尝试statemet.executeUpdate()在我的代码中执行时,我收到以下错误:
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statement最终,但声明给了我另一个错误.
dic*_*c19 85
你确实有一个范围问题,因为statement这里定义了一个局部方法变量:
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中访问局部变量,并且范围不够.所以它肯定必须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.
资料来源:
创建statement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这个变量,如@HotLicks所建议:
protected void createContents() {
...
Statement statement = null;
try {
statement = connect.createStatement();
final Statement innerStatement = statement;
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
...
}
重新考虑你的方法.如果在按下按钮statement之前不会使用变量,btnInsert那么在实际发生之前创建连接是没有意义的.你可以使用这样的所有局部变量:
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();
}
});
首先，我们不能将变量设为 final，因为它的状态在程序运行期间可能会发生变化，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程序变量，那么它不应该被列在类成员中。
但是 - 从 Java 8 开始 - 我们有第三种选择，描述如下：
https://docs.oracle.com/javase/tutorial/java/javaOO/localclasses.html
从 Java SE 8 开始，如果您在方法中声明本地类，则它可以访问该方法的参数。
所以现在我们可以简单地将包含新内部类及其方法覆盖的代码放入私有方法中，该方法的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：-
. . . .
. . . .
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;
}
. . . .
. . . .
. . . .
| 归档时间： |
|
| 查看次数： |
150306 次 |
| 最近记录： |