Special Case Pattern

Can we make the code that simple? It turns out that we can. We can change the ExpenseReportDAO so that it always returns a MealExpense object. If there are no meal expenses, it returns a MealExpense object that returns the per diem as its total:

public class PerDiemMealExpenses implements MealExpenses {
    public int getTotal() {
    	// return the per diem default
    }
}

This is called the SPECIAL CASE PATTERN [Fowler]. You create a class or configure an object so that it handles a special case for you. When you do, the client code doesn’t have to deal with exceptional behavior. That behavior is encapsulated in the special case object.

from The Clean Code, Uncle Bob Martin.