2 Why We Need Model-Based Testing

248 Modeling Objects
static class Contract
{
static Map salary = Map.EmptyMap;
// ...
}
}
In this example each employee has a salary. There is a state variable called salary
that maps Employee values to integers that represent an employee’s pay rate. The
value Employee(1) might be associated with the salary 200. The salary field is an
example of a field map, or table that relates an object identifier to an associated data
value. A field map is a way of encoding in a single state variable elements of state
that occur on a per-instance basis. Note that the salary field itself is global (static
in C# ).
Introducing an identifier class like Employee is a modeling idiom that allows us
to build up arbitrary object graphs, with independently updated fields. For example,
we use a field map to treat salary as a separate element of state. The type Employee
provides keys for the salary field map. The Employee class contains only fields that
establish identity. Other kinds of data are excluded.
The field map idiom is especially useful when we want to encode object graphs
with possibly circular references. For example, an employee might have a “backup
contact” who handles work in the employee’s absence. Backup contacts might
introduce circular references in the object graph. (A has B as backup contact and
vice versa.)
Note that it wouldn’t be possible to add a “backup contact” field to a compound
value type like the Employee class. You may recall from Section 10.3 that compound
values like Employee are immutable types with structural equality. Circular
references among fields of such types are not possible.
Since the idiom of independently updated per-instance fields occurs frequently,
object-oriented languages like C# provide built-in support for instance fields. The
NModel framework supports instance fields with a special class that implements
field maps in the background. This class is called LabeledInstance. You must
use this class (or a subclass) to program with objects in NModel. Here is an
example:
namespace Payroll2
{
class Employee : LabeledInstance
{
int salary;
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