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

public class ComboBoxIssue extends Application {

    public enum Fruit {
        APPLE, BANANA, CHERRY
    }

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

    @Override
    public void start(Stage primaryStage) throws Exception {
        ComboBox<Fruit> fruits = new ComboBox<>();
        fruits.getItems().addAll(Fruit.APPLE, Fruit.BANANA);
        fruits.setValue(Fruit.APPLE);
        Button button = new Button("Change");
        button.setOnAction(event -> {
            fruits.setValue(Fruit.CHERRY);
        });
        primaryStage.setScene(new Scene(new VBox(fruits, button)));
        primaryStage.show();
    }

}