1. Answer the following questions about Java language concepts: (20 marks total) (a) Java is a statically typed language. Define what is meant by statically typed, and de￾scribe the difference between implicit type conversion and explicit type conversion, giving an example of each. [5] (b) How do constructors differ from normal methods when it comes to inheritance? How is the superclass constructor invoked from a subclass? What happens automatically if you do not call the superclass constructor, and what problems can this cause? [4] (c) Name and describe two classes or interfaces involved in writing multi-threaded code in Java. [4] (d) Describe the steps involved in creating GUI elements in Swing and defining the be￾haviour associated with them. [3] (e) What is the main distinguishing feature of an immutable class in Java? Give an exam￾ple of a commonly used immutable class. List two advantages of using an immutable class. [4] 1 CONTINUED OVERLEAF 2. This question concerns the Java programming language. (15 marks total) (a) This question involves information about phone calls made from a particular mobile phone over one calendar month. Details of calls are stored in a list, in which each element contains the number called and the duration of the call in minutes, represented as an instance of the Call class below: public class Call { public String number; public int duration; } An example of a call list would be as follows: [ [“01234567890”, 5], [“09876543201”, 17], [“01234567890”, 14], . . . ] The following function is supposed to calculate the phone bill for the month, based on the following charging scheme. • Each call is charged at a rate of 2p per minute. • The favourite phone number has a maximum total charge of 100p; in other words, after 50 minutes of calls have been made to the favourite number, subsequent minutes are free. The parameter calls is a list of Call objects in the format described above, and the parameter favourite is a string representing the favourite phone number. The code below contains two errors. Give a clear explanation of each error (including how it would affect the execution of the function) and say how to correct it. You may illustrate your answer with fragments of Java code, but this is not required. [6] 1 public int calculateBill (List calls, String favourite) { 2 Map totals = new HashMap<>(); 3 for (Call call : calls) { 4 totals.put(call.number, totals.get(call.number) + call.duration); 5 } 67 int totalCharge = 0; 8 for (String number : totals.keySet()) { 9 if (number.equals(favourite)) { 10 totalCharge += 100; 11 } else { 12 totalCharge += totals.get(number) * 2; 13 } 14 } 15 16 return totalCharge; 17 } 2 CONTINUED OVERLEAF (b) For each of the following Java code fragments, indicate exactly what will happen when it is executed. If it produces output, show the exact output; if it runs but produces an error, specify the error precisely; if it will not compile, describe what the problem(s) are. Assume that all necessary classes have been imported. (i) double d = 5; int i = 2; System.out.println(d / i); [1] (ii) String s1 = "hello"; String s2 = new String("hello"); System.out.println(s1 == s2); System.out.println(s1.equals(s2)); [1] (iii) List days = Arrays.asList ("MONDAY", "TUESDAY", "WEDNESDAY", "THURSDAY", "FRIDAY", "SATURDAY", "SUNDAY"); System.out.println(days.stream() .filter(s -> s.contains("T")) .count()); [1] (iv) for (int i = 0; i < 10; i++) { System.out.println(i--); } [1] (v) Scanner s = new Scanner(System.in); while (true) { System.out.print("> "); try { int i = s.nextInt(); System.out.println("You entered: " + i); break; } catch (Exception e) { s.next(); } } [1] 3 CONTINUED OVERLEAF (vi) ArrayList l = new List<>(); l.add("one"); l.add("two"); Collections.reverse(l); System.out.println(l); [1] (vii) // File A.java public abstract final class A { public String toString() { return "Hello"; } } // main method System.out.println(new A()); [1] (viii) // File B.java public class B { public static int i; public int j; public B(int k) { i = k; j = k; } } // main method B b1 = new B(4); B b2 = new B(-3); System.out.println(b1.i + " " + b2.i + " " + b1.j + " " + b2.j); [1] (ix) // File C.java public class C { public String s; public C(String s) { s = s; } } // main method C c = new C("five"); System.out.println(c.s); [1] 4 CONTINUED OVERLEAF 3. This question concerns Java class design. (15 marks total) First, read the following description of a bicycle sharing system. You are to design a set of classes to model a simplified bike-share system similar to Glasgow’s NextBike system or to the Santander Cycles in London. Each bicycle has the following properties: an identifier (a positive integer), a bicycle type (a string), as well as a Boolean flag indicating whether the bicycle is available for rental. A customer can rent only one bicycle at a time: when a bicycle is returned, the total cost of that rental is computed by multiplying the rental time by the base rental rate, which is currently £2 per hour. If a customer attempts to rent a second bicycle when they already have one rented, or to rent a bicycle that is already rented to another customer, an error is returned and the bicycle is not rented. (a) Write a full class definition for Bicycle following the specification above. Be sure to use appropriate data types and access modifiers. Include a constructor that initialises all fields to appropriate values. The initial value for available should be true. Also implement a getter for all fields, and a setter method for the available flag. [5] (b) Write a class definition for the Customer class. Your class definition should include implementations of the following two public methods: • void rentBike(Bicycle bike) – rents the given bike to the customer, or else throws an IllegalArgumentException if one of the above error conditions is en￾countered (i.e., customer already has a bike rental, or specified bike is unavailable). • double endRental() – ends the rental of the current bike and returns the total cost of the rental. If the customer is not currently renting a bike, this method should instead throw an IllegalArgumentException. As part of your answer, you may want to make use of the java.time.Instant class, which represents a single instantaneous point in time. You can obtain an Instant object corresponding to the current time as follows: Instant currentTime = Instant.now(); You can also compute the difference in hours between two Instant objects as follows (NB: this is a slight simplification to the real behavour of the Instant class): long difference = Duration.between(instant1, instant2).toHours(); Your Customer class only needs to include fields that are required to support the above behaviour, and only the above methods are required. There is no need to add extra fields or to write constructors, getters, or setters unless they are required as part of your implementation. [7] 5 CONTINUED OVERLEAF (c) The bike provider now wants to allow customers to rent more than one bike at a time. How would you modify your Customer class to meet this requirement? You may illustrate your answer with fragments of Java code, but it is not required. [3]