import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.stage.Stage;
import javafx.collections.ObservableList;

public class ComboBoxTest extends Application {

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

    @Override
    public void start(Stage stage) {
        ComboBox<Integer> integerComboBox = new ComboBox<>(FXCollections.observableArrayList(1, 2, 3));
		integerComboBox.getSelectionModel().select(0); 
		integerComboBox.valueProperty().addListener((observable, oldValue, newValue) -> { 
			if (newValue == 2) 
				integerComboBox.getSelectionModel().select(oldValue); 
		}); 		
		
        stage.setScene(new Scene(integerComboBox));
        stage.show();
    }

}