尝试在我的代码中执行statemet.executeUpdate()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");
}
}
我还尝试设置statement final，但声明给了我另一个错误。
答案 0 :(得分：76)
确实存在范围问题，因为statement是此处定义的本地方法变量：
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()方法中访问此变量时，您尝试从匿名内部类中访问本地变量，并且范围不够。因此，它必须是final（由于代码不可能）或声明为类成员，因此内部类可以访问此statement变量。
来源：
使statement成为一个类成员而不是局部变量：
public class A1 { // Note Java Code Convention, also class name should be meaningful
private Statement statement;
...
}
定义另一个最终变量并使用此变量，如@HotLicks所建议的那样：
protected void createContents() {
...
Statement statement = null;
try {
statement = connect.createStatement();
final Statement innerStatement = statement;
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
...
}
重新考虑你的方法。如果statement变量在按下btnInsert按钮之前不会被使用，那么在实际发生之前创建连接是没有意义的。您可以使用所有本地变量：
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();
}
});
答案 1 :(得分：3)
除了使其成为最终或将无法访问的变量声明为类成员之外，还有第三个解决此问题的方法，因为这些选项并非总是可行或不可取。
首先，我们不能将变量设为final，因为它的状态可能在程序运行期间发生变化，而我们在内部类覆盖中的决定可能取决于其当前状态。
其次，良好的面向对象编程实践建议仅使用对类定义至关重要的变量/常量作为类成员。 这意味着如果我们在匿名内部类覆盖中引用的变量只是一个实用变量，那么它不应该列在类成员中。
所以 - 从Java 8开始 - 我们现在有第三个选项，如下所述：
https://docs.oracle.com/javase/tutorial/java/javaOO/localclasses.html
从Java SE 8开始，如果在方法中声明本地类，它可以访问方法的参数。
所以现在我们可以简单地把包含新内部类和代码的代码放入它的方法重写为私有静态方法，其参数包括我们从覆盖内部调用的变量。 然后在btnInsert声明语句之后调用此静态方法： -
// Original code :
Button btnInsert = new Button(shell, SWT.NONE);
// Call to new private static method :
addMouseListener(Button btnInsert, Statement statement);
. . .
. . .
. . .
// New private static method to give access to query statement :
private static void addMouseListener(Button btn, Statement st)
{
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) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
}
});
return;
}
. . . .
. . . .
. . . .
答案 2 :(得分：0)
不是错误：
JSONObject json1 = getJsonX();
错误：
JSONObject json2 = null;
if(x == y)
json2 = getJSONX();
错误：在封闭范围内定义的局部变量语句必须是最终的或有效的最终。
但你可以写：
JSONObject json2 = (x == y) ? json2 = getJSONX() : null;
答案 3 :(得分：0)
我发现这种方法很有用。这样你就不需要课程也不需要最后的
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));