Looking for specific design pattern

Question

I'm looking for a pattern describing a certain situation. Maybe someone knows something and can give me a hint

Situation Object of type A, 1 : n association to objects of type B. Example: A Person and all his pets. The pet objects have an age data member. Now each person should have a data member for the average age of his pets. I don't want this information to be saved and loaded but to be computed during the loading process (all necessary information is already there, so it's kind of redundant, but I want the information in the person object).

Question So how can I do this in an object oriented way? Is there a pattern that describes this situation? My first idea was to have a class ComputedFieldsUpdater who takes a pet object and then computes all the field that have not been loaded. But I wonder if this is the right way or if there might be a more elegant solution?

Thanks for your thoughts.

Answer 1

If you create a class for the association to type B it's fairly simple: You need some kind of list anyway. So just create a "PetList" or something like that, that has a Method "averageAge()" which computes this value from all it's pets.

Now you can access the average by calling person.getPets().averageAge().

From an object-oriented point of view the information about the average shouldn't be directly in the Person-object anyway.

Answer 2

Its not a Gang of Four design pattern, but the generally accepted solution when it is absolutely necessary to store data that is redundant to a calculation is to lazy-load it, and then keep a boolean value to describe whether it is still valid.

In other words:

class Person
{
List<Pet> myPets;
float averageAge;
boolean isAgeValid = false;

float getAverageAge()
{
    if(isAgeValid)
        return averageAge;
    else
        ...calculate age, save it in averageAge, set bool to true, and return derived float...
}

//all list operations should mark the bool false
void Add(Pet p)
{
    myPets.Add(p);
    isAgeValid=false;
}
}