Description

CS2030 Programming Methodology II

INSTRUCTIONS TO CANDIDATES
1. This assessment paper contains 13 questions and comprises 12 printed pages, including this page.
2. A 4-page answer sheet is also given. Write all your answers in the answer sheet. Submit your answersheet at the end of the assessment.
3. The total marks for this assessment is 50. Answer ALL questions.
4. This is an OPEN BOOK assessment.
5. All questions in this assessment paper use Java 8 unless specified otherwise.
6. State any additional assumption that you make.
Part I
Multiple Choice Questions (24 points)
• For each of the questions below, select the most appropriate answer and write your answer in the corresponding answer box on the answer sheet. Each question is worth 3 points.
• If multiple answers are equally appropriate, pick one and write the chosen answer in the answerbox. Do NOT write more than one answer in the answer box.
• If none of the answers are appropriate, write X in the answer box.
1. (3 points) Which of the following statements about inheritance in Java is FALSE?
A. We can use the extends keyword to specify inheritance
B. A class can extends from at most one other class
C. A class declared as final cannot be inherited
D. A method declared as final cannot be overridden
E. A field declared as final cannot be accessed by the subclass Write X in the answer box if none of the statements above is false.
2. (3 points) Recall that we can override the method equals in the class Point (as defined in CS2030) so that two Point objects are equal if they have the same x and y coordinates.
Which of the following property of equals, if specified, would cause Point to violate the LSP?
For two variables of type Object, o1 and o2, o1.equals(o2) is true if and only if
(i) o1 == o2
(ii) o2.equals(o1) is also true
(iii) o1.equals(o3) implies o3.equals(o2) for another variable o3.
A. (i) only B. (ii) only
C. (i) and (ii) only
D. (ii) and (iii) only
E. (i), (ii), and (iii)
Write X in the answer box if none of the combinations above is correct.
3. (3 points) Consider the definition of I, J, A, and B below. In order for B to be a concrete (non-abstract) class, what methods should B implements?
interface I {
void f(); }
interface J extends I {
void g(); }
abstract class A implements J {
public void g(int x) { return; }
abstract public void h();
}
class B extends A {
:
}
A. h only B. f and h only
C. f and g only
D. g and h only
E. f, g and h
Write X in the answer box if none of the combinations above is correct.
4. (3 points) Consider the definition of classes A, B, and C below.
class A {
void f(int x) {
System.out.println(“A”);
}
}
class B extends A {
void f(int x) {
System.out.println(“B”);
}
}
class C extends A {
void f(String x) {
System.out.println(“C”);
}
}
Which of the following declaration and initialization of variable x would cause x.f(1) to print the string “A”?
(i) A x = new B();
(ii) A x = new C();
(iii) B x = new B();
(iv) C x = new C();
A. (ii) only
B. (ii) and (iv) onlyC. (i) and (iii) only
D. (i), (ii), and (iii) only
E. (ii), (iii), and (iv) only
Write X in the answer box if none of the combinations above is correct.
5. (3 points) Note: Nested class is not within the scope of midterm after AY21/22.
Consider the following class Out which contains an inner class In and a local class Local
class Out { int x;
class In {
int y; }
void foo(int z) { x = 1; // (A) z = 1; // (B) class Local extends In {
void bar() {
int w; w = x; // (C) w = y; // (D) w = z; // (E)
}
}
}
}
Which of the following statement about the five statements labeled (A)-(E) above is FALSE:
A. Statement (A) causes a compilation error, as Java does not allow the value of x to be changed inside the method foo if x is captured by Local.
B. Statement (B) causes a compilation error, as Java does not allow the value of z to be changed inside the method foo if z is captured by Local.
C. Statement (C) compiles without error, as the method bar can access the field x.
D. Statement (D) compiles without error, as the method bar can access the field y
E. Statement (E) causes a compilation error, as Java does not allow variable capture of z, which is neither final or effectively final.
Write X in the answer box if none of the statements above is false.
6. (3 points) Suppose we have three types S, T, and U, with the following subtype relationship
U<: T <: S
Let A(X) be a complex type that depends on type X.
Which of the following statement is FALSE:
A. Assigning a variable of type U to a variable of type S is a form of widening type conversion.
B. Assigning a variable of type S to a variable of type T requires type casting in Java.
C. We can pass a variable of type S to a method expecting type T as an argument without type casting.
D. Passing a variable of type T to a method expecting an argument of type S will never raise a runtime ClassCastException.
E. If A(T) <: A(S), then we say that A is covariant Write X in the answer box if none of the statements above is false.
7. (3 points) Suppose we have a generic class with two type parameters:
class Pair<T, U> {
T first;
U second;
}
Which of the following code will lead to a compilation error?
(i) Pair<String, String> p = new Pair<>(); (ii) Pair<int, int> p = new Pair<>();
(iii) Pair<> p = new Pair<Object, Object>();
(iv) Pair<?, ?> p = new Pair<String, Object>();
A. (ii) only
B. (i) and (iv) onlyC. (ii) and (iii) only
D. (i), (iii), and (iv) only
E. (ii), (iii), and (iv) only
Write X in the answer box if none of the combinations above is correct.
