EECS2030Z Test 3

Fall 2017-18 students can ignore this question.

Consider the following three lines of Java code:

PhoneNumber n1 = PhoneNumber.getInstance(416, 967, 1111);
PhoneNumber n2 = PhoneNumber.getInstance(416, 967, 1111);
System.out.println(n1 == n2);

The Java code shown above prints true because PhoneNumber is a particular kind of class; what name is given to such classes?

multiton

Suppose that a client writes the following method:

public static void sort(List<Contact> contacts) {
    Collections.sort(contacts);
}

What method from the Contact class lets the client sort a list of contacts using Collections.sort?

compareTo(Contact) or just compareTo

The API for Contact says that every contact has a name (of type String) and a phone number (of type PhoneNumber). Would you implement Contact as an aggregation or a composition of a PhoneNumber? Explain your answer.

Aggregation. The contact can freely share its PhoneNumber instance because PhoneNumber is immutable.

4 marks for stating aggregation is safe because of immutability of PhoneNumber.
Up to 2 marks for stating composition should be used to preserve the state of the PhoneNumber.
0 marks for any answer that has no explanation.

Consider the constructor ContactSet(ContactSet other):

(Part 1): Based on the API of the constructor, does the constructor make a deep copy or a shallow copy of the set belonging to other?

(Part 2): Your answer here depends on your answer for (Part 1); see the two choices below:

If you answered deep copy in (Part 1) then show how to make a shallow copy of the set belonging to other.

If you answered shallow copy in (Part 1) then show how to make a deep copy of the set belonging to other.

(Part 1):
Deep copy.

// shallow copy
this.contacts = new HashSet<>(other.contacts);

or

// also a shallow copy
this.contacts = new HashSet<>();
for (Contact c : other.contacts) {
  this.contacts.add(c);
}

// deep copy
this.contacts = new HashSet<>();
for (Contact c : other.contacts) {
  this.contacts.add(new Contact(c));
}

2 marks for Part 1 (1 mark if the answer is shallow copy)
3 marks for Part 2 (deduct 2 marks if the shallow copy is actually a deep copy, or if the deep copy is actually a shallow copy).

(Part 1): What class invariant does ContactSet have regarding the contacts in its collection?

(Part 2): Based on the API for ContactSet which method allows a client to break the class invariant of ContactSet? Explain how a client can break the class invariant of ContactSet after using the method (showing actual Java code that does so is acceptable but not required).

(Part 1): A contact set guarantees that every contact is unique (i.e., no two contacts are equal)

(Part 2): None of the methods can be used to break the invariant.

2 marks for Part 1.
3 marks for Part 2.