📚 Java Technical Interview Q&A : All Programs & Explanations

[hirenpatel1903]

📚 Java Technical Interview Q&A : All Programs & Explanations

Master essential Java interview questions with clear explanations and practical program examples to strengthen your concepts and boost your confidence.

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1. Reverse a String

This program reverses the input string "Automation" by using StringBuilder (which is mutable) and its reverse() method, then prints the reversed string. It's concise and runs in O(n) time and O(n) extra space for the reversed result.

public class ReverseString {
    public static void main(String[] args) {
        String str = "Automation";
        
        StringBuilder reversed = new StringBuilder(str).reverse();
        
        System.out.println(reversed);
    }
}

Example Input

Automation

Output

noitamotuA

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2. Check for Palindrome

This Java program checks whether a given string is a palindrome or not.
A palindrome is a string that reads the same backward as forward, such as "madam", "racecar", or "level".

public class Palindrome {
    public static void main(String[] args) {
        String str = "madam";
        
        String reversed = new StringBuilder(str).reverse().toString();
        
        System.out.println(str.equals(reversed));
    }
}

Example Input

String str = "madam";
String str = "hello";

Output

madam is a palindrome.
hello is not a palindrome.

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3. Fibonacci Series

This Java program prints the Fibonacci series up to n terms.
The Fibonacci series is a sequence of numbers where each number is the sum of the two preceding ones. It typically starts with 0 and 1.

public class Fibonacci {
    public static void main(String[] args) {
        int n = 10, num1 = 0, num2 = 1;
        
        System.out.print("Fibonacci Series: " + num1 + ", " + num2);
        
        for (int i = 2; i < n; i++) {
            int num3 = num1 + num2;
            System.out.print(", " + num3);
            num1 = num2;
            num2 = num3;
        }
    }
} 

Example Input

int n = 10;

Output

Fibonacci Series: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34

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4. Factorial of a Number

This Java program calculates the factorial of a given number.
The factorial of a number n (denoted as n!) is the product of all positive integers from 1 to n.

public class Factorial {
    public static void main(String[] args) {
        int num = 5, factorial = 1;
        
        for (int i = 1; i <= num; i++) {
            factorial *= i;
        }
        
        System.out.println(factorial);
    }
}

Example Input

int num = 5;

Output

Factorial of 5 is: 120

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5. Prime Number Check

This Java program checks whether a given number is a prime number or not.
A prime number is a number greater than 1 that has no divisors other than 1 and itself.

public class PrimeCheck {
    public static void main(String[] args) {
        int num = 11;
        boolean isPrime = true;
        
        for (int i = 2; i <= Math.sqrt(num); i++) {
            if (num % i == 0) {
                isPrime = false;
                break;
            }
        }
        
        System.out.println(isPrime);
    }
}

Example Input

int num = 11;
int num = 15;

Output

11 is a prime number.
15 is not a prime number.

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6. Count Vowels and Consonants

This Java program counts the number of vowels and consonants in a given string.

public class VowelConsonantCount {
    public static void main(String[] args) {
        String str = "Automation";
        int vowels = 0, consonants = 0;
        
        for (char c : str.toCharArray()) {
            if ("aeiouAEIOU".indexOf(c) != -1) {
                vowels++;
            } else if (Character.isLetter(c)) {
                consonants++;
            }
        }
        
        System.out.println("Vowels: " + vowels + ", Consonants: " + consonants);
    }
}

Example Input

String str = "Automation";
String str = "Hello World";

Output

Vowels: 5, Consonants: 5
Vowels: 3, Consonants: 7

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7. Sort an Array

This Java program sorts an array of integers in ascending order.

import java.util.Arrays;

public class SortArray {
    public static void main(String[] args) {
        int[] arr = {5, 2, 8, 1, 3};
        
        Arrays.sort(arr);
        
        System.out.println(Arrays.toString(arr));
    }
}

Example Input

int[] arr = {5, 2, 8, 1, 3};
int[] arr = {10, 4, 6, 2};

Output

Sorted array: [1, 2, 3, 5, 8]
Sorted array: [2, 4, 6, 10] 

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8. Merge Two Arrays

This program demonstrates how to merge two integer arrays into a single array in Java. It uses System.arraycopy to copy elements from both arrays into a new array.

import java.util.Arrays;

public class MergeArrays {
    public static void main(String[] args) {
        int[] arr1 = {1, 3, 5};
        int[] arr2 = {2, 4, 6};
        
        int[] merged = new int[arr1.length + arr2.length];
        
        System.arraycopy(arr1, 0, merged, 0, arr1.length);
        System.arraycopy(arr2, 0, merged, arr1.length, arr2.length);
        
        System.out.println(Arrays.toString(merged));
    }
}

Example Input

arr1 = {1, 3, 5}
arr2 = {2, 4, 6}

Output

[1, 3, 5, 2, 4, 6]

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9. Find the Largest Element in an Array

This program finds the largest element in a given integer array. It initializes the largest element as the first element of the array and then iterates through the array, updating the largest element whenever a bigger number is found. Finally, it prints the largest number.

public class LargestInArray {
    public static void main(String[] args) {
        int[] arr = {1, 3, 5, 7, 9};
        int largest = arr[0];
        
        for (int num : arr) {
            if (num > largest) {
                largest = num;
            }
        }
        
