SimpleCalculator.java

import java.util.*;     // for StringTokenizer, Scanner

public class SimpleCalculator {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        String theString = "";
        int length;
        // num1, num2 and answer are declared as type double
        double num1 = 0;
        double num2 = 0;
        double answer = 0;

        String first = "";
        String operator = "";
        String second = "";

        boolean error;
        do {
            // 'theString' is the text the user types in
            System.out.print("Type your sum (q = quit): ");
            theString = scan.nextLine();
            if (theString.equals("q")) {
                // quit the program
                System.exit(0);
            } // end if
            // assign the variable 'length' to length of String 'theString'
            length = theString.length();

            StringTokenizer t = new StringTokenizer(theString, "+-*/%");
            // separate theString into two numbers, separated by the operator
                first = t.nextToken();
            try {
                second = t.nextToken();
                error = false;
            }
            catch (Exception e) {
                // there wasn't a second token because it didn't find an operator
                System.out.print("Error! Use the format: <number>+-*/%<number>");
                System.out.println();
                error = true;
            } // end try/catch
        } while (error);

        // convert strings to doubles
        num1 = Double.parseDouble(first);
        num2 = Double.parseDouble(second);
        // is the first letter a minus sign?
        String firstLetter;
        firstLetter = theString.substring(0, 1);
        // is the first number negative?
        if (firstLetter.equals("-" )) {
            num1 *= -1;
        } // end if

        // find which operator was used
        for (int i = 1; i < length; i++) {
            operator = theString.substring(i, i+1);
            if (operator.equals("+" ) || operator.equals("-" ) || operator.equals("*" ) ||
             operator.equals("/" )) {
                break;
            } // end if
        } // end for

        // compute answer
        switch(operator) {
            case "+" :  answer = num1+num2; break;
            case "-" :  answer = num1-num2; break;
            case "*" :  answer = num1*num2; break;
            case "/" :  if (num2 == 0) {
                        System.out.print("Division by zero error!");
                        System.exit(0);
                    } // end if
                    answer = num1/num2; break;
            case "%" :  answer = num1%num2; break;
            default  :  System.out.print("Wrong operator.");
                    System.exit(0);
        } // end switch

        // display original sum and answer
        System.out.print(num1+operator+num2+"="+answer);

    } // end main()
} // end class SimpleCalculator