AllergyRegister

public class AllergyRegister {

    private static AllergyRegister instance;
    private Map<Integer, Allergy> allergies;

    public static AllergyRegister getInstance(){
        if(instance == null)
            instance = new AllergyRegister();
        return instance;
    }

    //constructor private houden
    private AllergyRegister(){
        this.allergies = new TreeMap<>();
    }

    public void Register(Allergy allergy){
        this.allergies.put(allergy.getId(), allergy);
    }

    public Allergy getById(int id){
        return this.allergies.containsKey(id) ? this.allergies.get(id) : null;
    }


    public Map<Integer, Allergy> getAllAllergies(){
        return  this.allergies;
    }

}