Domain-Driven Design: Tackling Complexity in the Heart of Software

Figure 10.15. The 
SPECIFICATION
 for a gasoline container has been
tightened.
In the language of 
SPECIFICATION
, we would say that the new 
SPECIFICATION
subsumes
the old
SPECIFICATION
, because any candidate that would satisfy the new SPEC would also satisfy the old.
If each of these 
SPECIFICATIONS
is viewed as a predicate, subsumption is equivalent to logical
implication. Using conventional notation, 
A
B
means that statement 
A
implies statement 
B
, so
that if 
A 
is true, 
B
is also true.
Let's apply this logic to our container-matching needs. When a SPECIFICATION is being changed,
we would like to know if the proposed new SPEC meets all the conditions of the old one.
New Spec 
Old Spec
That is, if the new spec is true, then the old spec is also true. Proving a logical implication in a
general way is very difficult, but special cases can be easy. For example, particular parameterized
SPECS
can define their own subsumption rule.
public class MinimumAgeSpecification {
int threshold;
public boolean isSatisfiedBy(Person candidate) {
return candidate.getAge() >= threshold;
}
public boolean subsumes(MinimumAgeSpecification other) {
return threshold >= other.getThreshold();
}
}
A JUnit test might contain this:

---

drivingAge = new MinimumAgeSpecification(16);
votingAge = new MinimumAgeSpecification(18);
assertTrue(votingAge.subsumes(drivingAge));
Another practical special case, one suited to address the 
Container Specification
problem, is a
SPECIFICATION
interface combining subsumption with the single logical operator AND.
public interface Specification {
boolean isSatisfiedBy(Object candidate);
Specification and(Specification other);
boolean subsumes(Specification other);
}
Proving implication with only the AND operator is simple:
A
AND 
B
A
or, in a more complicated case:
A
AND 
B
AND C 
A
AND 
B
So if the