Description

INSTRUCTIONS TO CANDIDATES
1. This assessment paper contains TWENTY SEVEN (27) questions and comprises TWENTY (20) printed pages.
2. Write all your answers in the answer sheet provided.
3. The total mark for this assessment is 70.
4. This is an OPEN BOOK assessment. You are only allowed to refer to hardcopies materials.
5. All questions in this assessment paper uses Java 17.
6. Answer ALL questions.
7. Write your matric number below

Marks
Section Subtotal
Java Basic 6
Interface and Abstract Class 4
Inheritance, Polymorphism, and Dynamic Binding 8
Exceptions 4
Types 14
Generics 12
OOP Principles 12
Stack and Heap 10
Total 70
Good Luck!
1
Section I: Java Basic (6 Points)
1. Select ALL statements that are true about Java type systems.
(A) Java is statically typed
(B) Java is dynamically typed
(C) Java is strongly typed
(D) Java is weakly typed
1 points
Answer:
2. Select ALL constructs that are resolved via static binding instead of dynamic binding.
(A) Non-static method
(B) Static method
(C) Non-static field
(D) Static field
1 points
Answer:
3. Select ALL statements that are false about Java final keyword.
(A) final keyword can be used to prevent a method from being inherited
(B) final keyword can be used to prevent a method from being overloaded
(C) final keyword can be used to prevent a method from being overridden
(D) final keyword can be used to prevent a class from being inherited
1 points
Answer:
4. Consider the following classes
class A {
public void foo(A param) { .. }
}
class B {
public void foo(A param) { .. } public void foo(B param) { .. }
}
Select ALL concepts that are used in the code above.
(A) Method overriding
(B) Method overloading
(C) Inheritance (D) Polymorphism
1 points
Answer:
5. Select ALL statements that are true about complex types in Java.
(A) Java generic is covariant
(B) Java generic is contravariant
(C) Java generic is invariant (D) Java array is covariant
(E) Java array is contravariant
(F) Java array is invariant
1 points
Answer:
6. Select ALL the statements that are true regarding access modifier in Java.
(A) Fields/methods with private access modifier can be accessed by code inside the class
(B) Fields/methods with private access modifier can be accessed by code in the subclass
(C) Fields/methods with public access modifier can be accessed by code inside the class
(D) Fields/methods with public access modifier can be accessed by code in the subclass
1 points
Answer:
Section II: Interface and Abstract Class (4 points)
For this section, you are given the following interface
interface I1 { void f1(); void g();
}
interface I2 { void f2(); void g();
}
interface I3 extends I1, I2 { }
Additionally, you are given the following classes where the abstract modifier is omitted.
/* abstract modifier omitted */ class C1 implements I1 { public void g() { }
}
/* abstract modifier omitted */ class C2 implements I2 { public void f1() { } public void g() { }
}
/* abstract modifier omitted */ class C3 implements I3 { public void f2() { } public void g() { }
}
/* abstract modifier omitted */ class C4 extends C2 implements I1 { public void f2() { }
}
7. Select ALL classes that must have the abstract modifier in the class declaration.
(A) Class C1
(B) Class C2
(C) Class C3
(D) Class C4
2 points
Answer:
8. Assume that all abstract modifiers have been added correctly such that the code compiles. Select ALL valid subtype relationships.
(A) C1<: I2
(B) C2<: I1
(C) C3<: I1
2 points
(D) C4<: I2 Answer:
Section III: Inheritance, Polymorphism, and Dynamic Binding (8 points)
For this section, you are given the following classes where the @Override annotations have been omitted
class A {
public B foo(D arg) { .. }
}
class B extends A {
public C foo(D arg) { .. } public A foo(A arg) { .. }
}
class C extends B {
public B foo(A arg) { .. }
}
class D extends A {
public A foo(C arg) { .. }
}
Additionally, you are given the following variable declarations
A a = new C();
B b = new B();
C c = new C();
D d = new D();
9. Select ALL methods that can be annotated with @Override annotation.
(A) public B foo(D arg) in class A (B) public C foo(D arg) in class B (D) public B foo(A arg) in class C (E) public A foo(C arg) in class D
(C) public A foo(A arg) in class B
(F) none, the @Override annotation cannot be added to any methods
2 points
Answer:
10. Consider the following code fragment
a.foo(b);
Which of the following foo method is invoked by the code fragment above?
