PAPER TITLE: OBJECT-ORIENTED JAVA

DATE: Friday, 8 June 2018

Question 1 (40 marks)

(a) (i) Write a class called RoomCode with the following features:

• a private instance variable of type String called code, which will be used to reference a room code as outlined in the scenario above
• a public zero-argument constructor which initializes code to an empty string
• a public getter method getCode() that returns the current value of code (4 marks)

ANSWERS(Purchase full paper to get all the solution)

    //(i)
    //a private instance variable of type String called code
    private String code;

    //a public zero-argument constructor which initializes code to an empty string 
    RoomCode(){
        this.code = "";
    }

    //a public getter method getCode() that returns the current value of code 
    public String getCode(){
        return this.code;
    }

(ii) The class requires further public getter methods that will return the building letter, the floor and the room of the receiver object, based on the current value of code. You should assume that the code value will be valid in all cases.

• a method getBuilding() that returns the building letter part of the current room code as a char
• a method getFloor() that returns the floor number part of the current room code as a char
• a method getRoom() that returns the room part of the current room code as a String (6 marks)

(iii) Write a public instance method toString(), which returns a String representation of a RoomCode object in the format shown by the following example:

  Building J Floor 3 Room 1B 

(2 marks)

(b) (i)        Write a private helper method isValidCode(String s) for the class RoomCode. The method takes a single String argument representing a possible room code and returns true if the argument is not null, has four characters, and represents a valid room code, as described in the Scenario above. Otherwise the method returns false.

(Remember that characters can be compared using the > and < operators and this offers one way to carry out some of the checks that are needed.)  (8 marks)

(ii) Write a public method for the RoomCode class called setCode(String s), which takes a single String argument. The method determines if the argument represents a valid code. If so, the receiver’s code variable is set to the argument, otherwise no change is made. (2 marks)

(iii) Write a method isEqualCode(RoomCode r)for the RoomCode class, which takes a single argument of type RoomCode, and tests to see if the receiver and the argument have the same codes.

If both codes are null the method should return true.

If exactly one code is null the method should return false.

Otherwise the method should return true if the receiver’s code and the argument’s code are equal, and return false otherwise.  (5 marks)

(c) Based on the RoomCode class written so far, answer the following questions:

(i)             The RoomCode class has more attributes than it has instance variables. How many of each does it have? Explain your answer.  (2 marks)

(ii) How many reference type and primitive type instance variables does the RoomCode class have? Explain your answer. (1 mark)                

(iii) Write down any examples of overloading in the class RoomCode, or state ‘none’.  (1 mark)

(iv) Write down any examples of overriding in the class RoomCode, or state ‘none’.  (1 mark)       

(v) Explain how the class RoomCode illustrates the idea of encapsulation. (2 marks)

(d) A class called RoomFinder includes the following declarations:

 public class RoomFinder

 {

   private final int MAX_ROOMS = 105;     private RoomCode[] rooms;       }                

(i)  What is the nature of the object-oriented relationship between classes RoomCode and RoomFinder? Explain your answer. (2 marks)

(ii) Is the variable rooms a reference variable or a primitive variable? Explain your answer. (2 marks)

(iii) Suppose that a colleague of yours adds a method getRooms() in the RoomFinder class with the following header:

                                                      public RoomCode[] getRooms()

Write a short explanation to your colleague explaining why it may be undesirable to provide this method.  (2 marks)

Question 2 (30 marks)

(a) Suppose that Feedable is a Java interface that specifies a single  method with the signature feed(). This method takes no argument and returns a value of either true or false. Write down the Feedable interface. (2 marks)

(b) Suppose that HungryFrog is a subclass of the OU library class Frog and that HungryFrog implements the Feedable interface. Write down the header for the HungryFrog class.  (1 mark)

(c) Assume HungryFrog has an additional instance variable energyLevel of type int, and methods for getting and setting the value of this instance variable.       

Also, assume HungryFrog has a class constant FULL which has the value 2000.                

(i) When an instance of HungryFrog is created its inherited instance variables are to be initialised to the same values as for instances of Frog. The additional instance variable energyLevel is to be initialised to the class constant FULL.

                                                      Write a zero-argument constructor for HungryFrog. (2 marks)

(ii) Instances of HungryFrog expend energy when their position is changed, according to how far they move. The energy required is 100 units per stone moved.

For example, if a frog is on stone 1 and it moves to stone 5, it uses 4 x 100 = 400 units of energy.

However, if an instance of HungryFrog does not have enough energy to move to a specified stone it does not move. In this case, it remains on the stone it is currently on and the message "Insufficient energy to move" is printed to the Display Pane. 

This behaviour is to be achieved by overriding a single method in the Frog class. 

                                                      Write the overriding method for HungryFrog. (5 marks)

(iii) Write the feed() method for HungryFrog to the following specification:

The feed() method should first check that the hungry frog is not standing on a feeding stone, which for hungry frogs will be stone 1 and stone 10. 

If the hungry frog is not currently standing on a feeding stone, an attempt is made to move it directly to its nearest feeding stone using setPosition().

If the hungry frog is on a feeding stone, the feed() method should increase the hungry frog’s energy level to FULL by increments of 100. On each increment, the hungry frog should croak once.

Finally, if the hungry frog is on a feeding stone, the method should return true, otherwise false. (6 marks)

(d) Suppose that we want to keep a count of the number of HungryFrog objects that have been created. Explain using code fragments how we could achieve this. (3 marks)

(e) Briefly explain one advantage of using an interface type. (1 mark)

(f)  Suppose that a class called Snake, which is unrelated to HungryFrog, also implements the Feedable interface, and that a class called ZooKeeper has a constructor that takes a formal argument of type Feedable.

1. Briefly explain why the following lines of code are valid:

                                                      HungryFrog hf = new HungryFrog();

                                                      ZooKeeper zk1 = new ZooKeeper(hf);

                                                      Snake s = new Snake();

                                                      ZooKeeper zk2 = new ZooKeeper(s);

2. Explain, with reference to the ZooKeeper class, how the Feedable interface supports polymorphism. (4 marks)

(g) Write down three differences between abstract classes and interfaces. For each difference, you should state what is true of an abstract class, and what is true of an interface. (6 marks)

Question 3 (30 marks)