Java programs for common interview questions

This guide covers frequently asked Java programs, including pattern printing, string manipulation, sorting algorithms, and OOP concepts. Prepare effectively with hands-on coding examples and explanations to boost your confidence and land your dream job! 🚀

1.Find Odd or Even Number

import java.util.Scanner;

public class OddEven {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter any number: ");
        int number = scanner.nextInt();
        if (number % 2 == 0) {
            System.out.println(number + " is even.");
        } else {
            System.out.println(number + " is odd.");
        }
    }
}

2.Check if a Number is Prime

import java.util.Scanner;

public class PrimeNumber {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int number = scanner.nextInt();

        if (isPrime(number)) {
            System.out.println(number + " is a prime number.");
        } else {
            System.out.println(number + " is not a prime number.");
        }
    }

    public static boolean isPrime(int num) {
        if (num <= 1) return false;
        for (int i = 2; i <= num / 2; i++) {
            if (num % i == 0) return false;
        }
        return true;
    }
}

3.Fibonacci Series

import java.util.Scanner;

public class FibonacciSeries {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter the number of terms: ");
        int number = scanner.nextInt();

        int first = 0, second = 1;
        System.out.print("Fibonacci Series: " + first + " " + second);

        for (int i = 2; i < number; i++) {
            int next = first + second;
            System.out.print(" " + next);
            first = second;
            second = next;
        }
    }
}

4.Swap Two Numbers Without Using a Third Variable

import java.util.Scanner;

public class SwapNumbers {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter the first number: ");
        int a = scanner.nextInt();
        System.out.print("Enter the second number: ");
        int b = scanner.nextInt();

        System.out.println("Before Swap: a=" + a + ", b=" + b);
        a = a + b;
        b = a - b;
        a = a - b;
        System.out.println("After Swap: a=" + a + ", b=" + b);
    }
}

5.Find Factorial of a Number

import java.util.Scanner;

public class FactorialNumber {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int number = scanner.nextInt();

        int factorial = 1;
        for (int i = 1; i <= number; i++) {
            factorial *= i;
        }
        System.out.println("Factorial of " + number + " is: " + factorial);
    }
}

6.Reverse a Number

import java.util.Scanner;

public class ReverseNumber {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter any number: ");
        int no = scanner.nextInt();

        int rev = 0;
        while (no > 0) {
            int r = no % 10;
            rev = rev * 10 + r;
            no = no / 10;
        }
        System.out.println("Reversed Number: " + rev);
    }
}

7.Check if a Number is an Armstrong Number

import java.util.Scanner;

public class ArmstrongNumber {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter any number: ");
        int no = scanner.nextInt();

        int d = no, arm = 0;
        while (no > 0) {
            int a = no % 10;
            no = no / 10;
            arm = arm + (a * a * a);
        }

        if (d == arm) {
            System.out.println(d + " is an Armstrong number.");
        } else {
            System.out.println(d + " is not an Armstrong number.");
        }
    }
}

8.Count Number of Digits in a Given Number

import java.util.Scanner;

public class NumberOfDigits {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter any number: ");
        int no = scanner.nextInt();

        int count = 0;
        while (no > 0) {
            no = no / 10;
            count++;
        }
        System.out.println("Number of digits: " + count);
    }
}

9.Check if a Number is a Palindrome

import java.util.Scanner;

public class PalindromeNumber {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int number = scanner.nextInt();

        if (isPalindrome(number)) {
            System.out.println(number + " is a palindrome.");
        } else {
            System.out.println(number + " is not a palindrome.");
        }
    }

    public static boolean isPalindrome(int num) {
        int originalNumber = num, reversedNumber = 0;
        while (num > 0) {
            int digit = num % 10;
            reversedNumber = reversedNumber * 10 + digit;
            num /= 10;
        }
        return originalNumber == reversedNumber;
    }
}

10.Calculate Sum of Digits of a Number

import java.util.Scanner;

public class SumOfDigits {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int number = scanner.nextInt();

        int sum = calculateSumOfDigits(number);
        System.out.println("Sum of digits of " + number + " is: " + sum);
    }

    public static int calculateSumOfDigits(int number) {
        int sum = 0;
        while (number > 0) {
            int digit = number % 10;
            sum += digit;
            number /= 10;
        }
        return sum;
    }
}