(A) public B foo(D arg) in class A (B) public C foo(D arg) in class B (D) public B foo(A arg) in class C (E) public A foo(C arg) in class D
(C) public A foo(A arg) in class B
(F) none, it cannot compile
2 points
Answer:
11. Consider the following code fragment
c.foo(b);
Which of the following foo method is invoked by the code fragment above?
(A) public B foo(D arg) in class A (B) public C foo(D arg) in class B (D) public B foo(A arg) in class C (E) public A foo(C arg) in class D
(C) public A foo(A arg) in class B
(F) none, it cannot compile
2 points
Answer:
12. Consider the following code fragment
d.foo(b);
Which of the following foo method is invoked by the code fragment above?
(A) public B foo(D arg) in class A (B) public C foo(D arg) in class B (D) public B foo(A arg) in class C (E) public A foo(C arg) in class D
(C) public A foo(A arg) in class B
(F) none, it cannot compile
2 points
Answer:
Section IV: Exceptions (4 points)
For this section, you are given the following exceptions
class ExcA extends RuntimeException {} class ExcB extends ExcA {} class ExcC extends ExcB {}
Additionally, you are given the following code fragment
public static void f() {
try { // Line A
g();
// Line B
} catch(ExcC e) { // Line C
} catch(ExcA e) {
// Line D
} finally {
// Line E
}
}
public static void g() { h();
// Line F
}
public static void h() {
// Line G throw new ExcB();
// Line H
}
13. Consider the following code fragment
f();
Select ALL the lines that will be executed.
(A) Line A
(B) Line B
(C) Line C (D) Line D
(E) Line E
(F) Line F (G) Line G
(H) Line H
2 points
Answer:
14. If we change the declaration of ExcA to extend Exception instead of RuntimeException , the following code fragment will not compile
public static void foo() {
try {
bar();
} catch(ExcA e) { // do nothing
} catch(ExcC e) {
// do nothing
}
}
public static void bar() { throw new ExcB();
}
Explain in no more than three sentences ALL the changes that you need to make to the code fragment above so that the code fragment will compile when we changed ExcA to extend Exception instead of RuntimeException .
Answer: 2 points
Section V: Types (14 points)
In some questions in this section, you are given a set of subtyping relationships and you need to come up with a possible object-oriented design for each of the set such that:
• All the explicitly stated subtyping relationship are satisfied.
• All the implicitly (i.e., derived) stated subtyping relationship are satisfied.
• No additional subtyping relationship not explicitly/implicitly stated.
• Only a minimal number of interface can be used.
However, there is a possibility that the set of subtyping relationships is invalid. An example of a valid set of subtyping relationships is shown below.
A1 <: A2
A2 <: A3
A3 <: A4
A possible implementation is as follows
interface A4 {} class A3 implements A4 {} class A2 extends A3 {} class A1 extends A2 {}
An example of an invalid set of subtyping relationships is shown below.
A1 <: A2
A2 <: A3
A3 <: A1
If the set is
• Valid: Answer true for the question and give a possible design. In your design, you must minimise the number of interfaces.
• Invalid: Answer false and briefly explain in one sentence why the set is impossible.
15. Is the following set of subtyping relationship valid? Give an example of a possible design for types A1, A2, A3, and A4 if the set is valid. Otherwise, briefly explain why the set is invalid.
A2 <: A1
A3 <: A1
A3 <: A2
A3 <: A4
No marks will be awarded if no design or explanation is given.
Answer:
True/False 3 points
16. Is the following set of subtyping relationship valid? Give an example of a possible design for types A1, A2, A3, and A4 if the set is valid. Otherwise, briefly explain why the set is invalid.
A1 <: A2
A2 <: A3
A4 <: A1
A3 <: A4
No marks will be awarded if no design or explanation is given.
Answer:
True/False 3 points
17. Is the following set of subtyping relationship valid? Give an example of a possible design for types A1, A2, A3, and A4 if the set is valid. Otherwise, briefly explain why the set is invalid.
A2 <: A1
A3 <: A4
A2 <: A3
A1 <: A4
No marks will be awarded if no design or explanation is given.
