Problems with local variable scope How to solve it

Question

I m getting the following error when trying to execute statemet executeUpdate   in my code   Local variable statement defined in an enclosing scope must be final or effectively final    This is my code so far   import java sql Connection  import java sql DriverManager  import java sql ResultSet  import java sql SQLException  import java sql Statement    import org eclipse swt SWT  import org eclipse swt events MouseAdapter  import org eclipse swt events MouseEvent  import org eclipse swt widgets Button  import org eclipse swt widgets Display  import org eclipse swt widgets Label  import org eclipse swt widgets Shell  import org eclipse swt widgets Text   public class a1        protected Shell shell      private Text text      private Text text 1      private Text text 2      private Text text 3                  Launch the application          param args             public static void main String   args            try               a1 window   new a1                window open              catch  Exception e                e printStackTrace                                    Open the window              public void open             Display display   Display getDefault            createContents            shell open            shell layout            while   shell isDisposed                  if   display readAndDispatch                      display sleep                                                  Create contents of the window              protected void createContents              Connection connect   null           ResultSet resultSet   null           try               Class forName  com mysql jdbc Driver              catch  ClassNotFoundException e                   TODO Auto-generated catch block             e printStackTrace                      try               connect   DriverManager getConnection  jdbc mysql   localhost railwaydb    root                  catch  SQLException e                   TODO Auto-generated catch block             e printStackTrace                       Statement statement   null             statements allow to issue SQL queries to the database         try               statement   connect createStatement              catch  SQLException e                   TODO Auto-generated catch block             e printStackTrace                       shell   new Shell            shell setSize 450  300           shell setText  SWT Application             Label lblName   new Label shell  SWT NONE           lblName setBounds 10  43  47  15           lblName setText  Name             Label lblFrom   new Label shell  SWT NONE           lblFrom setBounds 10  74  55  15           lblFrom setText  From             Label lblTo   new Label shell  SWT NONE           lblTo setBounds 10  105  55  15           lblTo setText  To             Label lblPrice   new Label shell  SWT NONE           lblPrice setBounds 10  137  55  15           lblPrice setText  Price             text   new Text shell  SWT BORDER           text setBounds 64  43  76  21            text 1   new Text shell  SWT BORDER           text 1 setBounds 64  74  76  21            text 2   new Text shell  SWT BORDER           text 2 setBounds 64  105  76  21            text 3   new Text shell  SWT BORDER           text 3 setBounds 64  137  76  21            Label lblRailwayDatabase   new Label shell  SWT NONE           lblRailwayDatabase setBounds 174  10  97  15           lblRailwayDatabase setText  Railway Database             Label lblCreateView   new Label shell  SWT NONE           lblCreateView setBounds 189  43  76  15           lblCreateView setText  Create View             Button btnName   new Button shell  SWT CHECK           btnName setBounds 189  73  93  16           btnName setText  Name             Button btnFrom   new Button shell  SWT CHECK           btnFrom setBounds 189  105  93  16           btnFrom setText  From             Button btnTo   new Button shell  SWT CHECK           btnTo setBounds 189  137  93  16           btnTo setText  To             Button btnPrice   new Button shell  SWT CHECK           btnPrice setBounds 189  171  93  16           btnPrice setText  Price             Button btnInsert   new Button shell  SWT NONE           btnInsert addMouseListener new MouseAdapter                  Override             public void mouseDown MouseEvent e                    String name   text getText                    String from   text 1 getText                    String to   text 2 getText                    String price   text 3 getText                     String query    INSERT INTO booking  name  fromst  tost  price  VALUES     name        from        to        price                       try                       statement executeUpdate query                     catch  SQLException e1                           TODO Auto-generated catch block                     e1 printStackTrace                                                        btnInsert setBounds 10  171  75  25           btnInsert setText  Insert             Button btnView   new Button shell  SWT NONE           btnView setBounds 307  74  75  25           btnView setText  View             Button btnIndex   new Button shell  SWT NONE           btnIndex setBounds 307  127  75  25           btnIndex setText  Index               I also tried to set statement final but the declaration gives me another error

