CS1410 Java Program Development

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 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 1 public abstract class Aircraft { 2 protected int code; 3 4 public Aircraft(int code) { 5 = code; 6 } 7 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. 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 method. Write down a definition of this class. (8 marks) Model Answer: 1 OF 11 Downloaded by Austine Wanjala () lOMoARcPSD| ANSWERS CS1410 Java Program Development Beginning of code 1 public class Plane extends Aircraft { 2 private int flyingCapacity; 3 4 public Plane(int code, int flyingCapacity) { 5 super(code); 6 gCapacity = flyingCapacity; 7 } 8 9 public int getFlyingCapacity() { 10 return flyingCapacity; 11 } 12 } End of code (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 Sln(FlyingCapacity()); 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: 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 FlyingCapacity() occurs on line 3. (7 marks) Model Answer: 2 OF 11 Downloaded by Austine Wanjala () lOMoARcPSD| ANSWERS CS1410 Java Program Development Method overriding occurs when a subclass and superclass both declare a method with the same signature. When this method is called on a variable whose static type is that of the superclass and dynamic type that of the subclass, then the method defined in the subclass is executed (4 marks) . a:Aircraft Plane Aircraft +getFlyingCapacity : int +getFlyingCapacity : int The diagram shows how this process works in this example: when FlyingCapacity is called, the inheritance tree is searched from the class with the correct dynamic type (Plane in this case) upwards (3 marks) . e) Suppose that it were decided to model gliders as a subclass of Aircraft. Gliders don’t use fuel, but can only fly during daylight. For this reason, the getFlyingCapacity method for Glider takes a parameter to represent the number of minutes until darkness: Beginning of code 1 public int getFlyingCapacity(int minsTillDark) End of code Briefly write down the changes that would be needed to Aircraft and Plane and explain why they are necessary. (You may assume in the rest of the exam paper that gliders are NOT being modelled, so that the definition of Aircraft is as printed above.) (5 marks) Model Answer: Both Aircraft and Plane must change the declaration of getFlyingCapacity to match that of Glider (3 marks) . This is required so that method overriding still works (2 marks) . 3 OF 11 Downloaded by Austine Wanjala ()