Question

我知道如何使用final关键字。假设如果将一个类设为final，那么它不能被继承，如果一个方法是final，那么它不能被覆盖，如果一个变量是final，那么该值不能被改变。但在这种情况下我有点困惑

    final TextField urlTextField = new TextField();

我认为如果将urlTextField设为最终版，则无法重新制作

urlTextField = new textField（）。但是在下面的例子中为什么它是最终的

Source

package org.carlfx;

import javafx.application.Application;
import javafx.beans.property.BooleanProperty;
import javafx.beans.property.SimpleBooleanProperty;
import javafx.beans.value.ChangeListener;
import javafx.concurrent.Worker.State;
import javafx.print.*;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TextField;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.transform.Scale;
import javafx.scene.web.WebEngine;
import javafx.scene.web.WebView;
import javafx.stage.Stage;

/**
 * Demo to use JavaFX 8 Printer API.
 *
 * @author cdea
 */
public class PrintDemo extends Application {
    @Override
    public void start(Stage primaryStage) {

        final TextField urlTextField = new TextField();
        final Button printButton = new Button("Print");
        final WebView webPage = new WebView();
        final WebEngine webEngine = webPage.getEngine();

        HBox hbox = new HBox();
        hbox.getChildren().addAll(urlTextField, printButton);
        BorderPane borderPane = new BorderPane();
        borderPane.setTop(hbox);
        borderPane.setCenter(webPage);
        Scene scene = new Scene(borderPane, 300, 250);
        primaryStage.setTitle("Print Demo");
        primaryStage.setScene(scene);

        // print button pressed, page loaded
        final BooleanProperty printButtonClickedProperty = new SimpleBooleanProperty(false);
        final BooleanProperty pageLoadedProperty = new SimpleBooleanProperty(false);

        // when the a page is loaded and the button was pressed call the print() method.
        final BooleanProperty printActionProperty = new SimpleBooleanProperty(false);
        printActionProperty.bind(pageLoadedProperty.and(printButtonClickedProperty));

        // WebEngine updates flag when finished loading web page.
        webEngine.getLoadWorker()
                 .stateProperty()
                 .addListener( (ChangeListener) (obsValue, oldState, newState) -> {
                    if (newState == State.SUCCEEDED) {
                        pageLoadedProperty.set(true);
                    }
                 });

        // When user enters a url and hits the enter key.
        urlTextField.setOnAction( aEvent ->  {
            pageLoadedProperty.set(false);
            printButtonClickedProperty.set(false);
            webEngine.load(urlTextField.getText());
        });

        // When the user clicks the print button the webview node is printed
        printButton.setOnAction( aEvent -> {
            printButtonClickedProperty.set(true);
        });

        // Once the print action hears a true go print the WebView node.
        printActionProperty.addListener( (ChangeListener) (obsValue, oldState, newState) -> {
            if (newState) {
                print(webPage);
            }
        });

        primaryStage.show();

    }

    /** Scales the node based on the standard letter, portrait paper to be printed.
     * @param node The scene node to be printed.
     */
    public void print(final Node node) {
        Printer printer = Printer.getDefaultPrinter();
        PageLayout pageLayout = printer.createPageLayout(Paper.NA_LETTER, PageOrientation.PORTRAIT, Printer.MarginType.DEFAULT);
        double scaleX = pageLayout.getPrintableWidth() / node.getBoundsInParent().getWidth();
        double scaleY = pageLayout.getPrintableHeight() / node.getBoundsInParent().getHeight();
        node.getTransforms().add(new Scale(scaleX, scaleY));

        PrinterJob job = PrinterJob.createPrinterJob();
        if (job != null) {
            boolean success = job.printPage(node);
            if (success) {
                job.endJob();
            }
        }
    }

    /**
     * The main() method is ignored in correctly deployed JavaFX application.
     * main() serves only as fallback in case the application can not be
     * launched through deployment artifacts, e.g., in IDEs with limited FX
     * support. NetBeans ignores main().
     *
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        launch(args);
    }
}

Answer 1

在Java 7之前，你需要声明一个局部变量final来从内部类访问它。

从Java 8开始不再需要：the variable only need to be effectively final（即未更改）。这种改变与lambda语法一致。

从技术上讲，如果您使用的是Java FX 8，那么此处不需要final关键字。

Answer 2

在Java 7中，当你在一个匿名类中并且你正在尝试使用/接近包装类的变量时 - 该变量应该被声明为final或者编译器会抱怨。同样的事情适用于Lambda表达式（从Java 8支持）。尝试从声明中删除final，您将在第4行收到编译器错误：

    urlTextField.setOnAction( aEvent ->  {
        pageLoadedProperty.set(false);
        printButtonClickedProperty.set(false);
        webEngine.load(urlTextField.getText()); // <-- compile error 
    });

来自documentation：

匿名类无法访问其封闭的局部变量未被宣布为最终或有效最终的范围。

Lambda expressions和匿名类共享相似的属性（lambda表达式可以定义为两件事的组合：代码和范围）。除此之外，Lambda表达式（如内部类）只能使用在lambda外声明的final（或“有效最终”）变量。以下是 documentation ：

中的示例

        // The following statement causes the compiler to generate
        // the error "local variables referenced from a lambda expression
        // must be final or effectively final" in statement A:
        //
        // x = 99;

        Block<Integer> myBlock = (y) -> 
        {
            System.out.println("x = " + x); // Statement A
            System.out.println("y = " + y);
            System.out.println("this.x = " + this.x);
            System.out.println("LambdaScopeTest.this.x = " +
                LambdaScopeTest.this.x);
        };

在对象的情况下使用final关键字

2 个答案: