Java Code

  Check whether a given number is prime or not:

public class PrimeChecker { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); System.out.print("Enter a number: "); int number = scanner.nextInt(); scanner.close(); boolean isPrime = true; if (number <= 1) { isPrime = false; } else { for (int i = 2; i <= Math.sqrt(number); i++) { if (number % i == 0) { isPrime = false; break; } } } if (isPrime) { System.out.println(number + " is a prime number."); } else { System.out.println(number + " is not a prime number."); } } }

1. How do you reverse a string in Java?

public static void main(String[] args) { String originalString = "Hello, world!"; // Create a StringBuilder object and pass the original string to its constructor StringBuilder reversedString = new StringBuilder(originalString); // Use the reverse() method of the StringBuilder class to reverse the string reversedString.reverse(); // Convert the StringBuilder object back to a string using toString() method String result = reversedString.toString(); // Print the reversed string System.out.println("Reversed string: " + result); }

public static void main(String[] args) {
		String str = "123";

		System.out.println(reverse(str));
	}

	public static String reverse(String in) {
		if (in == null)
			throw new IllegalArgumentException("Null is not valid input");

		StringBuilder out = new StringBuilder();

		char[] chars = in.toCharArray();

		for (int i = chars.length - 1; i >= 0; i--)
			out.append(chars[i]);

		return out.toString();
	}




2. How do you swap two numbers without using a third variable in Java?

public static void main(String[] args) {
	int a = 10;
	int b = 20;

    System.out.println("a is " + a + " and b is " + b);

	a = a + b;
	b = a - b;
	a = a - b;

    System.out.println("After swapping, a is " + a + " and b is " + b);
    }

3. Write a Java program to check if a vowel is present in a string

public class StringContainsVowels {

	public static void main(String[] args) {
		System.out.println(stringContainsVowels("Hello")); // true
		System.out.println(stringContainsVowels("TV")); // false
	}

	public static boolean stringContainsVowels(String input) {
		return input.toLowerCase().matches(".*[aeiou].*");
	}

}

4. Write a Java program to check if the given number is a prime number.

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

		return true;
	}

5. Write a Java program to print a Fibonacci sequence using recursion.

public class PrimeNumberCheck {

	public static void main(String[] args) {
		System.out.println(isPrime(19)); // true
		System.out.println(isPrime(49)); // false
	}

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

		return true;
	}

}

5. Write a Java program to print a Fibonacci sequence using recursion.

public class PrintFibonacci {

	public static void printFibonacciSequence(int count) {
		int a = 0;
		int b = 1;
		int c = 1;

		for (int i = 1; i <= count; i++) {
			System.out.print(a + ", ");

            a = b;
			b = c;
			c = a + b;
		}
	}

	public static void main(String[] args) {
    	printFibonacciSequence(10);
	}

}

The following example class shows how to use recursion to calculate a Fibonacci sequence that is 10 numbers long:

public class PrintFibonacciRecursive {

    public static int fibonacci(int count) {
		if (count <= 1)
			return count;

		return fibonacci(count - 1) + fibonacci(count - 2);
	}

	public static void main(String args[]) {
    	int seqLength = 10;

    	System.out.print("A Fibonacci sequence of " + seqLength + " numbers: ");

    	for (int i = 0; i < seqLength; i++) {
      	    System.out.print(fibonacci(i) + " ");
    	}
  	}

}

6. How do you check if a list of integers contains only odd numbers in Java?

public static boolean onlyOddNumbers(List<Integer> list) {
	for (int i : list) {
		if (i % 2 == 0)
			return false;
	}

	return true;
}

public static boolean onlyOddNumbers(List<Integer> list) {
	return list.parallelStream() // parallel stream for faster processing
			.anyMatch(x -> x % 2 != 0); // return as soon as any elements match the condition
}

7. How do you check whether a string is a palindrome in Java?

boolean checkPalindromeString(String input) {
	boolean result = true;
	int length = input.length();

	for (int i = 0; i < length/2; i++) {
		if (input.charAt(i) != input.charAt(length - i - 1)) {
			result = false;
			break;
		}
	}

	return result;
}

public class PalindromeCheck {
    public static void main(String[] args) {
        String str = "radar";
        
        if (isPalindrome(str)) {
            System.out.println(str + " is a palindrome.");
        } else {
            System.out.println(str + " is not a palindrome.");
        }
    }
    
    public static boolean isPalindrome(String str) {
        str = str.toLowerCase();
        
        // Remove non-alphanumeric characters
        str = str.replaceAll("[^a-z0-9]", "");
        
        // Check if the string is equal to its reverse
        return str.equals(new StringBuilder(str).reverse().toString());
    }
}

10. How do you sort an array in Java?

int[] array = {1, 2, 3, -1, -2, 4};

Arrays.sort(array);

System.out.println(Arrays.toString(array));

12. How can you find the factorial of an integer in Java?

public static long factorial(long n) {
	if (n == 1)
		return 1;
	else
		return (n * factorial(n - 1));
}