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

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.

This question is related to java scope

The answer is


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:


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();
       }
});

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 & 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;
}

 . . . .
 . . . .
 . . . .

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;

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));