21-50 Java Coding Questions with Answers (With Code Examples)

This collection of 50 Java coding questions is designed to help you master the basics and build strong problem-solving skills. Whether you’re preparing for a Java interview, studying for an exam, or simply practicing your logic, this guide is a great companion.

Each question comes with:

• A clear problem statement

• Humanized logic explanation

• Java code

• Output example

🔹 21. Find Duplicate Elements in Array

int[] arr = {1, 2, 3, 2, 4, 5, 1};
Set<Integer> set = new HashSet<>();
for (int num : arr) {
    if (!set.add(num)) {
        System.out.println("Duplicate: " + num);
    }
}

---

🔹 22. Count Occurrence of Each Character

String s = "programming";
Map<Character, Integer> map = new HashMap<>();
for (char c : s.toCharArray()) {
    map.put(c, map.getOrDefault(c, 0) + 1);
}
System.out.println(map);

---

🔹 23. Print Star Pyramid

int n = 5;
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= i; j++) System.out.print("*");
    System.out.println();
}

---

🔹 24. Check Leap Year

int year = 2024;
boolean isLeap = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
System.out.println(isLeap ? "Leap Year" : "Not Leap Year");

---

🔹 25. GCD of Two Numbers

int a = 24, b = 18;
while (b != 0) {
    int temp = b;
    b = a % b;
    a = temp;
}
System.out.println("GCD: " + a);

---

🔹 26. LCM of Two Numbers

int a = 15, b = 20;
int lcm = (a * b) / gcd(a, b);
System.out.println("LCM: " + lcm);

static int gcd(int a, int b) {
    while (b != 0) {
        int t = b;
        b = a % b;
        a = t;
    }
    return a;
}

---

🔹 27. Check Anagram Strings

String s1 = "listen", s2 = "silent";
char[] a = s1.toCharArray(), b = s2.toCharArray();
Arrays.sort(a);
Arrays.sort(b);
System.out.println(Arrays.equals(a, b) ? "Anagram" : "Not Anagram");

---

🔹 28. Convert String to Integer

String s = "123";
int num = Integer.parseInt(s);
System.out.println(num);

---

🔹 29. Convert Integer to String

int num = 456;
String s = String.valueOf(num);
System.out.println(s);

---

🔹 30. Remove White Spaces

String s = "Hello World Java";
s = s.replaceAll("\\s", "");
System.out.println(s);

---

🔹 31. Count Words in String

String s = "Java is fun";
String[] words = s.trim().split("\\s+");
System.out.println("Words: " + words.length);

---

🔹 32. Find Second Largest in Array

int[] arr = {10, 20, 25, 15};
Arrays.sort(arr);
System.out.println("Second Largest: " + arr[arr.length - 2]);

---

🔹 33. Sum of Digits in Number

int n = 1234, sum = 0;
while (n != 0) {
    sum += n % 10;
    n /= 10;
}
System.out.println("Sum: " + sum);

---

🔹 34. Generate Random Number

int rand = (int)(Math.random() * 100); // 0 to 99
System.out.println("Random: " + rand);

---

🔹 35. Swap Without 3rd Variable

int a = 5, b = 10;
a = a + b;
b = a - b;
a = a - b;
System.out.println("a = " + a + ", b = " + b);

---

🔹 36. Simple Calculator

char op = '+';
int a = 10, b = 5;
switch(op) {
    case '+': System.out.println(a + b); break;
    case '-': System.out.println(a - b); break;
    case '*': System.out.println(a * b); break;
    case '/': System.out.println(a / b); break;
    default: System.out.println("Invalid");
}

---

🔹 37. Reverse Linked List (Concept)

class Node {
    int data;
    Node next;
    Node(int d) { data = d; }
}
Node reverse(Node head) {
    Node prev = null, curr = head;
    while (curr != null) {
        Node next = curr.next;
        curr.next = prev;
        prev = curr;
        curr = next;
    }
    return prev;
}

---

🔹 38. Check Armstrong for n-digits

int num = 9474, original = num, sum = 0;
int len = String.valueOf(num).length();
while (num != 0) {
    int d = num % 10;
    sum += Math.pow(d, len);
    num /= 10;
}
System.out.println(sum == original ? "Armstrong" : "Not Armstrong");

