Here are more Java interview programs to help you prepare for your interviews. These problems explore additional Java concepts, including recursion, sorting algorithms, and data structures. Mastering these will enhance your coding proficiency and problem-solving skills.

11. Find the Missing Number in an Array

• Problem: Given an array of integers from 1 to n with one number missing, find the missing number.

• Example:

  java

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  public class MissingNumber {
public static void main(String[] args) {
int[] arr = {1, 2, 4, 5, 6};
int n = 6; // Maximum number in the array
int total = n * (n + 1) / 2;
int sum = 0;for (int i : arr) {
  sum += i;
  }

  System.out.println(“Missing number is: “ + (total – sum));
  }
  }

• Expected Output:

  csharp

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  Missing number is: 3


12. Find the Second Largest Number in an Array

• Problem: Write a Java program to find the second largest element in an array.

• Example:

  java

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  public class SecondLargest {
public static void main(String[] args) {
int[] arr = {10, 20, 4, 45, 99};
int largest = arr[0];
int secondLargest = Integer.MIN_VALUE;for (int i = 1; i < arr.length; i++) {
  if (arr[i] > largest) {
  secondLargest = largest;
  largest = arr[i];
  } else if (arr[i] > secondLargest && arr[i] != largest) {
  secondLargest = arr[i];
  }
  }

  System.out.println(“Second Largest: “ + secondLargest);
  }
  }

• Expected Output:

  sql

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  Second Largest: 45


13. Armstrong Number for N Digits

• Problem: Write a program to check if a number is an Armstrong number for n digits.

• Example:

  java

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  public class ArmstrongNDigits {
public static void main(String[] args) {
int num = 1634;
int originalNum = num;
int numDigits = String.valueOf(num).length();
int sum = 0;while (num > 0) {
  int digit = num % 10;
  sum += Math.pow(digit, numDigits);
  num /= 10;
  }

  if (sum == originalNum) {
  System.out.println(originalNum + ” is an Armstrong number”);
  } else {
  System.out.println(originalNum + ” is not an Armstrong number”);
  }
  }
  }

• Expected Output:

  csharp

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  1634 is an Armstrong number


14. Merge Two Sorted Arrays

• Problem: Write a Java program to merge two sorted arrays into one sorted array.

• Example:

  java

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  public class MergeSortedArrays {
public static void main(String[] args) {
int[] arr1 = {1, 3, 5, 7};
int[] arr2 = {2, 4, 6, 8};
int[] merged = new int[arr1.length + arr2.length];
int i = 0, j = 0, k = 0;while (i < arr1.length && j < arr2.length) {
  if (arr1[i] < arr2[j]) {
  merged[k++] = arr1[i++];
  } else {
  merged[k++] = arr2[j++];
  }
  }

  while (i < arr1.length) {
  merged[k++] = arr1[i++];
  }

  while (j < arr2.length) {
  merged[k++] = arr2[j++];
  }

  System.out.print(“Merged Array: “);
  for (int num : merged) {
  System.out.print(num + ” “);
  }
  }
  }

• Expected Output:

  javascript

  CopyEdit

  Merged Array: 1 2 3 4 5 6 7 8


15. Find Duplicate Elements in an Array

• Problem: Write a Java program to find all duplicate elements in an array.

• Example:

  java

  CopyEdit

  import java.util.HashSet;

  public class FindDuplicates {
  public static void main(String[] args) {
  int[] arr = {1, 2, 3, 4, 5, 3, 6, 7, 8, 9, 5};
  HashSet<Integer> set = new HashSet<>();

  System.out.print(“Duplicate Elements: “);
  for (int i = 0; i < arr.length; i++) {
  if (!set.add(arr[i])) {
  System.out.print(arr[i] + ” “);
  }
  }
  }
  }

• Expected Output:

  yaml

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  Duplicate Elements: 3 5


16. Calculate the Factorial Using Recursion

• Problem: Write a Java program to calculate the factorial of a number using recursion.

• Example:

  java

  CopyEdit

  public class FactorialRecursion {
public static void main(String[] args) {
int num = 5;
System.out.println("Factorial of " + num + " is " + factorial(num));
}public static int factorial(int n) {
  if (n == 0 || n == 1) {
  return 1;
  } else {
  return n * factorial(n – 1);
  }
  }
  }

• Expected Output:

  csharp

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  Factorial of 5 is 120


17. Find the Sum of Natural Numbers

• Problem: Write a Java program to find the sum of the first n natural numbers.

• Example:

  java

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  public class SumNaturalNumbers {
public static void main(String[] args) {
int n = 10;
int sum = n * (n + 1) / 2;
System.out.println("Sum of first " + n + " natural numbers is " + sum);
}
}


• Expected Output:

  sql

  CopyEdit

  Sum of first 10 natural numbers is 55


18. Count Words in a String

• Problem: Write a Java program to count the number of words in a given string.

