Computer Science 1 Bh

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1 UNIVERSITY OF EDINBURGH course CS0077 FACULTY OF SCIENCE AND ENGINEERING DIVISION OF INFORMATICS SCHOOL OF COMPUTER SCIENCE Computer Science 1 Bh Degree Examination Specimen Solutions Date: Saturday 26th May 2001 Time: 12:00 13:30 (one and a half hours) Place: Roxburgh Place Room: Halls A and B Board of examiners Chair: Stuart Anderson External Examiner: Muffy Calder Notes about Specimen Solutions 1. These specimen solutions are issued for guidance only, to help as a revision aid. They do not represent a complete picture of how the exam was marked; they are specimen solutions, not a marking guide. 2. The solutions typically present one answer from a set of possible answers. Often, answers which have alternative wording or different technical details are equally acceptable. 3. If you have any questions concerning these solutions, please contact the course organiser. Please note that the course organiser will not be able to answer questions about the marking of the exam.

2 Question 1 Suppose we have a program which implements the following class hierarchy fragment: ClassA / \ ClassB ClassC / \ ClassD ClassE Suppose further that we have five variables declared and assigned by this program: ClassA a = new ClassA(); ClassB b = new ClassB(); ClassC c = (classc) (new ClassE()); ClassD d = null; ClassE e = new ClassE(); Fill in the result of executing the following program fragments. If you think that the Java compiler will reject the fragment, write does not compile. Remember that the Java compiler knows the class hierarchy and will reject programs where the types are always incompatible, whatever the run-time value of variables. System.out.println(a instanceof ClassA); true System.out.println(a instanceof ClassB); false System.out.println(b instanceof ClassA); true System.out.println(b instanceof ClassC); does not compile System.out.println(c instanceof ClassA); true System.out.println(c instanceof ClassB); does not compile System.out.println(c instanceof ClassE); true System.out.println(d instanceof ClassD); false System.out.println(d instanceof Object); false System.out.println(e instanceof Object); true Page 1 of 13

3 Question 2 A library class ArrayBinaryTree has an array field a declared by: private Object a[]; This array is used to represent a binary search tree of objects. The public methods of ArrayBinaryTree ensure that the tree is always balanced, which means that it has the smallest depth possible for a given number of elements stored in the tree. The tree is arranged in the array like this: a[0] a[1] a[2] a[3] a[4] a[5] a[6] If the tree does not have a node at a particular position, then the corresponding element in the array is null. i. The diagram shows an array big enough to hold a tree with up to 7 elements. If the number of elements stored in the tree is n, what is the size of the array needed? [2 marks] 2 log n +1 1 ii. Show, as trees, the possible ways of storing the integers 1, 2, 3, 4, 5, 6 in an ArrayBinaryTree object. [3 marks] / \ / \ / \ / \ / / \ \ / \ / \ \ / \ / iii. Write a method findleast which returns the integer index of the least element in the tree, making use of the binary search tree property. You may assume that the tree always has at least one node. [5 marks] Here is one possible solution. The counter i counts powers of 2. Since the array has at least the first position non-null, the result is 0 or more. class ArrayBinaryTree { Object a[]; public int findleast() { int i = 1; while ((i-1) < a.length && a[i-1]!= null) { i *= 2; return i/2-1; Page 2 of 13

4 Question 3 (a) This question concerns interfaces in Java. Recall that the purpose of an interface is to describe some methods which are to be implemented by a class. i. All methods in an interface are abstract. What does this mean? An abstract method is one which has no body, in other words, no implementation. ii. Since all methods are abstract, what else should be provided to explain an interface? An essential part of an interface is some accompanying documentation or comments to describe the purpose of the methods. iii. Show how you would write the standard Comparable interface in Java, which declares the compareto method. This method takes an object parameter and returns an integer, defining an ordering relation on objects in a class. [3 marks] public interface Comparable { // Compares this object with the specified object for order. // Returns a negative integer, zero, or a positive integer // as this object is less than, equal to, or greater than // the specified object. public int compareto(object o); (b) This question concerns abstract classes in Java. i. What is the difference between an abstract class and an ordinary class? Non- An abstract class is a class which contains at least one abstract method. abstract classes must provide implementations of all their methods. ii. Show how you would write an abstract class AbstractReverseComparable which implements the Comparable interface and contains a concrete definition of a method reversecompareto method in terms of compareto, which reverses the sense of the compareto comparison. This means that if a.compareto(b) = 1, then a.reversecompareto(b) = 1, and similarly for 1 and 1. [4 marks] abstract class AbstractReverseComparable implements Comparable { abstract public int compareto(object o); public int reversecompareto(object o) { return -1 * compareto(o); Page 3 of 13

