如果我无法从内部 class 访问非最终变量，你如何在 Java 中进行“计数器”计数？

Question

I'm working on a small coding exercise for my course but I can't seem to overcome an issue I'm having with it. I want to count everytime someone clicks a button, but I can't create a variable to store the information because I have to create the variable outside the event handler so it doesn't reset to 0 everytime the button is pressed.

However, whenever I create a variable outside the event handler and try to add onto it by 1, it says it cannot be accessed from within the inner class.

public class HelloApplication extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("Counter Application");
        Label label = new Label("Counter");
        Button btn = new Button();
        btn.setText("Count");
         int count = 0;

        btn.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {
                count++;
            }
        });

        StackPane root = new StackPane();
        root.getChildren().add(btn);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }
}

Answer 1

You can make it an instance variable in the event handler:

public class HelloApplication extends Application {
    public static void main(String[] args) {
        launch(args);
    }
    
    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("Counter Application");
        Label label = new Label("Counter");
        Button btn = new Button();
        btn.setText("Count");

        btn.setOnAction(new EventHandler<ActionEvent>() {

            private int count = 0 ;

            @Override
            public void handle(ActionEvent event) {
                count++;
            }
        });

        StackPane root = new StackPane();
        root.getChildren().add(btn);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }
}

Or you can make the counter an instance variable in the surrounding class:

public class HelloApplication extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    private int count = 0 ;
    
    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("Counter Application");
        Label label = new Label("Counter");
        Button btn = new Button();
        btn.setText("Count");

        btn.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {
                count++;
            }
        });

        StackPane root = new StackPane();
        root.getChildren().add(btn);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }
}

Or you can define a wrapper class holding a mutable int:

public class HelloApplication extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    
    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("Counter Application");
        Label label = new Label("Counter");
        Button btn = new Button();
        btn.setText("Count");

        class IntWrapper {
            private int x = 0 ;
            public int getX() {
                return x;
            }
            public void increment() {
                x++;
            }
        }

        IntWrapper count = new IntWrapper();

        btn.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {
                count.increment();
            }
        });

        StackPane root = new StackPane();
        root.getChildren().add(btn);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }
}

The last two options let you implement the event handler as a lambda expression

btn.setOnAction(event -> count.increment());

while the first doesn't let you do this.

Answer 2

Make count a member of class HelloApplication and use a method reference .

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class HelloApplication extends Application {
    private int  count;

    public static void main(String[] args) {
        launch(args);
    }
    
    @Override public void start(Stage primaryStage) throws Exception {
        primaryStage.setTitle("Counter Application");
        Label label = new Label("Counter");
        Button btn = new Button();
        btn.setText("Count");
        btn.setOnAction(this::buttonClickCounter);
        StackPane root = new StackPane();
        root.getChildren().add(btn);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }

    private void buttonClickCounter(ActionEvent event) {
        count++;
    }
}

As you can see, you can name the method anything you like and give it any access modifier you like. Just make sure it returns the same value and accepts the same parameters as method handle in interface EventHandler .