Strings

1.) Java program to reverse a string

import java.util.Scanner;
public class Test {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a string: ");
        String input = scanner.nextLine();
        char ch;
        String nstr = "";
        for (int i = 0; i < input.length(); i++) {
            ch = input.charAt(i);
            nstr = ch + nstr;
        }
        System.out.println("Reversed String is: " + nstr);
    }
}

2.) Java program to reverse each word of a given string

public static void main(String[] args) {
    reverseEachWordOfString("Java is good programming languages");
}

static void reverseEachWordOfString(String inputString) {
    String[] words = inputString.split(" ");
    String reverseString = "";

    for (int i = 0; i < words.length; i++) {
        String word = words[i];
        String nstr = "";
        char ch;
        for (int j = 0; j < word.length(); j++) {
            ch = word.charAt(j);
            nstr = ch + nstr;
        }
        reverseString = reverseString + nstr + " ";
    }

    System.out.println(inputString);
    System.out.println(reverseString);
}

3.) Java program to find duplicate characters in a string

import java.util.HashMap;
import java.util.Set;

public class Main {

    public static void main(String[] args) {
        duplicateCharacterCount("Learn Java Programming");
    }

    static void duplicateCharacterCount(String inputString) {

        HashMap<Character, Integer> charCountMap = new HashMap<>();
        char[] strArray = inputString.toCharArray();
        
        for (char c : strArray) {
            if (charCountMap.containsKey(c)) {
                charCountMap.put(c, charCountMap.get(c) + 1);
            } else {
                charCountMap.put(c, 1);
            }
        }

        Set<Character> charsInString = charCountMap.keySet();
        System.out.println("Duplicate Characters in : " + inputString);
        for (Character ch : charsInString) {
            if (charCountMap.get(ch) > 1) {
                System.out.println(ch + " : " + charCountMap.get(ch));
            }
        }
    }
}

4.) Java program to count occurrences of each character in a string

import java.util.HashMap;

public class Main {

    public static void main(String[] args) {
        CharacterCount("Test Automation Java Automation");
    }

    static void CharacterCount(String inputString) {
        HashMap<String, Integer> charCountMap = new HashMap<>();
        
        for (String s : inputString.split(" ")) {
            if (charCountMap.containsKey(s)) {
                charCountMap.put(s, charCountMap.get(s) + 1);
            } else {
                charCountMap.put(s, 1);
            }
        }

        System.out.println("Count of Characters in a given string : " + charCountMap);
    }
}

5.) Java program to count the number of words in a string

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        System.out.println("Enter the String");
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        int count = 1;
        
        for (int i = 0; i < s.length() - 1; i++) {
            if ((s.charAt(i) == ' ') && (s.charAt(i + 1) != ' ')) {
                count++;
            }
        }
        
        System.out.println("Number of words in a string: " + count);
    }
}

6.) Java program to find all permutations of a given string

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        String str = "abc";
        permute(str, "");
    }

    static void permute(String str, String prefix) {
        if (str.length() == 0) {
            System.out.println(prefix);
        } else {
            for (int i = 0; i < str.length(); i++) {
                String rem = str.substring(0, i) + str.substring(i + 1);
                permute(rem, prefix + str.charAt(i));
            }
        }
    }
}

7.) Java program to find if a string is Palindrome

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        String str = "madam";
        System.out.println(isPalindrome(str));
    }

    static boolean isPalindrome(String str) {
        int start = 0;
        int end = str.length() - 1;

        while (start < end) {
            if (str.charAt(start) != str.charAt(end)) {
                return false;
            }
            start++;
            end--;
        }
        return true;
    }
}

8.) Java program to determine if Two Strings are Anagrams

public class Main {
    public static void main(String[] args) {
        String str1 = "listen";
        String str2 = "silent";
        System.out.println(areAnagrams(str1, str2));
    }

    static boolean areAnagrams(String str1, String str2) {
        if (str1.length() != str2.length()) {
            return false;
        }
        
        int[] charCount = new int[256];
        for (int i = 0; i < str1.length(); i++) {
            charCount[str1.charAt(i)]++;
            charCount[str2.charAt(i)]--;
        }

        for (int count : charCount) {
            if (count != 0) {
                return false;
            }
        }
        return true;
    }
}

