要实现橡皮筋效果，可以使用JavaFX的Canvas和MouseEvent来实现。下面是一个示例代码：

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class RubberBandEffect extends Application {
    private double startX, startY, endX, endY;
    private boolean isDrawing;

    @Override
    public void start(Stage primaryStage) {
        Canvas canvas = new Canvas(400, 400);
        GraphicsContext gc = canvas.getGraphicsContext2D();

        Pane root = new Pane(canvas);

        canvas.setOnMousePressed(event -> {
            startX = event.getX();
            startY = event.getY();
            isDrawing = true;
        });

        canvas.setOnMouseDragged(event -> {
            if (isDrawing) {
                endX = event.getX();
                endY = event.getY();
                redraw(gc);
            }
        });

        canvas.setOnMouseReleased(event -> {
            isDrawing = false;
        });

        Scene scene = new Scene(root, 400, 400);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    private void redraw(GraphicsContext gc) {
        gc.clearRect(0, 0, gc.getCanvas().getWidth(), gc.getCanvas().getHeight());
        gc.setStroke(Color.BLACK);
        gc.setLineWidth(1);
        gc.strokeRect(startX, startY, endX - startX, endY - startY);
        gc.setFill(Color.LIGHTGRAY);
        gc.fillRect(startX, startY, endX - startX, endY - startY);
    }

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

在这个示例中，我们使用Canvas来绘制矩形，当鼠标按下时记录起始点坐标，鼠标拖动时更新终点坐标并重新绘制矩形，鼠标释放时停止绘制。通过清除画布并重新绘制矩形来实现橡皮筋效果。