Top 20 Java Coding Interview Questions with Real Solutions – 2025 Edition

Top 20 Java Coding Interview Questions with Real Solutions – 2025 Edition

Here are 20 hand-picked Java coding problems frequently asked in interviews (TCS, Infosys, Capgemini, etc.) for 1–3 years experience. Each includes a brief explanation and clean code solution.

1. Reverse a String

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

2. Check if a String is Palindrome

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;
}

3. Find Duplicates in Array

Set set = new HashSet<>();
for (int num : arr) {
    if (!set.add(num)) System.out.println("Duplicate: " + num);
}

4. Check if Two Strings Are Anagrams

public boolean isAnagram(String s1, String s2) {
    char[] a1 = s1.toCharArray(), a2 = s2.toCharArray();
    Arrays.sort(a1); Arrays.sort(a2);
    return Arrays.equals(a1, a2);
}

5. Frequency of Characters in String

Map map = new HashMap<>();
for (char c : str.toCharArray()) {
    map.put(c, map.getOrDefault(c, 0) + 1);
}

6. First Non-Repeating Character

Map freq = new LinkedHashMap<>();
for (char c : str.toCharArray()) freq.put(c, freq.getOrDefault(c, 0) + 1);
for (Map.Entry entry : freq.entrySet()) {
    if (entry.getValue() == 1) return entry.getKey();
}

7. Remove Duplicates from List

List 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 v = 0, c = 0;
for (char ch : str.toLowerCase().toCharArray()) {
    if ("aeiou".indexOf(ch) != -1) v++;
    else if (Character.isLetter(ch)) c++;
}

11. Check Armstrong Number

int num = 153, temp = num, result = 0;
while (temp != 0) {
    int d = temp % 10;
    result += d * d * d;
    temp /= 10;
}
System.out.println(result == num);

12. Factorial using Recursion

public int factorial(int n) {
    return (n == 0) ? 1 : 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, 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> grouped =
    employees.stream().collect(Collectors.groupingBy(Employee::getDept));

20. Read File Line by Line

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

Pro Tip: Practice these in a local IDE, explain each step out loud (for interviews), and create variants of each to boost confidence.

📅 Last Updated: 19 May 2025
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