Project Patterns form mapping "consolidated properties&

djmbrasil · **Posted:** Mon Jan 21, 2008 10:30 am

I've searched for some project patterns for mapping what I called "derived properties". I'm posting some solutions and the problems of each of them and I want the opinion of everybody who had these same problems.

The Problem:

Code:

public class Cart {
  double itemsValue;
  double freight; 
  double totalValue; \\items value + freight
  List<CarteItem> itemList;
}
public class CartItem {
  double itemValue;
}


The question is: How to map the totalValue and the itemsValue properties?

The Solutions:
1 - Update the total value and itemsValue for all changes in cart object.

Code:

//<property name="itemsValue" access="field" />
setItemsValue(double v) {
 itemsValue = v;
 recalculate();
}
recalculate() {
totalValue = itemsValue + freight;
}

Problem: Imagine a property that is derived from the value of 15 other properties ... Each set method should've to be modified to set the attribute and recalculate the values.
2 - Map the consolidated attributes without the existence of the corresponding fields. Ex:

Code:

getItemsValue() {
 double v = 0;
   for (CartItem  item:itemList) {
     v+=item.itemValue;
   }
  return v;
}
setItemsValue(double v) {}
 getTotalValue() { return itemsValue + freight; }
setTotalVallue(double v) { }

Problem: Problem with dirty check. Depending on the database the value stored by hibernate to do the check is deferent from the value calculated causing a database update even without any change. Another problem is the need for initialization of all items to get the total items value.

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