9.) Java program to Count Vowels and Consonants in a given string

public class Main {
    public static void main(String[] args) {
        String str = "Hello World";
        VowelConsonantCount(str);
    }

    static void VowelConsonantCount(String str) {
        int vowels = 0, consonants = 0;
        str = str.toLowerCase();
        
        for (char c : str.toCharArray()) {
            if (c >= 'a' && c <= 'z') {
                if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {
                    vowels++;
                } else {
                    consonants++;
                }
            }
        }
        
        System.out.println("Vowels : " + vowels);
        System.out.println("Consonants : " + consonants);
    }
}

10.) Java program to print unique characters

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a string: ");
        String input = scanner.nextLine();

        System.out.println("Unique characters in \"" + input + "\":");
        printUniqueCharacters(input);
    }

    public static void printUniqueCharacters(String str) {
        // Assume ASCII characters (0-127), use boolean array to track character occurrences
        boolean[] unique = new boolean[128];  
        
        for (int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            if (!unique[ch]) {
                unique[ch] = true;
                System.out.print(ch + " ");
            }
        }
    }
}

11.) Java program to print even indexed characters

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a string: ");
        String input = scanner.nextLine();

        System.out.println("Even indexed characters in \"" + input + "\":");
        printEvenIndexedCharacters(input);
    }

    public static void printEvenIndexedCharacters(String str) {
        for (int i = 0; i < str.length(); i++) {
            if (i % 2 == 0) {
                System.out.print(str.charAt(i));
            }
        }
    }
}

12.) Java program to remove space from a given string

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a string with spaces: ");
        String input = scanner.nextLine();

        String stringWithoutSpaces = removeSpaces(input);
        System.out.println("String without spaces: " + stringWithoutSpaces);
    }

    public static String removeSpaces(String str) {
        StringBuilder result = new StringBuilder();
        for (int i = 0; i < str.length(); i++) {
            if (str.charAt(i) != ' ') {
                result.append(str.charAt(i));
            }
        }
        return result.toString();
    }
}

13.) Java program to print each letter twice from a given string

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a string: ");
        String input = scanner.nextLine();

        String doubledString = doubleCharacters(input);
        System.out.println("Doubled characters: " + doubledString);
    }

    public static String doubleCharacters(String str) {
        StringBuilder doubled = new StringBuilder();
        for (int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            doubled.append(ch).append(ch); // Append each character twice
        }
        return doubled.toString();
    }
}

14.) Swapping Two Strings Without Using a Third Variable

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter first string: ");
        String str1 = scanner.nextLine();
        System.out.print("Enter second string: ");
        String str2 = scanner.nextLine();
        
        System.out.println("Before swapping: str1 = " + str1 + ", str2 = " + str2);

        // Swapping without using a third variable
        str1 = str1 + str2;  // Concatenate str1 and str2 and store in str1
        str2 = str1.substring(0, str1.length() - str2.length()); // Extract original str1 from concatenated string
        str1 = str1.substring(str2.length()); // Extract original str2

        System.out.println("After swapping: str1 = " + str1 + ", str2 = " + str2);
    }
}

15.) Java Program to Compress Consecutive Characters in a String

(Example: Input: “aabbcccd”, Output: “a2b2c3d2”)

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a string: ");
        String input = scanner.nextLine();

        String output = getCharacterCount(input);
        System.out.println("Output: " + output);
    }

    public static String getCharacterCount(String str) {
        StringBuilder result = new StringBuilder();
        int count = 1;

        for (int i = 0; i < str.length(); i++) {
            // If the next character is the same, increase the count
            if (i + 1 < str.length() && str.charAt(i) == str.charAt(i + 1)) {
                count++;
            } else {
                // Append the character and its count to the result
                result.append(str.charAt(i)).append(count);
                count = 1; // Reset the count
            }
        }
        return result.toString();
    }
}

16.) Java Program to Separate Uppercase and Lowercase Letters from a String

(Example: Input: “aBACbcEDed”, Output: “abcde” (lowercase) and “ABCDE” (uppercase))

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a string: ");
        String input = scanner.nextLine();

        System.out.println("Original String is: " + input);
        separateCharacters(input);
    }

    public static void separateCharacters(String input) {
        StringBuilder lowerCase = new StringBuilder();
        StringBuilder upperCase = new StringBuilder();

        for (char ch : input.toCharArray()) {
            if (Character.isLowerCase(ch)) {
                lowerCase.append(ch);
            } else {
                upperCase.append(ch);
            }
        }

        System.out.println("Output in lowercase: " + lowerCase);
        System.out.println("Output in uppercase: " + upperCase);
    }
}

 

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