Java Programming Coding Questions

Practice most asked Java programming coding interview questions with solutions for freshers and experienced developers.

1. Find factorial of a number

Easy 
Input: 5
Output: 120

import java.util.Scanner;

public class Factorial {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        
        long fact = 1;
        for(int i = 1; i <= n; i++) {
            fact *= i;
        }
        
        System.out.println(fact);
        sc.close();
    }
}

2. Check whether a number is prime

Easy 
Input: 7
Output: Prime

import java.util.Scanner;

public class PrimeCheck {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        
        if(n <= 1) {
            System.out.println("Not Prime");
            sc.close();
            return;
        }
        
        for(int i = 2; i * i <= n; i++) {
            if(n % i == 0) {
                System.out.println("Not Prime");
                sc.close();
                return;
            }
        }
        
        System.out.println("Prime");
        sc.close();
    }
}

3. Print Fibonacci series

Easy 
Input: 7
Output: 0 1 1 2 3 5 8

import java.util.Scanner;

public class Fibonacci {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        
        int a = 0, b = 1;
        
        for(int i = 1; i <= n; i++) {
            System.out.print(a + " ");
            int c = a + b;
            a = b;
            b = c;
        }
        
        sc.close();
    }
}

4. Reverse a number

Easy 
Input: 12345
Output: 54321

import java.util.Scanner;

public class ReverseNumber {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        
        int rev = 0;
        while(n > 0) {
            rev = rev * 10 + n % 10;
            n /= 10;
        }
        
        System.out.println(rev);
        sc.close();
    }
}

5. Check whether a number is palindrome

Easy 
Input: 121
Output: Palindrome

import java.util.Scanner;

public class PalindromeNumber {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int temp = n;
        
        int rev = 0;
        while(n > 0) {
            rev = rev * 10 + n % 10;
            n /= 10;
        }
        
        if(temp == rev) {
            System.out.println("Palindrome");
        } else {
            System.out.println("Not Palindrome");
        }
        
        sc.close();
    }
}

6. Find largest of three numbers

Easy 
Input: 10 25 15
Output: 25

import java.util.Scanner;

public class LargestOfThree {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        
        int max = Math.max(a, Math.max(b, c));
        System.out.println(max);
        
        sc.close();
    }
}

7. Check Armstrong number

Medium 
Input: 153
Output: Armstrong

import java.util.Scanner;

public class ArmstrongNumber {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int temp = n;
        
        int digits = String.valueOf(n).length();
        
        int sum = 0;
        while(temp > 0) {
            int digit = temp % 10;
            sum += Math.pow(digit, digits);
            temp /= 10;
        }
        
        if(sum == n) {
            System.out.println("Armstrong");
        } else {
            System.out.println("Not Armstrong");
        }
        
        sc.close();
    }
}

8. Find sum of array elements

Easy 
Input: 5
       10 20 30 40 50
Output: 150

import java.util.Scanner;

public class ArraySum {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        
        int sum = 0;
        for(int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
            sum += arr[i];
        }
        
        System.out.println(sum);
        sc.close();
    }
}

9. Linear search in array

Easy 
Input: 5
       10 20 30 40 50
       30
Output: Found

import java.util.Scanner;

public class LinearSearch {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        
        for(int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
        }
        
        int key = sc.nextInt();
        boolean found = false;
        
        for(int i = 0; i < n; i++) {
            if(arr[i] == key) {
                System.out.println("Found");
                found = true;
                break;
            }
        }
        
        if(!found) {
            System.out.println("Not Found");
        }
        
        sc.close();
    }
}

10. Count digits of a number

Easy 
Input: 12345
Output: 5

import java.util.Scanner;

public class CountDigits {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        
        int count = 0;
        while(n > 0) {
            count++;
            n /= 10;
        }
        
        System.out.println(count);
        sc.close();
    }
}

11. Swap two numbers without using third variable

Medium 
Input: 10 20
Output: 20 10

import java.util.Scanner;

public class SwapNumbers {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        a = a ^ b;
        b = a ^ b;
        a = a ^ b;
        
        System.out.println(a + " " + b);
        sc.close();
    }
}

12. Reverse a string

Easy 
Input: hello
Output: olleh

import java.util.Scanner;

public class ReverseString {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
        
        String reversed = new StringBuilder(str).reverse().toString();
        System.out.println(reversed);
        
        sc.close();
    }
}

13. Check if string is palindrome

Easy 
Input: madam
Output: Palindrome

import java.util.Scanner;

public class StringPalindrome {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
        
        String reversed = new StringBuilder(str).reverse().toString();
        
        if(str.equals(reversed)) {
            System.out.println("Palindrome");
        } else {
            System.out.println("Not Palindrome");
        }
        
        sc.close();
    }
}

14. Find largest element in array

Easy 
Input: 5
       10 45 23 67 34
Output: 67

import java.util.Scanner;

public class LargestInArray {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        
        for(int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
        }
        
        int max = arr[0];
        for(int i = 1; i < n; i++) {
            if(arr[i] > max) {
                max = arr[i];
            }
        }
        
        System.out.println(max);
        sc.close();
    }
}

15. Count vowels in a string

Easy 
Input: programming
Output: 3

import java.util.Scanner;

public class CountVowels {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next().toLowerCase();
        
        int count = 0;
        for(int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            if(ch == 'a' || ch == 'e' || ch == 'i' || 
               ch == 'o' || ch == 'u') {
                count++;
            }
        }
        
