Question at position 48 The subtype discriminator is a composite attribute when there is a(n):disjoint rule.partial specialization.overlap rule.full specialization.单项选择题
A
disjoint rule.
B
partial specialization.
C
overlap rule.
D
full specialization.

Question

Question at position 48 The subtype discriminator is a composite attribute when there is a(n):disjoint rule.partial specialization.overlap rule.full specialization.

BlackTom AI · Accepted Answer

A subtype discriminator is a composite attribute when there is an overlap rule.

1. Relevant concepts
- Subtype discriminator: an attribute in the superclass that identifies which subtype(s) an instance belongs to.
- Composite attribute (in this context): an attribute made up of multiple components (e.g., multiple Boolean flags or separate indicators) so it can represent more than one subtype membership.
- Overlap vs. Disjoint rules: overlap allows an entity instance to belong to multiple subtypes simultaneously; disjoint requires membership in at most one subtype.
- Partial vs. Full specialization: completeness constraints indicating whether every superclass instance must belong to some subtype (full) or not (partial); these do not determine whether the discriminator must be composite.

2. Step-by-step reasoning
- If the rule is disjoint, each instance can belong to only one subtype. A single-valued discriminator (e.g., a type code with values A, B, C) suffices.
- If the rule is overlap, an instance may belong to multiple subtypes at once. A single-valued attribute cannot represent multiple simultaneous memberships.
- Therefore, for overlap you need multiple components (for example, isTypeA, isTypeB, isTypeC) or a composite structure that can record multiple subtype memberships simultaneously.

3. Example (intermediate steps)
- Superclass: Person; subtypes: Student, Employee, Athlete.
- Overlap allowed: Alice can be Student and Athlete.
- Simple discriminator (single code) cannot represent {Student, Athlete} simultaneously.
- Composite discriminator: (isStudent = true, isEmployee = false, isAthlete = true) correctly represents Alice.

4. Why the correct answer is correct
- Overlap requires representing multiple true/false memberships, which necessitates a composite discriminator composed of multiple components.

5. Why other options are incorrect
- Disjoint rule: allows only one subtype membership, so a simple single-valued discriminator suffices.
- Partial specialization: concerns completeness (whether all instances must be in subtypes); it does not force a composite discriminator.
- Full specialization: same as partial but the opposite completeness constraint; it also does not require a composite discriminator.

Conclusion: The correct choice is "overlap rule," which matches the provided answer.

Question at position 48 The subtype discriminator is a composite attribute when there is a(n):disjoint rule.partial specialization.overlap rule.full specialization.单项选择题

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