Ddoocp Ms March2012 Final

DESIGNING AND DEVELOPING

OBJECT-ORIENTED COMPUTER PROGRAMS

11TH MARCH 2012

MARKING SCHEME

This Marking Scheme has been prepared as a guide only to markers. This is not a set of model answers, nor is the Marking Scheme exclusive, for there will frequently be alternative responses which will provide a valid answer. Unless a question specifies that an answer be produced in a particular form, then an answer that is correct, factually or in practical terms, must be given the available marks. If there is doubt as to the correctness of an answer the relevant NCC Education materials and associated module textbook should be the first authority.

Throughout the question, please credit any valid alternative point.

Notice to Markers

Where markers award half marks in any part of a question they should ensure that the total mark recorded for a task is rounded up to a whole mark.

QUESTIONS CONTINUE ON NEXT PAGE Page 2 of 11

Designing and Developing Object-Oriented Computer Programs March 2012 Formatted/HW © NCC Education Ltd 2012

ANSWER ALL QUESTIONS QUESTION 1

Marks

a) Explain what purpose the main method in a Java program serves, and provide the code for a main method that creates an instance of the class in which it is contained.

6

The main method is the starting point of a java program (1 mark) and is the method that java will look for when a program is executed (1 mark). 4 marks for the example which should include the correct method signature of the main method (2 marks) and the correct code for creating an instance of its container class (2 marks)

b) Explain what is meant by ‘primitive’ data type and ‘reference’ data type, and give an example of each.

4

Primitive data types are the building blocks of data representation and are provided by the language (1 mark). Reference data types are instances of objects, although some of them are also handled by java keywords (1 mark). An int is a primitive data type (1 mark), while a String is a reference data type (1 mark).

Total 10 Marks



Marks QUESTION 2

a) Define the words ‘Class’ and ‘Object’, and explain the relationship between them. 4 A class defines the methods and variables for a particular class of object (1 mark) and an

object defines the state of those variables (1 mark). An object is an instantiation of a class (1 mark), and the class acts as the blueprint for the object (1 mark).

b) Explain what is meant by an event, and outline what is required for a Java program to make use of them.

6

An event is a non-typical occurrence in a program’s lifecycle (1 mark). When the user interacts in some way with a Java program, an event will be generated (1 mark), and information about that event will be sent to all interested listener objects (1 mark). In order for a java program to use the event, it must first implement the required listener methods (1 mark), register itself as a listener for that kind of event (1 mark), and then implement the handling code (1 mark).

Total 10 Marks



QUESTION 3

Marks

a) Define the term ‘encapsulation’ and give an example of its use. 5

Encapsulation is a feature of data-centric languages, and involves the packaging of data (1 mark) along with the methods that act on that data (1 mark). Encapsulation also often implies a black box design of methods (1 mark). 2 marks for a suitable example.

b) Define the term ‘inheritance’ in the context of object oriented programming and give an example where it could profitably be used.

5

Inheritance is the programming process whereby one object gains a copy of another’s methods and variables (2 marks) so as to ensure reusability (1 mark). 2 marks for a suitable example.

Total 10 Marks



QUESTION 4

Marks

a) Define what is meant by a strongly typed language and explain what benefits strong typing provides for software developers.

4

Strongly typed languages limit how variables of different data types can be intermixed (1 mark), ensuring that invalid operations are caught as early as possible (1 mark). Benefits include finding errors earlier in the development process (1 mark), and the ability of the compiler to generate meaningful errors (1 mark).

b) Assume you are given an array of the numbers 30, 50, 20 and 10. Outline the process by which you could generate a bar chart from this data.

6

Assuming a fixed origin of 0,0 (the top left), we would get the size of the user’s screen and find where the bottom left was located relative to the origin (1 mark). We would then define the width and height of our bars based on a proportion of the user’s screen (1 mark), and draw the bars of equal width but of a height indicated by the numbers we have (1 mark). For each bar, we move to the right a number of pixels equal to the width of the bar we decided upon (1 mark). Drawing the bars themselves is handled by fillRect (1 mark), and the labels underneath or over the bars would be handled by drawString (1 mark).

