Meh calc

import java.util.Scanner;

public class Calc {

    private double a, b;
    private boolean running = true;

    private final Scanner mScanner;

    public Calc() {
        mScanner = new Scanner(System.in);
        start();
    }

    private void start() {
        while (running) {
            getInput();
            processInput();
        }
        mScanner.close();
    }

    private void processInput() {
        System.out.println("1. Arithmetic");
        System.out.println("2. Boolean");
        System.out.println("3. Bitwise (Integers only!)");
        System.out.println("4. Exit");
        System.out.print("Select an operation: ");
        int op = mScanner.nextInt();
        switch (op) {
            case 1:
                doArithmetic();
                break;
            case 2:
                doBoolean();
                break;
            case 3:
                doBitwise();
                break;
            case 4:
                System.out.println("Exiting...");
                running = false;
                break;
            default:
                System.err.println("Invalid operation");
                processInput();
                break;
        }
    }


    private void doArithmetic() {
        System.out.println("1. Add");
        System.out.println("2. Subtract");
        System.out.println("3. Multiply");
        System.out.println("4. Divide");
        System.out.println("5. Modulo");
        System.out.println("6. Power");
        System.out.println("7. Root");
        System.out.println("8. Natural log");
        System.out.print("Select an operation: ");

        int op = mScanner.nextInt();

        if (op < 9 && op > 0) {
            System.out.print("The result is: ");
        }

        switch (op) {
            case 1:
                System.out.println(a + b);
                break;
            case 2:
                System.out.println(a - b);
                break;
            case 3:
                System.out.println(a * b);
                break;
            case 4:
                System.out.println(a / b);
                break;
            case 5:
                System.out.println(a % b);
                break;
            case 6:
                System.out.println(Math.pow(a, b));
                break;
            case 7:
                System.out.println(Math.pow(a, 1 / b));
                break;
            case 8:
                System.out.println(Math.log(a) + ", " + Math.log(b));
                break;
            default:
                System.err.println("Invalid operation");
                doArithmetic();
                break;
        }
    }

    private void doBoolean() {
        System.out.println("1. Greater");
        System.out.println("2. Greater than or equal");
        System.out.println("3. Smaller");
        System.out.println("4. Smaller than or equal");
        System.out.println("5. Logical equal");
        System.out.print("Select an operation: ");

        int op = mScanner.nextInt();

        if (op < 6 && op > 0) {
            System.out.print("The result is: ");
        }

        switch (op) {
            case 1:
                System.out.println(a > b);
                break;
            case 2:
                System.out.println(a >= b);
                break;
            case 3:
                System.out.println(a < b);
                break;
            case 4:
                System.out.println(a <= b);
                break;
            case 5:
                System.out.println(a == b);
                break;
            default:
                System.err.println("Invalid operation");
                doBoolean();
                break;
        }
    }

    private void doBitwise() {
        System.out.println("1. AND");
        System.out.println("2. OR");
        System.out.println("3. XOR");
        System.out.println("4. NAND");
        System.out.println("5. NOR");
        System.out.println("6. Shift left");
        System.out.println("7. Shift right");
        System.out.print("Select an operation: ");

        int op = mScanner.nextInt();

        if (op < 8 && op > 0) {
            System.out.print("The result is: ");
        }

        int iA = (int) a;
        int iB = (int) b;

        switch (op) {
            case 1:
                System.out.println(iA & iB);
                break;
            case 2:
                System.out.println(iA | iB);
                break;
            case 3:
                System.out.println(iA ^ iB);
                break;
            case 4:
                // TODO: Implement NAND
                break;
            case 5:
                System.out.println(~(iA | iB));
                break;
            case 6:
                System.out.println(iA << iB);
                break;
            case 7:
                System.out.println(iA >> iB);
                break;
            default:
                System.err.println("Invalid operation");
                doBitwise();
                break;
        }

    }

    private void getInput() {
        System.out.print("Enter the first number: ");
        a = mScanner.nextDouble();
        System.out.print("Enter the second number: ");
        b = mScanner.nextDouble();
    }

    public static void main(String[] args) {
        new Calc();
    }
}