• Example:

  java

  CopyEdit

  public class CountWords {
public static void main(String[] args) {
String str = "Java is fun to learn";
String[] words = str.split(" ");
System.out.println("Number of words: " + words.length);
}
}


• Expected Output:

  javascript

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  Number of words: 5


19. Find the GCD of Two Numbers

• Problem: Write a Java program to find the GCD (Greatest Common Divisor) of two numbers using recursion.

• Example:

  java

  CopyEdit

  public class GCD {
public static void main(String[] args) {
int a = 56, b = 98;
System.out.println("GCD of " + a + " and " + b + " is: " + findGCD(a, b));
}public static int findGCD(int a, int b) {
  if (b == 0) {
  return a;
  }
  return findGCD(b, a % b);
  }
  }

• Expected Output:

  csharp

  CopyEdit

  GCD of 56 and 98 is: 14


20. Bubble Sort Implementation

• Problem: Implement bubble sort algorithm to sort an array.

• Example:

  java

  CopyEdit

  public class BubbleSort {
public static void main(String[] args) {
int[] arr = {64, 25, 12, 22, 11};
for (int i = 0; i < arr.length - 1; i++) {
for (int j = 0; j < arr.length - 1 - i; j++) {
if (arr[j] > arr[j + 1]) {
int temp = arr[j];
arr[j] = arr[j + 1];
arr[j + 1] = temp;
}
}
}System.out.print(“Sorted array: “);
  for (int num : arr) {
  System.out.print(num + ” “);
  }
  }
  }

• Expected Output:

  php

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  Sorted array: 11 12 22 25 64

• 21. Find the Length of a String Without Using Built-in Methods

  • Problem: Write a Java program to find the length of a string without using the built-in length() method.

  • Example:

    java

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    public class StringLength {
public static void main(String[] args) {
String str = "Hello, World!";
int length = 0;for (char c : str.toCharArray()) {
    length++;
    }

    System.out.println(“Length of string: “ + length);
    }
    }

  • Expected Output:

    csharp

    CopyEdit

    Length of string: 13


  22. Find the Prime Numbers in a Given Range

  • Problem: Write a Java program to find all prime numbers between two numbers.

  • Example:

    java

    CopyEdit

    public class PrimeInRange {
public static void main(String[] args) {
int start = 10, end = 50;for (int num = start; num <= end; num++) {
    boolean isPrime = true;
    for (int i = 2; i <= Math.sqrt(num); i++) {
    if (num % i == 0) {
    isPrime = false;
    break;
    }
    }
    if (isPrime && num > 1) {
    System.out.print(num + ” “);
    }
    }
    }
    }

  • Expected Output:

    CopyEdit

    11 13 17 19 23 29 31 37 41 43 47


  23. Find the Missing Element in an Array (Using XOR)

  • Problem: Find the missing element in an array containing all elements from 1 to n, except one.

  • Example:

    java

    CopyEdit

    public class MissingElementXOR {
public static void main(String[] args) {
int[] arr = {1, 2, 4, 5, 6};
int n = 6; // The maximum numberint xor1 = 0;
    int xor2 = 0;

    for (int i = 1; i <= n; i++) {
    xor1 ^= i; // XOR for all numbers from 1 to n
    }

    for (int num : arr) {
    xor2 ^= num; // XOR for all numbers in the array
    }

    System.out.println(“Missing element: “ + (xor1 ^ xor2));
    }
    }

  • Expected Output:

    yaml

    CopyEdit

    Missing element: 3


  24. Find the Most Frequent Element in an Array

  • Problem: Write a Java program to find the most frequent element in an array.

  • Example:

    java

    CopyEdit

    import java.util.HashMap;

    public class MostFrequentElement {
    public static void main(String[] args) {
    int[] arr = {1, 3, 2, 1, 4, 1, 2, 1, 3};

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

    for (int num : arr) {
    map.put(num, map.getOrDefault(num, 0) + 1);
    }

    int maxCount = 0;
    int mostFrequent = arr[0];

    for (int key : map.keySet()) {
    if (map.get(key) > maxCount) {
    maxCount = map.get(key);
    mostFrequent = key;
    }
    }

    System.out.println(“Most frequent element: “ + mostFrequent);
    }
    }

  • Expected Output:

    yaml

    CopyEdit

    Most frequent element: 1


  25. Find the Sum of Even and Odd Numbers in an Array

  • Problem: Write a Java program to find the sum of even and odd numbers separately in an array.