8. (3 points) Consider the code below. InterruptedException is a subclass of Exception.
class Inception {
public static void main(String args[]) {
van();
}
static void van() {
try {
System.out.println(“van”); hotel();
} catch (Exception e) {
System.out.println(“exception (van)”);
} }
static void hotel() throws InterruptedException {
try {
System.out.println(“hotel”); snowFortress();
} catch (Exception e) {
System.out.println(“exception (hotel)”); } }
static void snowFortress() throws InterruptedException { System.out.println(“snow fortress”); limbo();
}
static void limbo() throws InterruptedException {
throw new InterruptedException();
}
}
Which of the following string will NOT be printed when we invoke the main class Inception?
A. van
B. hotel C. snow fortress
D. exception (van)
E. exception (hotel)
Write X in the answer box if every string above is printed.
Part II
Short Questions (24 points)
Answer all questions in the space provided on the answer sheet. Be succinct and write neatly.
9. (4 points) Modeling
Suppose you want to model the following scenario in an object-oriented program.
A module has multiple assessments. There are three types of assessments: lab assignment, test, and project, each to be graded differently.
(a) (3 points) List down the name of the five classes, and the relationship (either IS-A or HAS-A) between them.
(b) (1 point) Identify an opportunity to use polymorphism in the scenario above.Note: you do not have to write any Java code to answer this question.
10. (3 points) Hash Code.
Note: Hash code is not within the scope of midterm after AY21/22.
Recall that whenever we override the method equals() from the class Object, we must override the method hashCode() as well. It is required that two objects x and y satisfy the following property P:
if x.equals(y), then x.hashCode() == y.hashCode()
Someone presented to you the following implementation of hashCode() for the class A. The other parts of class A are omitted (including implementation of equals()).
class A {
: @Override int hashCode() {
return 8888;
}
}
(a) (1 point) Does the implementation of hashCode() above satisfy property P?
(b) (2 points) The implementation of hashCode() above is, however, considered a bad practice. Why?
11. (8 points) Method Overriding.
During the lectures, we have seen that, if we have two methods with the same method signature, one in the superclass and the other in a subclass, then the method in the subclass will override the method in the superclass. We, however, did not say much about the return type of the overridden and the overriding methods. We will explore more about that in this question.
Let’s construct a simple example. Suppose we have two classes, class A and class B inherits from A. Both classes A and B define a method A copy(), as seen below, that returns a copy of the object.
class A {
int x;
A(int x) { this.x = x;
}
public A copy() {
return new A(x);
}
}
class B extends A { int y;
B(int x, int y) { super(x); this.y = y;
}
@Override
public A copy() { // Line 22
return new B(x, y);
}
}
(a) (2 points) Why does the following code gives a compilation error? Fix the code below so that thecompilation error goes away.
B b1 = new B(1, 2); B b2 = b1.copy();
(b) (2 points) Which version of copy() will the line a1.copy() below invoke? The one in class A, or in B?
A a1 = new B(1, 2);
A a2 = a1.copy();
(c) (4 points) Suppose we change Line 22 above so that the return type of method copy() is B instead of A. Java compiler does not give any compilation error and allows copy() in class B to override copy() in class A. Explain why it is safe for Java to allow this.
12. (4 points) Type.
You are shown the implementation of a class with the following two methods.
void printPositiveBytesFromIntegers(List<Integer> list) {
for (Integer i : list) {
if (i.byteValue() > 0) {
System.out.println(i.byteValue());
}
} }
void printPositiveBytesFromLong(List<Long> list) {
for (Long i : list) {
if (i.byteValue() > 0) {
System.out.println(i.byteValue());
}
}
}
The methods go through, a list of Integer objects and a list of Long objects, round or truncate them to a value of type byte, and print out the value if it is positive. You are asked to copy-and-paste the methods given and change them to produce methods that perform the same action but on a list of other types. One for a list of Double objects, one for a list of Short objects, one for a list of Float objects, etc.
You recall the abstraction principle from CS2030, and you know that copying-and-pasting the code multiple times is not the best way to do this. You look up the Java API, and found that:
• Integer, Long, Double, Short, and Float are all subclasses of the abstract class Number.
• byteValue() is a non-abstract method defined in the class Number and it does exactly what the code above intended it to do.
With this information, and with what you learn about generic types, you are now ready to write only ONE method to replace the five methods that would have been produced if you naively replicate the methods, one for each type. Your method should be able to take in a list of type List<Integer>, List<Long>, List<Double>, List<Short>, or List<Float> as an argument. In fact, your method is so general that a list of any subtype of Number can be passed in as an argument.
Write this method in the space given on the answer sheet.
13. (5 points) Heap and Stack.
Consider the following definition of a Vector2D class:
class Vector2D {
private double x; private double y;
Vector2D(double x, double y) { this.x = x; this.y = y;
}
void add(Vector2D v) {
this.x += v.x; this.y += v.y;
// line A
}
}
class Main {
public static void main(String[] args) {
Vector2D v1 = new Vector2D(1, 1); Vector2D v2 = new Vector2D(2, 2); v1.add(v2);
}
}
We execute the Main class without any command line argument. Show the content on the stack and the heap when the execution reaches the line labeled A above. Label your variables and the values they hold clearly. You can use arrows to indicate object references. Draw boxes around the stack frames of the methods main and add and label them.
END OF PAPER
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