Practice Questions — Methods in Java

public class PowerCalc {
    static long power(int base, int exp) {
        long result = 1;
        for (int i = 0; i < exp; i++) {
            result *= base;
        }
        return result;
    }

    public static void main(String[] args) {
        System.out.println("2^10 = " + power(2, 10));
        System.out.println("5^3 = " + power(5, 3));
    }
}

public class CountDigits {
    static int countDigits(int n) {
        if (n < 10) return 1;
        return 1 + countDigits(n / 10);
    }

    public static void main(String[] args) {
        System.out.println("Digits in 12345: " + countDigits(12345));
        System.out.println("Digits in 7: " + countDigits(7));
    }
}

public class AreaCalculator {
    static double area(double radius) {
        return Math.PI * radius * radius;
    }

    static double area(double length, double width) {
        return length * width;
    }

    static double area(double base, double height, boolean isTriangle) {
        return 0.5 * base * height;
    }

    public static void main(String[] args) {
        System.out.printf("Circle area: %.2f%n", area(5.0));
        System.out.printf("Rectangle area: %.2f%n", area(5.0, 6.0));
        System.out.printf("Triangle area: %.2f%n", area(8.0, 6.0, true));
    }
}

public class SumOfDigits {
    static int sumOfDigits(int n) {
        if (n == 0) return 0;
        return n % 10 + sumOfDigits(n / 10);
    }

    public static void main(String[] args) {
        System.out.println("Sum of digits of 12345: " + sumOfDigits(12345));
        System.out.println("Sum of digits of 9999: " + sumOfDigits(9999));
    }
}

public class PalindromeRecursive {
    static boolean isPalindrome(String s, int left, int right) {
        if (left >= right) return true;
        if (s.charAt(left) != s.charAt(right)) return false;
        return isPalindrome(s, left + 1, right - 1);
    }

    public static void main(String[] args) {
        String s1 = "racecar";
        String s2 = "hello";
        System.out.println(s1 + " is a palindrome: " + isPalindrome(s1, 0, s1.length() - 1));
        System.out.println(s2 + " is a palindrome: " + isPalindrome(s2, 0, s2.length() - 1));
    }
}

public class BinarySearch {
    static int binarySearch(int[] arr, int target, int low, int high) {
        if (low > high) return -1;
        int mid = low + (high - low) / 2;
        if (arr[mid] == target) return mid;
        if (arr[mid] < target) return binarySearch(arr, target, mid + 1, high);
        return binarySearch(arr, target, low, mid - 1);
    }

    public static void main(String[] args) {
        int[] arr = {2, 5, 8, 12, 16, 23, 38, 56, 72, 91};
        System.out.println("Index of 23: " + binarySearch(arr, 23, 0, arr.length - 1));
        System.out.println("Index of 50: " + binarySearch(arr, 50, 0, arr.length - 1));
    }
}

public class TowerOfHanoi {
    static void hanoi(int n, char from, char to, char aux) {
        if (n == 0) return;
        hanoi(n - 1, from, aux, to);
        System.out.println("Disk " + n + ": " + from + " -> " + to);
        hanoi(n - 1, aux, to, from);
    }

    public static void main(String[] args) {
        hanoi(3, 'A', 'C', 'B');
    }
}

public class Permutations {
    static void permute(char[] arr, int index) {
        if (index == arr.length - 1) {
            System.out.println(new String(arr));
            return;
        }
        for (int i = index; i < arr.length; i++) {
            char temp = arr[index];
            arr[index] = arr[i];
            arr[i] = temp;

            permute(arr, index + 1);

            temp = arr[index];
            arr[index] = arr[i];
            arr[i] = temp;
        }
    }

    public static void main(String[] args) {
        permute("ABC".toCharArray(), 0);
    }
}

public class StatsCalc {
    static int min(int... nums) {
        int result = nums[0];
        for (int n : nums) {
            if (n < result) result = n;
        }
        return result;
    }

    static int max(int... nums) {
        int result = nums[0];
        for (int n : nums) {
            if (n > result) result = n;
        }
        return result;
    }

    static double average(int... nums) {
        int sum = 0;
        for (int n : nums) sum += n;
        return (double) sum / nums.length;
    }

    public static void main(String[] args) {
        System.out.println("Min: " + min(45, 78, 23, 91, 56));
        System.out.println("Max: " + max(45, 78, 23, 91, 56));
        System.out.printf("Average: %.2f%n", average(45, 78, 23, 91, 56));
    }
}