局部变量范围问题。怎么解决呢？

Question

尝试statemet.executeUpdate()在我的代码中执行时出现以下错误：

Local variable statement defined in an enclosing scope must be final or effectively final.

到目前为止，这是我的代码：

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");

    }
}

我也尝试设置statementfinal，但声明给了我另一个错误。

Answer 1

确实存在范围问题，因为statement这里定义了一个本地方法变量：

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();
            }
            ...
    });
}

当您尝试在mouseDown()方法内部访问此变量时，您尝试从匿名内部类内部访问局部变量，并且作用域不够。因此，它肯定是final（不可能给出您的代码）或声明为类成员，以便内部类可以访问此statement变量。

怎么解决呢？

你可以…

使statement一个类成员而不是一个局部变量：

public class A1 { // Note Java Code Convention, also class name should be meaningful   
    private Statement statement;
    ...
}

你可以…

定义另一个最终变量，并使用该变量，如@HotLicks建议：

protected void createContents() {
    ...
    Statement statement = null;
    try {
        statement = connect.createStatement();
        final Statement innerStatement = statement;
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    ...
}

但是你应该…

重新考虑您的方法。如果在按下按钮statement之前不使用变量，btnInsert那么 在 实际发生 之前
创建连接是没有意义的。您可以这样使用所有局部变量：

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();
       }
});