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  gt   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

User · Answer

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

[java] Read input from a JOptionPane.showInputDialog box

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