Calculator using JavaFX in a simple way

src_Calculator.css

.root{
    -fx-background-color: darkgrey;
}
ToggleButton{
    -fx-padding: 15;
    -fx-font-size: 15;
}

src_Calculator_Display.java

package Calculator;

import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.control.TextField;
import javafx.scene.control.ToggleButton;
import javafx.scene.control.ToggleGroup;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;

public class Display {
    static String a, b, operator;//a=first number   &&  b=second number
    static int time = 0;

    public static VBox numpad() {
        TextField input = new TextField();
        input.setPromptText("Enter a value");

        VBox lay = new VBox();
        lay.setAlignment(Pos.CENTER);
        lay.getChildren().add(input);

        ToggleGroup tg = new ToggleGroup();

        String[][] s = {
                {"9", "8", "7", "/"},
                {"6", "5", "4", "*"},
                {"1", "2", "3", "-"},
                {"0", ".", "=", "+"}
        };

        for (int i = 0; i < 4; i++) {
            HBox h = new HBox();
            for (int j = 0; j < 4; j++) {
                ToggleButton t = new ToggleButton(s[i][j]);
                t.setUserData(s[i][j]);
                t.setToggleGroup(tg);
                h.getChildren().add(t);
            }
            h.setPadding(new Insets(10, 0, 0, 0));
            h.setSpacing(10);
            h.setAlignment(Pos.CENTER);
            lay.getChildren().add(h);
        }

        tg.selectedToggleProperty().addListener((v, oldValue, newValue) -> {
            String data = newValue.getUserData().toString();
            if (data.equals("+") || data.equals("-") || data.equals("*") || data.equals("/") || data.equals("=")) {
                if (time == 0) {
                    a = input.getText();
                    operator=data;
                    input.clear();
                    time++;
                } else {
                    b = input.getText();
                    input.clear();
                    calculation(input);
                    time=0;
                }
            } else
                input.setText(input.getText() + data);
        });
        return lay;
    }

    public static void calculation(TextField input) {
        Double ans=0.0;
        switch (operator) {
            case "+": ans = Double.parseDouble(a) + Double.parseDouble(b);
            break;
            case "-": ans = Double.parseDouble(a) - Double.parseDouble(b);
            break;
            case "*": ans = Double.parseDouble(a) + Double.parseDouble(b);
            break;
            case "/": ans = Double.parseDouble(a) + Double.parseDouble(b);
            break;
        }
        input.setText(""+ans);
    }
}

src_Calculator_Main.java

package Calculator;

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

public class Main extends Application{
    public static void main(String[] args){
        launch(args);
    }
    public void start(Stage window) throws Exception{
        VBox lay = Display.numpad();

        window.setTitle("Calculator");
        Scene sc = new Scene(lay, 230, 300);
        sc.getStylesheets().add("Calculator.css");
        window.setScene(sc);
        window.show();
    }

}