Read input from a JOptionPane.showInputDialog box

Question

I am almost done with this project. It's my very first one and I need to read a little better the inputs from the JOption box. For example, I want to go back to the previous page if the user clicks on "cancel" or close the program. So far, every time I click on "cancel" my program crashes. help me out! I just started learning Java about a month ago. I think I am doing pretty alright. Thanks so very much!

Here is the code,

import java.util.Scanner;
import javax.swing.JOptionPane;

/**
 * Write a description of class Operation here.
 *
 * @author Charles Kossivi
 * @version 1.0
 */
public class Operation {

    static String firstNumber; //first number or coefficient
    static String secondNumber; //second number or coefficient
    static String thirdNumber; //third number or coefficient
    static String operationType = ""; //Operation type
    static boolean anotherOperation = false;
    static boolean goodType = true; //true=run program false=stop program
    static boolean numberTypeA = true; // another operation?
    static boolean numberTypeB = true; // another operation? 
    static boolean numberTypeC = true; // another operation? 
    static Scanner keyInput = new Scanner(System.in);
    static String inputType; // temperature type
    static double a; // first number
    static double b; // second number
    static double c; // third number
    static double s; // sum of a and b
    static double d; // difference of a and b
    static double p; // product of a and b
    static double r; // ratio of a and b
    static double D; // discriminant
    static double numType;

    public static void main(String[] args) {
        while (goodType) {
            operationType = JOptionPane.showInputDialog("Welcome! \nPlease, Select the Desired Operation Type"
                    + "(\nA=Addition, \nD=Division, \nE=Quadratic Equation, "
                    + "\nM=Multiplication, \nS=Subtraction, \nQ=Quit");
            if (operationType.equalsIgnoreCase("A")) {
                newAddition();
            }
            if (operationType.equalsIgnoreCase("S")) {
                newSubtraction();
            }
            if (operationType.equalsIgnoreCase("M")) {
                newMultiplication();
            }
            if (operationType.equalsIgnoreCase("D")) {
                newDivision();
            }
            if (operationType.equalsIgnoreCase("E")) {
                newQuadratic();
            }
            if (operationType.equalsIgnoreCase("Q")) {
                // quit   
                JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
                goodType = false;
            }
        }
        goodType = false;
    }

    //Start Addition class here
    private static void newAddition() {
        //read in first coefficient from user as a string
        firstNumber = JOptionPane.showInputDialog("Please, Enter First Number");
        if (firstNumber.equalsIgnoreCase("Q")) {
            // quit   
            JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
            goodType = false;
        } else {
            a = Double.parseDouble(firstNumber);
            //read in second coefficient from user as a string
            secondNumber = JOptionPane.showInputDialog("Please, Enter Second Number");
            if (secondNumber.equalsIgnoreCase("Q")) {
                // quit   
                JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
                goodType = false;
            } else {
                b = Double.parseDouble(secondNumber);
                //Adding the numbers
                s = a + b;
                //Display results
                JOptionPane.showMessageDialog(null, "The sum is " + s);
            }
        }
    }

    //End Addition class
    //Start Addition class here
    private static void newSubtraction() {
        //read in first coefficient from user as a string
        firstNumber = JOptionPane.showInputDialog("Please, Enter First Number");
        if (firstNumber.equalsIgnoreCase("Q")) {
            // quit   
            JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
            goodType = false;
        } else {
            a = Double.parseDouble(firstNumber);
            //read in second coefficient from user as a string
            secondNumber = JOptionPane.showInputDialog("Please, Enter Second Number");
            if (secondNumber.equalsIgnoreCase("Q")) {
                // quit   
                JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
                goodType = false;
            } else {
                b = Double.parseDouble(secondNumber);
                //Subtraction the numbers
                d = a - b;
                //Display results
                JOptionPane.showMessageDialog(null, "The sum is " + d);
            }
        }
    }//End Subtraction class

    // Begin Multiplication class
    private static void newMultiplication() {
        //read in first coefficient from user as a string
        firstNumber = JOptionPane.showInputDialog("Please, Enter First Number");
        if (firstNumber.equalsIgnoreCase("Q")) {
            // quit   
            JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
            goodType = false;
        } else {
            a = Double.parseDouble(firstNumber);
            //read in second coefficient from user as a string
            secondNumber = JOptionPane.showInputDialog("Please, Enter Second Number");
            if (secondNumber.equalsIgnoreCase("Q")) {
                // quit   
                JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
                goodType = false;
            } else {
                b = Double.parseDouble(secondNumber);
                //multiplying the numbers
                p = a * b;
                //Display results
                JOptionPane.showMessageDialog(null, "The product is " + p);
            }
        }
    } // End Multiplication class

    //Start Division class
    private static void newDivision() {
        //read in first coefficient from user as a string
        firstNumber = JOptionPane.showInputDialog("Please, Enter First Number");
        if (firstNumber.equalsIgnoreCase("Q")) {
            // quit   
            JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
            goodType = false;
        } else {
            a = Double.parseDouble(firstNumber);
            //read in second coefficient from user as a string
            secondNumber = JOptionPane.showInputDialog("Please, Enter Second Number");
            if (secondNumber.equalsIgnoreCase("Q")) {
                // quit   
                JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
                goodType = false;
            } else {
                b = Double.parseDouble(secondNumber);
                //Dividing the numbers
                r = a / b;
                //Display results
                JOptionPane.showMessageDialog(null, "The ratio is " + r);
            }
        }
    } //End Division class

    //Start Quadratic class
    private static void newQuadratic() {
        //read in first coefficient from user as a string
        firstNumber = JOptionPane.showInputDialog("Please, Enter First Number");
        if (firstNumber.equalsIgnoreCase("Q")) {
            // quit   
            JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
            goodType = false;
        } else {
            a = Double.parseDouble(firstNumber);
            //read in second coefficient from user as a string
            secondNumber = JOptionPane.showInputDialog("Please, Enter Second Number");
            if (secondNumber.equalsIgnoreCase("Q")) {
                // quit   
                JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
                goodType = false;
            } else {
                b = Double.parseDouble(secondNumber);
                //read in third coefficient from user as a string
                thirdNumber = JOptionPane.showInputDialog("Please, Enter Third coefficient");
                if (thirdNumber.equalsIgnoreCase("Q")) {
                    // quit   
                    JOptionPane.showMessageDialog(null, "\nProgram ended. \nThank You!");
                    goodType = false;
                } else {
                    c = Double.parseDouble(thirdNumber);
                    //Quadratic formula
                    D = (Math.sqrt(b * b - 4 * a * c)) / (2 * a);
                    //Display results
                    if (D >= 0) {
                        JOptionPane.showMessageDialog(
                                null, "The equation's first real root is " + ((-b + D) / (2 * a)) + "\n"
                                + "The equation's second real root is " + ((-b - D) / (2 * a)) + "\n");
                    } else {
                        JOptionPane.showMessageDialog(
                                null, "The equation has no real solution");
                    }
                }
            }
        }
    }// end class quadratics
}

Answer 1

Your problem is that, if the user clicks cancel, operationType is null and thus throws a NullPointerException. I would suggest that you move

if (operationType.equalsIgnoreCase("Q"))

to the beginning of the group of if statements, and then change it to

if(operationType==null||operationType.equalsIgnoreCase("Q")).

This will make the program exit just as if the user had selected the quit option when the cancel button is pushed.

Then, change all the rest of the ifs to else ifs. This way, once the program sees whether or not the input is null, it doesn't try to call anything else on operationType. This has the added benefit of making it more efficient - once the program sees that the input is one of the options, it won't bother checking it against the rest of them.