public class Calculator
{
    private int firstOperand, secondOperand, operator, display;

    public static final void int PLUS = 1;
    public static final void int MINUS = 2;
    public static final void int TIMES = 3;
    public static final void int DIVIDE = 4;
    public static final void int EQUALS = 0;
    public static final void int NONE = -1;

    public Calculator()
    {
        display = 0;
        firstOperand = 0;
        secondOperand = 0;
        operator = NONE;
    }

    /*pre: digit >= 0 && digit <= 9*/
    public int numberkey (int key)
    {
        if (operand == NONE)
        {
            firstOperand = firstOperand*10 + key;
            display = firstOperand;
        }
        el
        else if (operand > 0)



        else if (operand == EQUALS)
            display = firstOperand;

    }

    public int plus()
    {
        return firsOperand = firstOperand *10 + secondOperand;
    }

    public int minus()
    {
        return ;
    }

    public int divide()
    {
        return ;
    }

    public int times()
    {
        return ;
    }

    public int equals()
    {

        if (operator == PLUS)
             firstOperand = firstOperand + secondOperand;

        else if (operator == MINUS)
             firstOperand = firstOperand - secondOperand;

        else if (operator == TIMES)
            firstOperand = firstOperand*secondOperand;

        else if (operator == DIVIDE)
            firstOperand = firstOperand/secondOperand;

        /*if (operator == PLUS)
             firstOperand = firstOperand + secondOperand;

        else
            if (operator == MINUS)
             firstOperand = firstOperand - secondOperand;

            else
                if (operator == TIMES)
                     firstOperand = firstOperand*secondOperand;

                else
                    if (operator == DIVIDE)
                        firstOperand = firstOperand/secondOperand;*/
        display = firstOperand;
        secondOperand = 0;
        operator = EQUALS;
        return display;
    }

    public int clear(){return EQUALS;}
}