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.

Answer 1

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?

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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();        } });