Answer:
True/False 3 points
18. Is the following set of subtyping relationship valid? Give an example of a possible design for types A1, A2, A3, and A4 if the set is valid. Otherwise, briefly explain why the set is invalid.
A2 <: A1
A4 <: A1
A4 <: A2
A3 <: A2
No marks will be awarded if no design or explanation is given.
Answer:
True/False 3 points
19. Consider the following code fragment
Integer i = Integer.valueOf(3); X x = i;
Select ALL valid options for the type X ?
(A) Object
(B) int
(C) double
2 points
(D) String Answer:
Section VI: Generics (12 points)
20. Consider the following generic class
Integer i = Integer.valueOf(3); class Tesla<T, S extends Car<T>> {
T obj1;
S obj2;
}
What will be the type of obj1 after type erasure?
(A) Object
(B) Car
(C) Car<T>
(D) Car<Object>
2 points
Answer:
21. Consider the following generic class
Integer i = Integer.valueOf(3); class Tesla<T, S extends Car<T>> {
T obj1;
S obj2;
}
What will be the type of obj2 after type erasure?
(A) Object
(B) Car
(C) Car<T>
(D) Car<Object>
2 points
Answer:
Explain your reasoning in at most four sentences.
class A<T, S> { S a; public T foo() { return null;
}
}
class B<T, S extends T> extends A<T, S> {
public S foo() { return a;
}
}
No marks will be awarded if no explanation is given.
Answer:
True/False 4 points
class X {} class Y extends X {}
class A<T> {
public T foo(T a, X b) { return null;
}
public T foo(T a, Y b) { return null;
}
}
No marks will be awarded if no explanation is given.
Answer:
True/False 4 points
Section VII: OOP Principles (12 points)
For this section, consider the following Java program
class Point { private double x; private double y; public Point(double x, double y) {
this.x = x; this.y = y;
}
@Override public boolean equals(Object obj) { if (this == obj) { return true; } if (obj instanceof Point) { Point p = (Point) obj; return this.x == p.x && this.y == p.y;
}
return false;
}
}
class Circle extends Point { private double r; public Circle(double x, double y, double r) { super(x, y); this.r = r;
}
@Override public boolean equals(Object obj) { if (this == obj) { return true; } if (obj instanceof Circle) { Circle c = (Circle) obj; return this.r == c.r && super.equals(c);
}
return false;
}
}
An important property of the equals method as written in the Java documentation is reproduced here:
The equals method implements an equivalence relation on non-null object references:
• It is reflexive: for any non-null reference value x, x.equals(x) should return true.
• It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
• It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
• It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
• For any non-null reference value x, x.equals(null) should return false.
24. The program above violates information hiding. True or False? Answer True if the program violates information hiding and answer False if the program does not violate information hiding.
Explain your reasoning in no more than two sentences with reference to the program above. No marks will be awarded if no explanation is given.
Answer:
True/False 4 points
25. The program above violates LSP. True or False? Answer True if the program violates LSP and answer False if the program does not violate LSP.
Explain your reasoning in no more than two sentences with reference to the program above. No marks will be awarded if no explanation is given.
Answer:
True/False 4 points
26. The program above violates the principle of “Tell, Don’t Ask”. True or False? Answer True if the program violates “Tell, Don’t Ask” and answer False if the program does not violate “Tell, Don’t Ask”.
Explain your reasoning in no more than two sentences with reference to the program above. No marks will be awarded if no explanation is given.
Answer:
True/False 4 points

Section VIII: Stack and Heap (10 points)
Consider the following complete program
class Interval { private int begin; private int end; public Interval(int begin, int end) { this.begin = begin; this.end = end;
}
public Interval union(Interval time) { return new Interval(this.begin, time.end);
}
}
class Event { private Interval time; public Event(Interval time) { this.time = time;
}
public void merge(Event event) {
Interval temp = this.time.union(event.time); this.time = temp; event.time = temp;
// Line A
}
}
public class StudentLife { public static void main(String[] args) {
Interval morning = new Interval(8, 12); Interval noon = new Interval(12, 15);
Event midterm = new Event(morning); Event lecture = new Event(noon); midterm.merge(lecture); }
}

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