  • Example:

    java

    CopyEdit

    public class EvenOddSum {
public static void main(String[] args) {
int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9};int evenSum = 0;
    int oddSum = 0;

    for (int num : arr) {
    if (num % 2 == 0) {
    evenSum += num;
    } else {
    oddSum += num;
    }
    }

    System.out.println(“Sum of even numbers: “ + evenSum);
    System.out.println(“Sum of odd numbers: “ + oddSum);
    }
    }

  • Expected Output:

    yaml

    CopyEdit

    Sum of even numbers: 20
Sum of odd numbers: 25


  26. Implement a Linked List (Basic Operations)

  • Problem: Write a Java program to implement a singly linked list and perform basic operations such as insertion and traversal.

  • Example:

    java

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    class Node {
int data;
Node next;public Node(int data) {
    this.data = data;
    this.next = null;
    }
    }

    public class LinkedList {
    Node head;

    public void insert(int data) {
    Node newNode = new Node(data);
    if (head == null) {
    head = newNode;
    } else {
    Node temp = head;
    while (temp.next != null) {
    temp = temp.next;
    }
    temp.next = newNode;
    }
    }

    public void display() {
    Node temp = head;
    while (temp != null) {
    System.out.print(temp.data + ” “);
    temp = temp.next;
    }
    }

    public static void main(String[] args) {
    LinkedList list = new LinkedList();
    list.insert(10);
    list.insert(20);
    list.insert(30);
    list.insert(40);

    System.out.print(“Linked List: “);
    list.display();
    }
    }

  • Expected Output:

    yaml

    CopyEdit

    Linked List: 10 20 30 40


  27. Convert a String to an Integer Without Using Integer.parseInt()

  • Problem: Write a program to convert a string to an integer without using Integer.parseInt().

  • Example:

    java

    CopyEdit

    public class StringToInt {
public static void main(String[] args) {
String str = "12345";
int result = 0;for (int i = 0; i < str.length(); i++) {
    result = result * 10 + (str.charAt(i) – ‘0’);
    }

    System.out.println(“Converted Integer: “ + result);
    }
    }

  • Expected Output:

    sql

    CopyEdit

    Converted Integer: 12345


  28. Sum of Digits Using Recursion

  • Problem: Write a Java program to find the sum of digits of a number using recursion.

  • Example:

    java

    CopyEdit

    public class SumOfDigitsRecursion {
public static void main(String[] args) {
int num = 1234;
System.out.println("Sum of digits: " + sumOfDigits(num));
}public static int sumOfDigits(int n) {
    if (n == 0) {
    return 0;
    }
    return n % 10 + sumOfDigits(n / 10);
    }
    }

  • Expected Output:

    yaml

    CopyEdit

    Sum of digits: 10


  29. Remove Duplicates from an Array

  • Problem: Write a program to remove duplicate elements from an array.

  • Example:

    java

    CopyEdit

    import java.util.HashSet;

    public class RemoveDuplicates {
    public static void main(String[] args) {
    int[] arr = {1, 2, 2, 3, 4, 4, 5};
    HashSet<Integer> set = new HashSet<>();

    for (int num : arr) {
    set.add(num);
    }

    System.out.print(“Array after removing duplicates: “);
    for (int num : set) {
    System.out.print(num + ” “);
    }
    }
    }

  • Expected Output:

    pgsql

    CopyEdit

    Array after removing duplicates: 1 2 3 4 5


  30. Sort an Array Using Merge Sort

  • Problem: Implement the merge sort algorithm to sort an array.

  • Example:

    java

    CopyEdit

    public class MergeSort {
public static void main(String[] args) {
int[] arr = {12, 11, 13, 5, 6, 7};
mergeSort(arr, 0, arr.length - 1);System.out.print(“Sorted array: “);
    for (int num : arr) {
    System.out.print(num + ” “);
    }
    }

    public static void mergeSort(int[] arr, int left, int right) {
    if (left < right) {
    int mid = (left + right) / 2;
    mergeSort(arr, left, mid);
    mergeSort(arr, mid + 1, right);
    merge(arr, left, mid, right);
    }
    }

    public static void merge(int[] arr, int left, int mid, int right) {
    int n1 = mid – left + 1;
    int n2 = right – mid;

    int[] L = new int[n1];
    int[] R = new int[n2];

    System.arraycopy(arr, left, L, 0, n1);
    System.arraycopy(arr, mid + 1, R, 0, n2);

    int i = 0, j = 0, k = left;

    while (i < n1 && j < n2) {
    if (L[i] <= R[j]) {
    arr[k++] = L[i++];
    } else {
    arr[k++] = R[j++];
    }
    }

    while (i < n1) {
    arr[k++] = L[i++];
    }

    while (j < n2) {
    arr[k++] = R[j++];
    }
    }
    }

  • Expected Output:

    php

    CopyEdit

    Sorted array: 5 6 7 11 12 13


  Conclusion:

  These core Java interview programs cover various essential topics such as recursion, sorting, string manipulation, data structures (arrays, linked lists), and more. Practicing these problems will improve your coding and problem-solving skills, making you well-prepared for Java-related job interviews.core java interview programs