        System.out.println(count);
        sc.close();
    }
}

16. Find longest palindromic substring

Hard 
Input: babad
Output: bab (or aba)

import java.util.Scanner;

public class LongestPalindrome {
    public static String expandAroundCenter(String s, int left, int right) {
        while(left >= 0 && right < s.length() && 
              s.charAt(left) == s.charAt(right)) {
            left--;
            right++;
        }
        return s.substring(left + 1, right);
    }
    
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        
        if(s == null || s.length() < 1) {
            System.out.println("");
            sc.close();
            return;
        }
        
        String longest = "";
        for(int i = 0; i < s.length(); i++) {
            String odd = expandAroundCenter(s, i, i);
            String even = expandAroundCenter(s, i, i + 1);
            
            if(odd.length() > longest.length()) longest = odd;
            if(even.length() > longest.length()) longest = even;
        }
        
        System.out.println(longest);
        sc.close();
    }
}

17. Implement merge sort

Hard 
Input: 5
       64 34 25 12 22
Output: 12 22 25 34 64

import java.util.Scanner;

public class MergeSort {
    public static void merge(int[] arr, int l, int m, int r) {
        int n1 = m - l + 1;
        int n2 = r - m;
        
        int[] L = new int[n1];
        int[] R = new int[n2];
        
        for(int i = 0; i < n1; i++)
            L[i] = arr[l + i];
        for(int j = 0; j < n2; j++)
            R[j] = arr[m + 1 + j];
        
        int i = 0, j = 0, k = l;
        while(i < n1 && j < n2) {
            if(L[i] <= R[j]) {
                arr[k] = L[i];
                i++;
            } else {
                arr[k] = R[j];
                j++;
            }
            k++;
        }
        
        while(i < n1) {
            arr[k] = L[i];
            i++;
            k++;
        }
        
        while(j < n2) {
            arr[k] = R[j];
            j++;
            k++;
        }
    }
    
    public static void mergeSort(int[] arr, int l, int r) {
        if(l < r) {
            int m = l + (r - l) / 2;
            mergeSort(arr, l, m);
            mergeSort(arr, m + 1, r);
            merge(arr, l, m, r);
        }
    }
    
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        
        for(int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
        }
        
        mergeSort(arr, 0, n - 1);
        
        for(int i = 0; i < n; i++) {
            System.out.print(arr[i] + " ");
        }
        
        sc.close();
    }
}

18. Find all permutations of a string

Hard 
Input: ABC
Output: ABC ACB BAC BCA CAB CBA

import java.util.Scanner;

public class StringPermutations {
    public static void permute(String str, String answer) {
        if(str.length() == 0) {
            System.out.print(answer + " ");
            return;
        }
        
        for(int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            String left = str.substring(0, i);
            String right = str.substring(i + 1);
            String rest = left + right;
            permute(rest, answer + ch);
        }
    }
    
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
        
        permute(str, "");
        
        sc.close();
    }
}

19. Solve N-Queens problem

Hard 
Input: 4
Output: 
. Q . .
. . . Q
Q . . .
. . Q .

import java.util.Scanner;

public class NQueens {
    public static boolean isSafe(int[][] board, int row, int col, int n) {
        for(int i = 0; i < col; i++)
            if(board[row][i] == 1) return false;
        
        for(int i = row, j = col; i >= 0 && j >= 0; i--, j--)
            if(board[i][j] == 1) return false;
        
        for(int i = row, j = col; i < n && j >= 0; i++, j--)
            if(board[i][j] == 1) return false;
        
        return true;
    }
    
    public static boolean solveNQueens(int[][] board, int col, int n) {
        if(col >= n) return true;
        
        for(int i = 0; i < n; i++) {
            if(isSafe(board, i, col, n)) {
                board[i][col] = 1;
                
                if(solveNQueens(board, col + 1, n))
                    return true;
                
                board[i][col] = 0;
            }
        }
        
        return false;
    }
    
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[][] board = new int[n][n];
        
        if(solveNQueens(board, 0, n)) {
            for(int i = 0; i < n; i++) {
                for(int j = 0; j < n; j++) {
                    System.out.print(board[i][j] == 1 ? "Q " : ". ");
                }
                System.out.println();
            }
        } else {
            System.out.println("No solution exists");
        }
        
        sc.close();
    }
}

20. Find longest common subsequence (LCS)

Hard 
Input: AGGTAB
       GXTXAYB
Output: 4 (GTAB)

import java.util.Scanner;

public class LongestCommonSubsequence {
    public static int lcs(String s1, String s2, int m, int n) {
        int[][] dp = new int[m + 1][n + 1];
        
        for(int i = 0; i <= m; i++) {
            for(int j = 0; j <= n; j++) {
                if(i == 0 || j == 0) {
                    dp[i][j] = 0;
                } else if(s1.charAt(i - 1) == s2.charAt(j - 1)) {
                    dp[i][j] = dp[i - 1][j - 1] + 1;
                } else {
                    dp[i][j] = Math.max(dp[i - 1][j], dp[i][j - 1]);
                }
            }
        }
        
        return dp[m][n];
    }
    
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s1 = sc.next();
        String s2 = sc.next();
        
        int result = lcs(s1, s2, s1.length(), s2.length());
        System.out.println(result);
        
        sc.close();
    }
}