---

🔹 39. Remove Duplicates from String

String s = "programming";
StringBuilder sb = new StringBuilder();
Set<Character> set = new LinkedHashSet<>();
for (char c : s.toCharArray()) set.add(c);
for (char c : set) sb.append(c);
System.out.println(sb);

---

🔹 40. Java Class Example (OOP)

class Car {
    String name;
    int speed;
    Car(String name, int speed) {
        this.name = name;
        this.speed = speed;
    }
    void show() {
        System.out.println(name + " runs at " + speed + "km/h");
    }
}

---

🔹 41. Abstract Class

abstract class Animal {
    abstract void sound();
}
class Dog extends Animal {
    void sound() { System.out.println("Bark"); }
}

---

🔹 42. Interface Example

interface Flyable {
    void fly();
}
class Bird implements Flyable {
    public void fly() {
        System.out.println("Flying...");
    }
}

---

🔹 43. StringBuilder Reverse

String s = "hello";
System.out.println(new StringBuilder(s).reverse());

---

🔹 44. Check if Number is Perfect

int n = 28, sum = 0;
for (int i = 1; i < n; i++) {
    if (n % i == 0) sum += i;
}
System.out.println(sum == n ? "Perfect" : "Not Perfect");

---

🔹 45. Print ASCII Values

for (char c = 'A'; c <= 'Z'; c++) {
    System.out.println(c + ": " + (int)c);
}

---

🔹 46. Sort Array in Descending

Integer[] arr = {3, 5, 1, 8};
Arrays.sort(arr, Collections.reverseOrder());
System.out.println(Arrays.toString(arr));

---

🔹 47. Convert Char to Int and Vice Versa

char ch = 'A';
int code = ch; // ASCII
char back = (char) code;
System.out.println("ASCII: " + code + ", Back: " + back);

---

🔹 48. Find Missing Number (1 to n)

int[] arr = {1, 2, 4, 5};
int n = 5, total = n * (n + 1) / 2;
for (int num : arr) total -= num;
System.out.println("Missing: " + total);

---

🔹 49. Check String Rotation

String s1 = "abcd", s2 = "cdab";
System.out.println((s1.length() == s2.length()) && ((s1 + s1).contains(s2)) ? "Rotation" : "Not Rotation");

---

🔹 50. Find Frequency of Elements in Array

int[] arr = {1, 2, 2, 3, 1, 4};
Map<Integer, Integer> freq = new HashMap<>();
for (int num : arr) freq.put(num, freq.getOrDefault(num, 0) + 1);
System.out.println(freq);

---

Respective output for questions 21 to 50:

🔢 Q.No	Question Name	Output (Example)
21	Find Duplicate Elements in Array	Duplicate: 2, 1
22	Count Occurrence of Each Character	{p=1, r=2, o=1, g=2, a=1, m=2, i=1, n=1}
23	Print Star Pyramid	<br><br> **** *****
24	Check Leap Year	Leap Year
25	GCD of Two Numbers	GCD: 6
26	LCM of Two Numbers	LCM: 60
27	Check Anagram Strings	Anagram
28	Convert String to Integer	123
29	Convert Integer to String	456
30	Remove White Spaces	HelloWorldJava
31	Count Words in String	Words: 3
32	Find Second Largest in Array	Second Largest: 20
33	Sum of Digits in Number	Sum: 10
34	Generate Random Number	Random: 42 (varies each time)
35	Swap Without 3rd Variable	a = 10, b = 5
36	Simple Calculator	15 (for op = '+')
37	Reverse Linked List	New head node reference (logic only, no print)
38	Check Armstrong for n-digits	Armstrong
39	Remove Duplicates from String	progamin
40	Java Class Example (OOP)	BMW runs at 200km/h
41	Abstract Class	Bark
42	Interface Example	Flying...
43	StringBuilder Reverse	olleh
44	Check if Number is Perfect	Perfect
45	Print ASCII Values	A: 65 ... Z: 90
46	Sort Array in Descending	[8, 5, 3, 1]
47	Convert Char to Int and Vice Versa	ASCII: 65, Back: A
48	Find Missing Number (1 to n)	Missing: 3
49	Check String Rotation	Rotation
50	Find Frequency of Elements in Array