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Tutti i file elencati si trovano in src/main/java nel package application.

Per quanto riguarda il file Main.java:

package application;

public class Main {
    public static void main(String[] args) {
        MainApplication.main(args);
    }
}

Per quanto riguarda il file MainApplication.java (una sorta di main):

package application;

import javafx.application.Application;
import javafx.concurrent.WorkerStateEvent;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.ProgressBar;
import javafx.scene.control.TextField;
import javafx.scene.layout.FlowPane;
import javafx.stage.Stage;

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

    @Override
    public void start(Stage primaryStage) throws Exception {
        CheckPrime check = new CheckPrime();
        TextField field = new TextField();
        Button b = new Button("Check prime");
        b.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                String text = field.getText();
                long num = Long.parseLong(text);
                check.setNum(num);
                check.restart();
            }
        });

        Label result = new Label("Res: ");
        ProgressBar bar = new ProgressBar();

        FlowPane pane = new FlowPane();
        pane.getChildren().add(field);
        pane.getChildren().add(b);
        pane.getChildren().add(bar);
        pane.getChildren().add(result);
        bar.progressProperty().bind(check.progressProperty());

        check.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
            @Override
            public void handle(WorkerStateEvent event) {
                Boolean res = (Boolean) event.getSource().getValue();
                if(res)
                    result.setText("Res: prime");
                else
                    result.setText("Res: not prime");
            }
        });

        check.setOnFailed(new EventHandler<WorkerStateEvent>() {
            @Override
            public void handle(WorkerStateEvent event) {
                System.out.println("E' avvenuta un'eccezione nel check di primalità");
            }
        });

        check.setOnCancelled(new EventHandler<WorkerStateEvent>() {
            @Override
            public void handle(WorkerStateEvent event) {
                System.out.println("Interrupted");
            }
        });

        Scene scene = new Scene(pane, 800, 800);
        primaryStage.setScene(scene);
        primaryStage.show();
        field.setText(""+918278217);
        check.setNum(918278217);
        check.start();
    }
}

Per quanto riguarda il file CheckPrime.java:

package application;

import javafx.concurrent.Service;
import javafx.concurrent.Task;

public class CheckPrime extends Service<Boolean> {
    private long num;

    public void setNum(long num) { this.num = num; }

    @Override
    protected Task<Boolean> createTask() {
        return new Task<Boolean>() {
            @Override
            protected Boolean call() throws Exception {
                boolean res = true;
                for(long i = 2; i < num; i++) {
                    updateProgress(i, num);
                    if(num%i==0)
                        res = false;
                }
                return res;
            }
        };
    }
}