EECS2030Z Test 3

Suppose a client writes a main method that includes the following line of Java code:

CellPhone phone = new CellPhone(new CellNumber(416, 736, 2100));

How many CellNumber objects are created by the client's code? Explain how you determined the number of CellNumber objects.

There are 2 CellNumber objects. One is created by the client, and the other is created by the CellPhone constructor (because a CellPhone and its CellNumber form a composition).

1 mark for indicating there are 2 objects.
1 mark for noting the CellNumber created by the client.
1 mark for noting the CellNumber created by the CellPhone constructor.

Fall 2017-18 students can ignore this question.

What is the definition of a singleton class?

A singleton class is a class that ensures there is 0 or 1 instances of the class; ok if the answer says there is exactly 1 instance.

3 marks for definition.

The API for the CellPhone method named getCallLog says that the method returns a shallow copy of the list of called cell numbers. What code would you write if the API had said that method returned a deep copy of the list of called numbers?

List<CellNumber> result = new ArrayList<>();
for (CellNumber n : this.callLog) {
  result.add(new CellNumber(n));
}
return result;

2 marks for the new list.
2 marks for adding new CellNumber instances to the list.

(Part 1): Besides the fact that the call log is never equal to null, what class invariant does CellPhone have regarding its call log?

(Part 2): Explain why returning a shallow copy in getCallLog is sufficient to ensure that the invariant remains true (in other words, why is a deep copy of the call log not required to maintain the invariant?)

(Part 1): The order of contacts in the call log are from the oldest call to the most recent call.

(Part 2): Modifying the returned list does not affect the CellPhone because the returned list is a new list. It is impossible to modify the CellNumbers in the list because CellNumber is immutable. Because the CellPhone call log cannot be modified, and the numbers in the log cannot be modified a shallow copy is sufficient to preserve the invariant.

2 marks for Part 1.
3 marks for Part 2 (1 mark for stating that the returned list is a new list, and 2 marks for indicating that CellNumber is immutable).

(Part 1): What two conditions are required to be true for compareTo to be consistent with equals?

(Part 2): Explain why the CellNumber implementation of compareTo is consistent with equals. One sentence should be sufficient for your answer.

(Part 1):
if x.compareTo(y) == 0 then x.equals(y) == true
if x.equals(y) == true then x.compareTo(y) == 0

(Part 2): Both compareTo and equals use the same fields to perform the comparison.

2 marks for Part 1 (1 mark for each condition).
3 marks for Part 2.