Total 10 Marks



MarksQUESTION 5 a) Give TWO (2) examples of Swing components, and explain for what purposes they might be

used. 4

Any swing components are valid here, such as JTextArea and JScrollBar. One mark for each component, and one mark for the explanation of the components purpose.

b) Explain the role of a layout manager in setting up a user interface in Java, and why they are preferable to manual positioning.

6

Layout managers handle the automatic laying out of GUI components (1 mark) according to some rigorous regime (1 mark). Layout managers can be set on a frame or panel (1 mark), and combinations of panels and layout managers permit for sophisticated layouts (1 mark). Layout managers are more sophisticated than manually positioning (1 mark) because they preserve relationships, proportionality and sizing across applications when their dimensions are manipulated (1 mark).

Total 10 MarksJ



MarksQUESTION 6 a) Explain the purpose of UML, and what benefits accrue from adopting it in software engineering. 4 UML is a diagramming notation (1 mark) and is used to communicate between involved

parties in development (1 mark). Benefits include front-loading design issues (1 mark) and the ease with which designs can be translated into code (1 mark).

b) Define two kinds of UML diagram, and explain what role they fulfil. Provide examples of the diagrammatic notation of each.

6

Any two diagrams would be suitable for this – 1 mark for identifying the diagrams, 1 mark for the roles, and 1 mark for drawing the diagram.

Total 10 Marks



MarksQUESTION 7 a) Explain what is meant by ‘defensive programming’. 4 Defensive programming is the tactic of writing code to reduce future, unanticipated problems

(1 mark). It can be done by reducing code complexity (1 mark), ensuring data consistency throughout all operations in a program (1 mark), and treating all input from the user as potentially tainted and thus in need of validation (1 mark).

b) Explain what is meant by an exception. 2 An exception indicates that an exceptional event has occurred during the running of a java

program (1 mark), and indicates that an error has been encountered that could not be foreseen at compile-time (1 mark).

c) Provide a code example of a try-catch-finally block, explaining the responsibilities of each part of the structure.

4

try { // Some expression } Catch (SomeException) { } Finally { } 1 mark for the code, and up to 3 marks for explaining what each part of the structure is supposed to do.

Total 10 Marks



MarksQUESTION 8 a) Provide code examples of the getter and setter for an integer variable called ‘value’, and explain

why getters and setters are used in OO programming.6

2 marks each for the getter and the setter. Justifications should include the need for encapsulation (1 mark) and the need for controls to be placed around the manipulation of data elements so as to ensure consistency (1 mark).

b) Explain what is meant by the term ‘visibility’ and give TWO (2) examples of visibility modifiers in Java.

4

Visibility refers to how generous a program is with when a variable or method can be invoked (1 mark). The higher the visibility, the more objects can make use of the internal code of other objects (1 mark). Public is the highest visibility (1 mark), and private the lowest (1 mark).

Total 10 Marks



MarksQUESTION 9 a) Explain what is meant by a two dimensional array, and give an example of where one might be

appropriate. 4

A two dimensional array is one that has both length and height and each compartment is indicated by a pair of integers (1 mark). It is best used for storing grids of data as opposed to tables of data (1 mark). 2 marks for a suitable example.

b) One of the limitations of a standard array is that its length is fixed. State a data structure that can be used to give the benefits of an array without this limitation, and provide a code example in which an array and your suggested data structure perform the same duty.

6

An ArrayList would be an appropriate data structure (1 mark) as it is functionally identical to an array except that it dynamically resizes (1 mark). For the example, 2 marks are available for showing a scenario using an array, and another 2 for showing the same scenario using the student’s suggested data type.

Total 10 Marks

END OF EXAM Page 11 of 11


MarksQUESTION 10 a) Explain what is meant by a ‘stream’ in terms of input/output, and give an example where you

have previously used streams in the module. 4

A stream is a form of I/O in which all data is received in sequence (1 mark) and in one direction (1 mark). 2 marks for a suitable example.

b) Explain the phrase ‘buffering’, and why we should ensure we use BufferedReaders and BufferedWriters when constructing our file access routines.

6

Buffering refers to the process of waiting until we have enough read or write operations pending to justify the overhead of file I/O (2 marks). We should use BufferedReaders and BufferedWriters because file I/O is usually the performance bottleneck in a program (2 mark) and by reducing the overhead required to make a file operation we can have much more efficient computer programs (2 marks).

Total 10 Marks

Documents

Ddoocp Ms March2012 Final