User · Answer

not Error   JSONObject json1   getJsonX     Error   JSONObject json2   null  if x    y     json2   getJSONX      Error  Local variable statement defined in an enclosing scope must be final or effectively final   But you can write    JSONObject json2    x    y    json2   getJSONX     null

User · Answer

I found this approach useful  This way you do not need a class nor final   btnInsert addMouseListener new MouseAdapter             private Statement  statement           public MouseAdapter setStatement Statement  stmnt                         statement    stmnt              return this                     Override         public void mouseDown MouseEvent e                String name   text getText                String from   text 1 getText                String to   text 2 getText                String price   text 3 getText                 String query    INSERT INTO booking  name  fromst  tost  price  VALUES     name        from        to        price                   try                    statement executeUpdate query                 catch  SQLException e1                       TODO Auto-generated catch block                 e1 printStackTrace                                  setStatement statement

User · Answer

You have a scope problem indeed  because statement is a local method variable defined here   protected void createContents                 Statement statement   null     local variable              btnInsert addMouseListener new MouseAdapter        anonymous inner class          Override         public void mouseDown MouseEvent e                                try                   statement executeUpdate query      local variable out of scope here               catch  SQLException e1                    e1 printStackTrace                                              When you try to access this variable inside mouseDown   method you are trying to access a local variable from within an anonymous inner class and the scope is not enough  So it definitely must be final  which given your code is not possible  or declared as a class member so the inner class can access this statement variable   Sources    Anonymous Classes How are Anonymous  inner  classes used in Java      How to solve it   You could     Make statement a class member instead of a local variable   public class A1      Note Java Code Convention  also class name should be meaningful        private Statement statement              You could     Define another final variable and use this one instead  as suggested by  HotLicks   protected void createContents                 Statement statement   null      try           statement   connect createStatement            final Statement innerStatement   statement        catch  SQLException e               TODO Auto-generated catch block         e printStackTrace                      But you should     Reconsider your approach  If statement variable won t be used until btnInsert button is pressed then it doesn t make sense to create a connection before this actually happens  You could use all local variables like this   btnInsert addMouseListener new MouseAdapter         Override    public void mouseDown MouseEvent e           try              Class forName  com mysql jdbc Driver               try  Connection connect   DriverManager getConnection                       Statement statement   connect createStatement                          execute the statement here               catch  SQLException ex                   ex printStackTrace                           catch  ClassNotFoundException ex               e printStackTrace

User · Answer

Firstly  we just CAN T make the variable final as its state may be changing during the run of the program and our decisions within the inner class override may depend on its current state   Secondly  good object-oriented programming practice suggests using only variables constants that are vital to the class definition as class members   This means that if the variable we are referencing within the anonymous inner class override is just a utility variable  then it should not be listed amongst the class members   But     as of Java 8     we have a third option  described here    https   docs oracle com javase tutorial java javaOO localclasses html  Starting in Java SE 8  if you declare the local class in a method  it can access the method s parameters   So now we can simply put the code containing the new inner class  amp  its method override into a private method whose parameters include the variable we call from inside the override   This static method is then called after the btnInsert declaration statement  -                      Statement statement   null                                                        Button btnInsert   new Button shell  SWT NONE    addMouseListener Button btnInsert  Statement statement         Call new private method                           private static void addMouseListener Button btn  Statement st     New private method giving access to statement         btn addMouseListener new MouseAdapter                 Override       public void mouseDown MouseEvent e                   String name   text getText            String from   text 1 getText            String to   text 2 getText            String price   text 3 getText            String query    INSERT INTO booking  name  fromst  tost price  VALUES     name        from        to        price               try                        st executeUpdate query                      catch  SQLException e1                         e1 printStackTrace                                          TODO Auto-generated catch block                         return

[java] Problems with local variable scope. How to solve it?

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