//Calculator.java

import java.util.Scanner;

/**
 * Simple line-oriented calculator program. The class
 * can also be used to create other calculator programs.
 */
public class Calculator
{
    private double result;
    private double precision = 0.0001;
    //Numbers this close to zero are treated as if equal to zero.

    public static void main(String[] args)
    {
        Calculator clerk = new Calculator();
        try
        {
            System.out.println("Calculator is on.");
            System.out.print("Format of each line: ");
            System.out.println("operator space number");
            System.out.println("For example: + 3");
            System.out.println("To end, enter the letter e.");
            clerk.doCalculation();
        }
        catch (UnknownOpException e)
        {
            clerk.handleUnknownOpException(e);
        }
        catch (DivideByZeroException e)
        {
            clerk.handleDivideByZeroException(e);
        }
        System.out.println("The final result is " + clerk.getResult());
        System.out.println("Calculator program ending.");
    }

    public Calculator()
    {
        result = 0;
    }

    public void reset()
    {
        result = 0;
    }

    public void setResult
    (
        double newResult
    )
    {
        result = newResult;
    }

    public double getResult()
    {
        return result;
    }

    /**
     * The heart of a calculator. This does not give
     * instructions. Input errors throw exceptions.
     */
    public void doCalculation()
        throws DivideByZeroException, UnknownOpException
    {
        Scanner keyboard = new Scanner(System.in);

        boolean done = false;
        result = 0;
        System.out.println("result = " + result);
        while (!done)
        {
            char nextOp = (keyboard.next()).charAt(0);
            if ((nextOp == 'e') || (nextOp == 'E'))
                done = true;
            else
            {
                double nextNumber = keyboard.nextDouble();
                result = evaluate(nextOp, result, nextNumber);
                System.out.println("result " + nextOp + " "
                    + nextNumber + " = " + result);
                System.out.println("updated result = " + result);
            }
        }
    }

    /**
     * Returns n1 op n2, provided op is one of '+', '-', '*',or '/'.
     * Any other value of op throws UnknownOpException.
     */
    public double evaluate
    (
        char op,
        double n1,
        double n2
    ) throws DivideByZeroException, UnknownOpException
    {
        double answer;
        switch (op)
        {
        case '+':
            answer = n1 + n2;
            break;
        case '-':
            answer = n1 - n2;
            break;
        case '*':
            answer = n1 * n2;
            break;
        case '/':
            if ((-precision < n2) && (n2 < precision))
                throw new DivideByZeroException();
            answer = n1 / n2;
            break;
        default:
            throw new UnknownOpException(op);
        }
        return answer;
    }

    public void handleDivideByZeroException
    (
        DivideByZeroException e
    )
    {
        System.out.println("Dividing by zero.");
        System.out.println("Program aborted");
        System.exit(0);
    }

    public void handleUnknownOpException
    (
        UnknownOpException e
    )
    {
        System.out.println(e.getMessage());
        System.out.println("Try again from the beginning:");

        try
        {
            System.out.print("Format of each line: ");
            System.out.println("operator number");
            System.out.println("For example: + 3");
            System.out.println("To end, enter the letter e.");
            doCalculation();
        }
        catch (UnknownOpException e2)
        {
            System.out.println(e2.getMessage());
            System.out.println("Try again at some other time.");
            System.out.println("Program ending.");
            System.exit(0);
        }
        catch (DivideByZeroException e3)
        {
            handleDivideByZeroException(e3);
        }
    }
}