CS1410 Java Program Development
1. (Compulsory Question)
Read the following partial problem specification.
The task is to develop a simulation of an air traffic control system. There are several
different types of aircraft, so an abstract base class has been created. Each aircraft
has an integ...
AN
1. (Compulsory Question)
CS1410 Java Program Development
Read the following partial problem specification.
The task is to develop a simulation of an air traffic control system. There are several
different types of aircraft, so an abstract base class has been created. Each aircraft
has an integer code, initialised in the constructor. It is also possible to find out the
remaining flying time (or CAPACITY), measured in minutes, before the aircraft
must land.
The following block of code contains a skeleton implementation of the Aircraft
class.
Beginning of code
SW
1 public abstract class Aircraft {
2 protected int code;
3
8 public abstract int getFlyingCapacity();
9 }
End of code
a) Aircraft is an ‘abstract class’. What are the implications of this?
(4 marks)
Model Answer:
• It is impossible to instantiate Aircraft objects.
• Any subclass of Aircraft can only be concrete if all abstract methods are defined.
(2 marks) for each point.
ER
b) Define the difference between public and protected access. Explain why it is
appropriate to make the code field protected.
(6 marks)
Model Answer:
public members can be accessed by any other class (2 marks) ; protected members can only
be accessed by the same class, its subclasses (and other classes in the same package) (2 marks) . It is
appropriate to make code protected since subclasses may need direct access to it, and it shouldn’t be
public as that would break encapsulation (2 marks) .
c) Plane is a concrete subclass of Aircraft. In the constructor for Plane, as well as
the code, a field for the flying capacity (in minutes) corresponding to the initial fuel load
should be initialised. This field value should be returned by the getFlyingCapacity
S
method. Write down a definition of this class.
(8 marks)
Model Answer:
1 OF 11
Downloaded by Austine Wanjala (wanjala883@gmail.com)
, lOMoARcPSD|47166518
AN 1
CS1410 Java Program Development
Beginning of code
public class Plane extends Aircraft {
2 private int flyingCapacity;
3
4 public Plane(int code, int flyingCapacity) {
5 super(code);
6 this.flyingCapacity = flyingCapacity;
7 }
8
9 public int getFlyingCapacity() {
10 return flyingCapacity;
11 }
12 }
End of code
SW
(2 marks) for the class declaration, (1 mark) for the field declaration, (1 mark) for the
constructor declaration, (2 marks) for the call to super, (1 mark) for initialising the field, and
(1 mark) for the body of the method getFlyingCapacity.
d) Consider the following code fragment (arguments omitted on line 1, but you may as-
sume that they are correct).
Beginning of code
1 Plane p = new Plane(....);
2 Aircraft a = p;
3 System.out.println(a.getFlyingCapacity());
End of code
i) Define the ‘static type’ of a variable. In the above code fragment, what are the static
types of p and a?
(5 marks)
Model Answer:
ER
The static type of a variable is its declared type and is determined at compile time (3 marks) . p has
static type Plane and a has static type Aircraft; (1 mark) each.
ii) Define the ‘dynamic type’ of a variable. In the above code fragment, what are the
dynamic types of p and a?
(5 marks)
Model Answer:
The dynamic type of a variable is the type of the object to which it refers; this can change during the
execution of a programme (3 marks) . p has dynamic type Plane and a has dynamic type Plane;
(1 mark) each.
iii) Define the term ‘method overriding’. Explain, with the aid of a diagram, the se-
quence of events in the JVM when the method call a.getFlyingCapacity()
occurs on line 3.
(7 marks)
S
Model Answer:
2 OF 11
Downloaded by Austine Wanjala (wanjala883@gmail.com)
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