5 Question 4 (a) Client-server systems require a method of identification of particular services on particular hosts. What is the naming and numbering system which is used for this identification? How are connections between hosts implemented in the Java programming language? [5 marks] Hosts are identified using their numeric network address (in the case of an IP network this might be ) or its host name e.g. scar.dcs.ed.ac.uk. The textual form of the name will be translated to the network address by a domain name service (DNS). Additionally each service that the host provides is associated with a port number. Java programs can connect to particular services using Socket objects initialised with the host s name and port number. e.g. Socket sock = new Socket ("scar.dcs.ed.ac.uk", 5055); (b) A Java program in the client-server model passes in turn each element of an array of messages (msgs) from the client to the server. The server acknowledges all non-empty messages. As usual, the server s input stream (serverin) is the client s output stream (serverout) and the client s input stream (clientin) is the server s output stream (clientout). An extract of the code is shown below. On the Server: On the Client: while (line!= null) { for (int i = 0 ; i < msgs.length ; i++) { if (line.equals("")) { serverout.println(msgs[i]); // ignore empty lines serverout.flush(); else { System.out.println(clientIn.readLine()); System.out.println (line); clientout.println("acknowledged"); clientout.flush(); line = serverin.readline(); After the system has been in use for some time an error occurs. What do you think is the likely error? How are these sorts of errors usually dealt with in client-server systems? [5 marks] The client sends a message to the server and then waits for a reply. The server waits for a message and the then sends an acknowledgement. If either the client s message or the server s response was lost by the network both server and client would be stuck waiting indefinitely for a message from the other. This situation is called deadlock. The usual way of avoiding deadlock in client-server systems is to place a limit (called a timeout) on how long the program will wait before taking some kind of corrective action, such as resending its last message. Page 4 of 13

6 Question 5 (a) Provide an equivalent Java byte code version for the Java method below. [Hint: you should use the instructions iconst, ifge, iload and ireturn.] [3 marks] \textit{int minabs (\textit{int x) \ttlbrace\\ \textbf{if (x < 0)\\ \textbf{return 0;\\ \textbf{else\\ \textbf{return x;\\ \ttrbrace Method int minabs(int) 0 iload_1 1 ifge 4 2 iconst_0 3 ireturn 4 iload_1 5 ireturn (b) Consider a Java virtual machine which has the following values in memory and on the stack. 1 2 memory stack 2 Giving this starting state, show the intermediate states of the memory and the stack as each instruction is executed. [7 marks] 1 imul 2 istore 1 3 iload 2 4 iinc istore 2 6 iconst 1 7 istore 2 1. M: 5 4 S: 6 2. M: 6 4 S: 3. M: 6 4 S: 4 4. M: 6 5 S: 4 5. M: 6 4 S: 6. M: 6 4 S: 1 7. M: 6 1 S: Page 5 of 13

7 Question 6 (a) You are to implement the graphical interface of a Java program to play chess. Your interface is to draw a chessboard as shown below, made up of alternating black and white squares. The frame in which the chessboard is to be drawn is pixels, so each of the black and white squares above measures pixels. Recall that the Color class provides the constants Color.white and Color.black and a graphics context Graphics g provides the following methods: public void setcolor (Color c) public void fillrect (int x, int y, int width, int height) Using these ingredients, provide an implementation of the paint() method for a class which displays the chessboard. [6 marks] public void paint(graphics g) { for (int x = 0 ; x < 8 ; x++) { for (int y = 0 ; y < 8 ; y++) { if ((x + y) % 2 == 0) g.setcolor(color.white); else g.setcolor(color.black); g.fillrect(x * 100, y * 100, 100, 100); (b) The program needs to register mouse clicks in the frame in order to identify the squares on the chessboard by their row and column position. These positions are used to index into a two-dimensional array of ChessPiece objects and so their values are to range between 0 and 7. Implement the method public void mousepressed(mouseevent e) for the MouseAdapter class listening for these mouse clicks. Recall that the MouseEvent class provides methods getx() and gety(). Update the int variables row and column accordingly. [4 marks] public void mousepressed(mouseevent e) { row = e.gety() / 100; column = e.getx() / 100; System.out.println("row = " + row); System.out.println("column = " + column); Page 6 of 13

8 Question 7 (a) Briefly describe the four steps you would undertake in debugging a program. [4 marks] 1. Test the program to detect any incorrect behaviour. 2. Identify the cause of the error. 3. Redesign the program to remove the error. 4. Implement the new design. Repeat the above cycle until no more errors are detected, and after every change to the program. [ Question continues on next page ] Page 7 of 13

