How to eliminate chars in textfield JavaFx directly so the user can't type in chars?

Hello I want to have a textfield , where the user only can type in numbers and the user isn't able to type in a char. I'm confused because some want to do it with actionlistener some with Formatter and a few more options. Can someone tell me what how to do this? and where to place it in the code that it always checks if there is a letter and deletes it automatically? thx

package sample;

import javafx.fxml.FXML;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.control.TextFormatter;

import java.awt.*;
import java.util.function.UnaryOperator;

public class Controller {

    GuessingGame gg = new GuessingGame();
    //Main m = new Main();


    public Button playButton;
    public Button settingsButton;
    public Button exitButton;

    public TextField textfield;

    public Label countText;
    public Label notification;

    public int currentNumber = 0;
    public int numberBetweenLower = 0;
    public int numberBetweenHigher = 100;

    public void clickedOnExit() {
        System.exit(1);
    }

    ;

    public void clickedOnSettings() {
    }

    ;

    public void clickedOnPlay() {
        Main.pStage.setScene(Main.scene2);
    }

    ;

    public void clickedBackToMenu() {
        Main.pStage.setScene(Main.scene1);
    }

    ;

    public void enteredTextfield() {

        currentNumber = Integer.parseInt(textfield.getText());
        textfield.clear();

        GuessingGame.Result checkNumber;
        checkNumber = gg.evaluateEnteredNumber(currentNumber);
        if (checkNumber == GuessingGame.Result.HIGHER) {
            notification.setText("HIGHER");
        }
        ;
        if (checkNumber == GuessingGame.Result.LOWER) {
            notification.setText("LOWER");
        }
        ;
        if (checkNumber == GuessingGame.Result.EQUALS) {
            notification.setText("YOU GOT IT!");
        }
        ;

        countText.setText("" + gg.getCounter());
    }


    @FXML
    public void initialize() {
        // initialize controller and set text shown in the UI
    }

    private static final class NumberFormatUnaryOperator implements UnaryOperator<TextFormatter.Change> {

        /**
         * Applies this function to the given argument.
         *
         * @param change the function argument
         * @return the function result
         */
        @Override
        public TextFormatter.Change apply(TextFormatter.Change change) {
            var String = change.getControlNewText();

            if (!text.matches("\\d*")) {
                Toolkit.getDefaultToolkit().beep();
                return null;
            }
            return change;
        }
    }
}