无限 while 循环输入法 runTask1() 在课堂上 Task1 正在冻结javafx应用程序线程。把它取下来。
基本上是你的 Task1 上课是另一回事 Scene 所以当你点击按钮 newTask1Button 在课堂上 Tasks 你只需要设置一个新的 Scene .
这是上课时间 Task1 有必要的改变。

package tasks;

import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.HBox;
import javafx.scene.control.Button;

public class Task1 {
    private Stage window;
    private Scene task1Scene;
    private boolean intialised;

    private int state;

    public Task1() {
    }

    public Task1(Stage _window) {
        this.window = _window;
        this.task1Scene = null; // Will be set later
        this.intialised = false;
        this.state = -1;
    }

    private Scene createScene() {
        // Creates some GUI to interact

        // Buttons in End, to control exit
        HBox menu = new HBox();

        Button pauseButton = new Button("Pause");
        pauseButton.setOnAction(clickEvent -> this.state = 1);
        menu.getChildren().add(pauseButton);

        Button exitButton = new Button("Exit");
        exitButton.setOnAction(clickEvent -> this.state = 0);
        menu.getChildren().add(exitButton);

        Scene scene = new Scene(menu, 400, 600);

        return scene;
    }

    private void setupControls() {
        // To assign event handlers to interact with GUI
    }

    public int runTask1() {
        if (!this.intialised)
            this.task1Scene = this.createScene();
        this.window.setScene(this.task1Scene);

        this.setupControls();

//        while (this.state == -1)
//            ;

        return this.state;
    }
}

如你所见，我只是把 while 循环。javafx应用程序线程包含一个循环，它等待用户操作发生，例如移动鼠标或在键盘上键入键。你不必在你的代码里处理这个。

编辑

由于您的问题中的代码输入错误，您在对我的答案的评论中提到，并且您在随后的编辑中更正了您的问题，我正在编辑我的答案。
javafx应用程序的工作方式是对用户操作做出React。你要上课吗 Tasks 上课时要通知“状态” Task1 ，并且当用户单击 pauseButton 或者 exitButton 在课堂上 Task1 . 根据你发布的代码，你可以回调到类 Tasks 从的事件处理程序 pauseButton .
班级 Task1 .
（请注意此处的注解更改和构造函数中的额外参数。）

package tasks;

import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.HBox;
import javafx.scene.control.Button;

public class Task1 {
    private Tasks tasks;
    private Stage window;
    private Scene task1Scene;
    private boolean intialised;

    private int state;

    public Task1(Stage _window, Tasks tasks) {
        this.tasks = tasks;
        this.window = _window;
        this.task1Scene = null; // Will be set later
        this.intialised = false;
        this.state = -1;
    }

    private Scene createScene() {
        HBox menu = new HBox();
        Button pauseButton = new Button("Pause");
        pauseButton.setOnAction(clickEvent -> tasks.setState(this.state = 1)); // CHANGE HERE
        menu.getChildren().add(pauseButton);
        Button exitButton = new Button("Exit");
        exitButton.setOnAction(clickEvent -> this.state = 0);
        menu.getChildren().add(exitButton);
        Scene scene = new Scene(menu, 400, 600);
        return scene;
    }

    private void setupControls() {
        // To assign event handlers to interact with GUI
    }

    public int runTask1() {
        if (!this.intialised) {
            this.task1Scene = this.createScene();
        }
        this.window.setScene(this.task1Scene);
        this.setupControls();
        return this.state;
    }
}

班级 Tasks （新增方法） setState(int) .)

package tasks;

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.VBox;
import javafx.scene.control.Button;

public class Tasks extends Application {
    private Stage window;
    private Scene menuScene;
    private Task1 task1;

    public Tasks() {
        this.window = null;
        this.menuScene = null;
        this.task1 = null;
    }

    private void setMenu() {
        VBox menu = new VBox();
        Button newTask1Button = new Button("New Task 1");
        newTask1Button.setOnAction(clickEvent -> this.startNewTask1());
        menu.getChildren().add(newTask1Button);
        this.menuScene = new Scene(menu, 400, 600);
        this.window.setScene(this.menuScene);
    }

    private void startNewTask1() {
        this.task1 = new Task1(this.window, this);
        this.launchTask1();
    }

    private void launchTask1() {
        if (this.task1 != null) {
            this.task1.runTask1();
        }
    }

    @Override
    public void start(Stage primaryStage) {
        this.window = primaryStage;
        this.window.setTitle("Tasks");
        this.setMenu();
        this.window.show();
    }

    public static void main(String[] args) {
        Application.launch(args);
    }

    public void setState(int task1State) {
        System.out.println("Task1 is in state " + task1State); // In no way part of program,
                                                               // just for debugging.
    }
}