        System.out.println(largest);
    }
}

Example Input

Array: [1, 3, 5, 7, 9]

Output

9

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10. Remove Duplicates from an Array

This program demonstrates how to remove duplicate elements from an array using a HashSet in Java. A HashSet automatically ensures that all elements are unique.

import java.util.HashSet;

public class RemoveDuplicates {
    public static void main(String[] args) {
        int[] arr = {1, 2, 2, 3, 4, 4};
        
        HashSet<Integer> set = new HashSet<>();
        
        for (int num : arr) {
            set.add(num);
        }
        
        System.out.println(set);
    }
}

Example Input

{1, 2, 2, 3, 4, 4}

Output

{1, 2, 2, 3, 4, 4}

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11. Check if a Number is Armstrong

This program checks whether a given number is an Armstrong number. An Armstrong number of three digits is a number whose sum of the cubes of its digits is equal to the number itself.

public class ArmstrongNumber {
    public static void main(String[] args) {
        int num = 153, sum = 0, temp = num;
        
        while (temp != 0) {
            int digit = temp % 10;
            sum += Math.pow(digit, 3);
            temp /= 10;
        }
        
        System.out.println(num == sum);
    }
}

Example Input

153
123

Output

true
false

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12. Reverse a Number

Write a Java program to reverse the digits of a given integer. The program should take an integer, reverse its digits, and print the reversed number.

public class ReverseNumber {
    public static void main(String[] args) {
        int num = 12345, reversed = 0;
        
        while (num != 0) {
            reversed = reversed * 10 + num % 10;
            num /= 10;
        }
        
        System.out.println(reversed);
    }
}

Example Input

12345
1001
-456

Output

54321
1001
-654

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13. Calculate GCD of Two Numbers

Write a Java program to calculate the Greatest Common Divisor (GCD) of two integers using the Euclidean algorithm. The program should take two integers as input and print their GCD.

public class GCD {
    public static void main(String[] args) {
        int a = 60, b = 48;
        
        while (b != 0) {
            int temp = b;
            b = a % b;
            a = temp;
        }
        
        System.out.println(a);
    }
}

Example Input

60 48
101 103
36 60

Output

12
1
12

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14. Check for Anagram

Write a Java program to check whether two strings are anagrams of each other. Two strings are anagrams if they contain the same characters in a different order. The program should print true if they are anagrams, otherwise false.

import java.util.Arrays;

public class AnagramCheck {
    public static void main(String[] args) {
        String str1 = "listen", str2 = "silent";
        
        char[] arr1 = str1.toCharArray();
        char[] arr2 = str2.toCharArray();
        
        Arrays.sort(arr1);
        Arrays.sort(arr2);
        
        System.out.println(Arrays.equals(arr1, arr2));
    }
}

Example Input

listen silent
hello world
triangle integral

Output

true
false
true

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15. Count the Number of Digits in a Number

Write a Java program to count the number of digits in a given integer. The program should print the total count of digits.

public class CountDigits {
    public static void main(String[] args) {
        int num = 12345;
        
        int count = String.valueOf(num).length();
        
        System.out.println(count);
    }
}

Example Input

12345
0
-9876

Output

5
1
4

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16. Print the Prime Numbers in a Range

Write a Java program to print all prime numbers within a given range. A prime number is a number greater than 1 that has no positive divisors other than 1 and itself.

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 + " ");
            }
        }
    }
}

Example Input

10 50
1 10
50 60

Output

11 13 17 19 23 29 31 37 41 43 47
2 3 5 7
53 59

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17. Find the Second Largest Element in an Array

Write a Java program to find the second largest element in an array of integers. The program should handle arrays with duplicate values as well.

public class SecondLargest {
    public static void main(String[] args) {
        int[] arr = {12, 35, 1, 10, 34, 1};
        
        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;
            }
        }
        
        System.out.println(second);
    }
}

Example Input

12 35 1 10 34 1
5 5 5 5 2
10 20 20 30 30

Output

34
2
20

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18. Swap Two Numbers

Write a Java program to swap the values of two numbers without using a temporary variable.

public class SwapNumbers {
    public static void main(String[] args) {
        int a = 5, b = 10;
        
        a = a + b;
        b = a - b;
        a = a - b;
        
        System.out.println("a: " + a + ", b: " + b);
    }
}

Example Input

5 10
-3 7
0 100

Output

a: 10, b: 5
a: 7, b: -3
a: 100, b: 0

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19. Print the Pascal's Triangle

Write a Java program to print Pascal's Triangle up to a given number of rows. Each number in the triangle is the sum of the two numbers directly above it.

public class PascalsTriangle {
    public static void main(String[] args) {
        int rows = 5;
        
        for (int i = 0; i < rows; i++) {
            int num = 1;
            System.out.format("%" + (rows - i) * 2 + "s", "");
            
            for (int j = 0; j <= i; j++) {
                System.out.format("%4d", num);
                num = num * (i - j) / (j + 1);
            }
            
            System.out.println();
        }
    }
}

Example Input

5

Output

        1
      1   1
    1   2   1
  1   3   3   1
1   4   6   4   1

Example Input2

3

Output2

      1
    1   1
  1   2   1

Example Input3

6

Output3

          1
        1   1
      1   2   1
    1   3   3   1
  1   4   6   4   1
1   5  10  10   5   1

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