/******************************************************
 *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 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.resultValue());
        System.out.println("Calculator program ending.");
    }

    public Calculator()
    {
        result = 0; 
    }

    public void reset()
    {
        result = 0;
    } 

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

     public double resultValue()
    {
        return result;
    }

    /**********************************************
     *The heart of a calculator. This does not give 
     *instructions. Input errors throw exceptions.
     **********************************************/
    public void doCalculation() throws DivideByZeroException,
                                       UnknownOpException
    {
        char nextOp;
        double nextNumber;
        boolean done = false;
        result = 0;
        System.out.println("result = " + result);
        while (! done)
        {
            nextOp = SavitchIn.readNonwhiteChar();
            if ((nextOp == 'e') || (nextOp == 'E'))
                done = true;
            else
            {
               nextNumber = SavitchIn.readLineDouble();
               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); 
        }
    }
}