EasyCalc.java

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

public class EasyCalc {

    public static void main(String[] args) {

        /**
        *   Program:    EasyCalc.java
        *   Creator:    Chris Clarke
        *   Created:    11.01.2011
        *   Purpose:    Perform simple calculation by evaluating string.
        *   Notes:      The text entered can include spaces.
        *           For "times", you can use "*" or "x".
        */

        Scanner scan = new Scanner(System.in);
        // declare variable 'theString' to be of type String
        String theString = "";
        // 'length' is an Integer variable
        int length;
        // 'temp' is a String variable
        String temp;
        // num1, num2 and answer are all declared as type double
        double num1 = 0;
        double num2 = 0;
        double answer = 0;
        // 'first', 'operand' and 'second' are declared to be of type String.
        // 'operand' is given an arbitrary value
        String first = "";
        String operand = "&";
        String second = "";
        // declare Boolean flag 'error'
        boolean error;
        do {
            // 'theString' is the text the user types in
            System.out.print("Type your sum: ");
            theString = scan.nextLine();
            if (theString.equals("" )) {
                // user didn't type anything so quit the program
                System.exit(0);
            }
            // assign the variable 'length' the length of string 'theString'
            length = theString.length();
            // get last character of theString and assign it to temp
            temp = theString.substring(length-1, length);
            if (temp.equals("=" )) {
                // remove equals sign at the end, if there was one.
                theString = theString.substring(0, length-1);
            }
            StringTokenizer t = new StringTokenizer(theString, "+-*x/");
            // separate theString into two numbers, separated by the operand
            first = t.nextToken();
            try {
                second = t.nextToken();
                error = false;
            }
            catch (Exception e) {
                // there wasn't a second token because it didn't find...
                // ...an operand "+, -, * or x, /"
                System.out.print("Operand error! Use +, -, * or x, and /");
                System.out.println();
                error = true;
            }
            // end of try/catch statement
        }
        while (error);
        // convert strings to doubles, i.e. floating point numbers
        try {
            num1 = Double.parseDouble(first);
            num2 = Double.parseDouble(second);
        } catch (NumberFormatException e) {
            System.out.print("Number error!");
            System.exit(1);
        }
        // end of try/catch statement

        // is the first letter a minus sign?
        String firstLetter;
        firstLetter = theString.substring(0, 1);
        // is the first number negative?
        if (firstLetter.equals("-" )) {
            num1 *= -1;
        }
        // find which operand was used
        for (int i = 1; i < length; i++) {
            operand = theString.substring(i, i+1);
            if (operand.equals("+" ) || operand.equals("-" )
             || operand.equals("*" ) || operand.equals("x" )
             || operand.equals("/" )) {
                break;
            }
            // end if
        }
        // end for
        // compute answer
        if (operand.equals("+" )) {
            answer = num1+num2;
        }
        else if (operand.equals("-")) {
            answer = num1-num2;
        }
        else if (operand.equals("*") || operand.equals("x")) {
            answer = num1*num2;
        }
        else if (operand.equals("/")) {
            if (num2 == 0) {
                System.out.print("Division by zero error!");
                System.exit(0);
            }
            answer = num1/num2;
        }
        // end if

        // display original sum and the answer
        System.out.println(num1+" "+operand+" "+num2+" = "+answer);
    }
}