JavaFX

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.Pane;
import javafx.scene.shape.Line;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;

public class App extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        Pane pane = new AnchorPane();

        mountDemo(pane);

        stage.setScene(new Scene(pane));
        stage.setMaximized(true);
        stage.show();
    }

    private static void mountDemo(Pane pane) {
        Button buttonAdd = new Button("Add");
        Button buttonRemove = new Button("Remove");
        buttonRemove.setLayoutX(50);

        Line line = new Line(100, 100, 400, 100);
        Rectangle rectangle = new Rectangle(5, 5);

        pane.getChildren().add(buttonAdd);
        pane.getChildren().add(buttonRemove);

        buttonAdd.setOnMouseClicked((event)->pane.getChildren().add(line));
        buttonRemove.setOnMouseClicked((event)->pane.getChildren().remove(line));

        line.parentProperty().addListener((observable, oldParent, newParent)->{
            if(newParent != null)
                ((Pane)newParent).getChildren().add(rectangle);
            else
                ((Pane)oldParent).getChildren().remove(rectangle);
        });
    }

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

}