9 (b) The method given below is intended to find the number that is the minimum of all the numbers in the Vector v that are larger than the minimum of the elements in v. You may assume that v contains at least two elements, and that that the first two elements are different numbers. 1 int second (Vector v) { 2 int min; 3 int secmin; 4 5 if (((Integer)v.elementAt(0)).intValue()<= 6 ((Integer)v.elementAt(1)).intValue()){ 7 min = ((Integer)v.elementAt(0)).intValue(); 8 secmin = ((Integer)v.elementAt(1)).intValue(); 9 else { 10 min = ((Integer)v.elementAt(1)).intValue(); 11 secmin = ((Integer)v.elementAt(0)).intValue(); for (int i = 2; i < v.size(); i++) { 15 if (((Integer)v.elementAt(i)).intValue() <= min) { 16 int tmp = min; 17 min = ((Integer)v.elementAt(i)).intValue(); 18 secmin = tmp; 19 else if (((Integer)v.elementAt(i)).intValue() <= secmin) { 20 secmin = ((Integer)v.elementAt(i)).intValue(); return secmin; 24 i. Describe a vector of Integers that will give the correct result using the above method. [2 marks] 1, 2, 3, 4 ii. Describe a vector of Integers that will give incorrect results using the above method. [2 marks] 1, 2, 1, 1 iii. Outline how you would go about repairing the fault in the program so it would give correct results with both your tests. [2 marks] Change the tests at lines 15 and 19 to strict inequalities. Page 8 of 13

10 Question 8 Consider the following brief description of the Informatics Teaching Office (ITO): The ITO maintains information about each student taking a taught course in the Division of Informatics. Each student follows a programme of studies comprising a collection of individual courses and for each course an assessment record is maintained. Each student s director of studies maintains oversight over the selection of courses and the students progress. An analyst has begun to build a class model for this system and has identified: Student, Programme, Course, Assessment and DoS have been identified as class names in building the class model. (a) Explain what is meant by an association and how associations are used to make a more elaborate class model. Illustrate your answer by identifying some associations between the classes identified in the question. [3 marks] Associations between classes represent relationships between the entities represented by the classes. They frequently describe the way the classes will interact with each other. Examples from above: Student is directed by a DoS Student follows a Programme Course is part of a Programme (b) Explain how you might go about validating a class model. Use the example above to illustrate your answer. [3 marks] One method of validating a class model is to check the multiplicities of the associations. For example in the above we each Student should have exactly one DoS, while each DoS may have many Students. Our model should enforce these limits. Another method is the use of CRC cards. Each class get a card which lists its responsibilities and the other classes it collaborates with to carry them out. By role playing with the cards we can determine if the model allows each class to carry out its responsibilities. For example each DoS must oversee the course selection of their Students. Therefore the model should include an association between DoS and Course. (c) The DoS has the responsibility to check students they supervise are making satisfactory progress. Describe, using the classes identified above and the associations you have identified, how a DoS can carry out this responsibility. You may identify additional associations if necessary. [2 marks] In addition to the above we might add the following associations: Assessment is taken by a Student. DoS can check an Assessment. Now for each Student that DoS directs, DoS can check each Assessment that Student has taken. Page 9 of 13

11 (d) What is meant by refactoring in object-oriented design? Explain its role in the design process. [2 marks] Refactoring is a disciplined process of transforming the class model so it is better structured and cleaner in design. The aim of refactoring is to improve the code without changing its behaviour. It is used in OO design to ensure the quality of the model is maintained as it is developed. Page 10 of 13

12 Question 9 The Java language provides an operation + for concatenating two strings. (a) Using only the operations of addition, subtraction and concatenation, by repeated concatenation or otherwise, write a Java implementation of a method static String repeat(string s, int n) such that, if s is a String and n is a natural number (i.e. a non-negative integer), then repeat(s, n) is the result of concatenating n copies of s. [4 marks] static String repeat (String s, int n) { if (n == 0) return "; else return s + repeat(s, n-1); [ Question continues on next page ] Page 11 of 13

13 (b) The following function e defined on strings makes 2 n copies of the string s. The definition uses the function + to concatenate strings. e(s, 0) = s e(s, n + 1) = e(s, n) + e(s, n) The following Java method is intended to be an implementation of this function: static String exp(string s, int n) { if (n == 0) { return s; else { String res = exp(s,n-1); return res+res; [6 marks] Base case n = 0 : We must show that e(s, 0) = exp(s, 0). Using the definition of e we must show exp(s, 0) = s. This is true by computation on exp. Induction Step: Assume e(s, n) = exp(s, n). We have that exp(s, n + 1) = res + res, where res = exp(s, n). By the induction hypothesis, res = e(s, n) and so exp(s, n + 1) = e(s, n) + e(s, n) = e(s, n + 1) by definition of e. Page 12 of 13

14 Question 10 This question concerns stack-based evaluation of arithmetic expressions. (a) Draw syntax trees for the following expressions: 5 ((2 3) + 4) (5 2) (3 + 4) ((5 2) 3) + 4 [3 marks] * * + / \ / \ / \ 5 + * + * 4 / \ / \ / \ / \ * * 3 / \ / \ (b) Give the expressions derived from each of these trees by (i) infix and (ii) postfix tree traversal. [3 marks] (i) infix traversals: (ii) postfix traversals: (c) Using the Java byte code instructions imul, iadd, iconst, show the result of compiling the expression 5 ((2 3) + 4). [4 marks] 1 iconst 5 2 iconst 2 3 iconst 3 4 imul 5 iconst 4 6 iadd 7 imul Page 13 of 13