else if (i % 5 == 0) {
        System.out.println("Buzz");
    }
    else {
        System.out.println(i);
    }
}

9. Sort HashMap by Value


map.entrySet()
   .stream()
   .sorted(Map.Entry.comparingByValue())
   .forEach(e -> 
       System.out.println(e.getKey() + ": " + e.getValue()));

10. Count Vowels and Consonants


int vowels = 0;
int consonants = 0;

for (char ch : str.toLowerCase().toCharArray()) {

    if ("aeiou".indexOf(ch) != -1) {
        vowels++;
    }
    else if (Character.isLetter(ch)) {
        consonants++;
    }
}

11. Check Armstrong Number


int num = 153;
int temp = num;
int result = 0;

while (temp != 0) {

    int digit = temp % 10;
    result += digit * digit * digit;

    temp /= 10;
}

System.out.println(result == num);

12. Factorial using Recursion


public int factorial(int n) {

    if (n == 0) {
        return 1;
    }

    return n * factorial(n - 1);
}

13. Binary to Decimal Conversion


int decimal = Integer.parseInt("1010", 2);

14. Prime Number Check


boolean isPrime(int n) {

    if (n <= 1) {
        return false;
    }

    for (int i = 2; i <= Math.sqrt(n); i++) {

        if (n % i == 0) {
            return false;
        }
    }

    return true;
}

15. Find Second Largest Number


int first = Integer.MIN_VALUE;
int second = Integer.MIN_VALUE;

for (int num : arr) {

    if (num > first) {

        second = first;
        first = num;
    }

    else if (num > second && num != first) {
        second = num;
    }
}

16. Find Missing Number in Range


int sum = (n * (n + 1)) / 2;

for (int val : arr) {
    sum -= val;
}

System.out.println("Missing: " + sum);

17. Sort Custom Objects with Comparator


Collections.sort(list,
    (a, b) -> a.getAge() - b.getAge());

18. Java 8 Map and Filter


list.stream()
    .filter(s -> s.startsWith("A"))
    .map(String::toUpperCase)
    .forEach(System.out::println);

19. Group By Using Java Streams


Map<String, List<Employee>> grouped =
    employees.stream()
        .collect(Collectors.groupingBy(Employee::getDept));

20. Read File Line by Line


Files.lines(Paths.get("file.txt"))
     .forEach(System.out::println);

Frequently Asked Questions

Are Java coding questions asked in TCS interviews?

Yes, companies like TCS, Infosys, Wipro, and Capgemini frequently ask Java coding questions for freshers and experienced candidates.

Is Java 8 important for coding interviews?

Yes, Java 8 features like Streams, Lambda Expressions, and Functional Interfaces are commonly asked in modern interviews.

How can I practice Java coding questions?

Practice regularly using IntelliJ IDEA or Eclipse and focus on arrays, strings, collections, and Java 8 concepts.

Pro Tip: Practice these programs in your local IDE and explain the logic step-by-step during interviews.

📅 Last Updated: May 2025
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