212 f w2005 Sample Solution

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School of Computer Science

0360-212 Object-oriented programming using JAVA

Winter 2005 Final Examination Duration: 3 hours

Student information Last name ______________________

First name _____________________

Student ID# _____________________

Lab section ____________________

University regulations: conduct during the examination

i) A candidate must not give assistance to or receive assistance from, or communicate in any manner with any person other than the Presiding Examiner or invigilator, or copy, or have at the examination unauthorized aids of any kind. (This includes the use of cell phones and pagers.) A candidate who is involved in such activity will be subject to the disciplinary procedures of the University according to Senate Bylaw 31.

ii) Eating, drinking and smoking are not permitted in examination rooms. Personal recording/playback devices are not permitted in the examination room. iii) If it is necessary for a candidate to leave the room he or she may do so and return if accompanied by the Presiding Examiner or an invigilator.

iv) A candidate must not write on any paper, other than that in the answer book, and must keep all papers on the desk.

Students are allowed to take two textbooks on Java. No computers allowed.

All answers must be given in this booklet.

Part I Part II Part III Total /20

/ 20

/60

/100

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Answer Sheet for Part I and Part II. You must answer all the questions in part I and Part II on this page. Part I: True/False Questions. Question # 1 2 3 4 5 Answer Question # 6 7 8 9 10 Answer Part II: Multiple Choices. Question # 1 2 3 4 5 Answer Question # 6 7 8 9 10 Answer

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Part I True/False 20 marks For this part, clearly indicate if each assertion is TRUE or FALSE. You must write TRUE or FALSE, NOT T or F. 1. An interface reference can refer to any object of any class that implements

the interface. T

2. A try block must have at least one catch block, but could have many catch blocks, and may or may not have a finally block. F

3. If classes C1 and C2 both implement an interface Cint, which has a method whichIsIt, and if C1 c = new C1( ); is performed at one point of the program, then a later instruction c.whichIsIt( ); will invoke the whichIsIt method defined in C1. F

4. All parts of a container in BorderLayout must be used.

F 5. If a GUI container is resized by the user, all components of the GUI are

automatically increased or decreased by the same fraction to fit in the newly sized container. F

6. A constructor may contain a return statement so long as no value (or

expression) is returned. T

7. In order to handle a MouseEvent, one must implement either the MouseListener or MouseMotionListener interfaces, or both. F 8. An inner class is allowed to access directly all the instance variables and

methods of its enclosing class. T

9. One can always extends an existing class to produce a subclass.

F 10. The font of the text of a button can be changed using the setFont() method.

T

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Part II Multiple Choice Questions 20 marksFor this part, please note that each question below has only one answer. Choose the best answer. 1. What is the output of the following program?

public class Inherit { class Figure { void display( ) { System.out.println("Figure"); } } class Rectangle extends Figure { void display( ) { System.out.println("Rectangle"); } } class Box extends Rectangle { void display( ) { System.out.println("Box"); } } Inherit( ) { Figure f = new Figure( ); Rectangle r = new Rectangle( ); Box b = new Box( ); f.display( ); f = r; ((Figure) f).display( ); f = (Figure) b; f.display( ); } public static void main(String[ ] args) { new Inherit( ); } }

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A. Figure Rectangle Box

B. Figure Figure Figure

C. Figure Rectangle Figure

D. Rectangle

Figure

E. The program will not compile, therefore, no output.

A 2. An abstract class

A. is a class which cannot be instantiated B. is a class which has no methods C. is a class which has only abstract methods D. is a class which has only overridden methods

A For questions 3 5, use the following skeletal code.

public class ExceptionThrowerCode{ public void m1( ) { }

public void m2( ) { try{ m3( ); } catch(ArithmeticException ae) {} catch(NullPointerException npe) {} } public void m3( ) { try{ } catch(ArithmeticException ae) {} }

public static void main(String[ ] args){ try {

ExceptionThrowerCode etc = new ExceptionThrowerCode( ); etc.m1( ); etc.m2( );

} catch (ArithmeticException ae) { }

} }

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3. If an ArithmeticException arises in the try statement in m3, A. it is caught in main B. it is caught in m1 C. it is caught in m2 D. it is caught in m3 E. it is not caught leading to the program terminating

D 4. If a NullPointerException arises in the try statement in m3,

A. it is caught in main B. it is caught in m1 C. it is caught in m2 D. it is caught in m3 E. it is not caught leading to the program terminating

C

5. If a NullPointerException arises in the try statement in m1, A. it is caught in main B. it is caught in m1 C. it is caught in m2 D. it is caught in m3 E. it is not caught leading to the program terminating

E 6. Which of ing statement best describes a Java program in a file

named MyExam.java.

A. The file MyExam.java defines a class called MyExam. B. The file MyExam.java defines a public class called MyExam. C. The file MyExam.java defines a class which extends the class

Object. D. None of the above.

D 7. Consider the following method declared in a class whose header is below: public myMethod() throws AnException

Which of the following statements is true? A. There must have a try statement in the method myMethod. B. The exception AnException must be a checked exception. C. The exception AnException was thrown by a throw statement in

myMethod. D. None of the above.

D

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8. What is the output of running class Test?

A. ABCD B. BACD C. CBAE D. AEDC E. BEDC

E

public class Test { public static void main(String[] args) { new Circle9(); } } public abstract class GeometricObject { protected GeometricObject() { System.out.print("A"); } protected GeometricObject(String color, boolean filled) { System.out.print("B"); } } public class Circle9 extends GeometricObject { /** Default constructor */ public Circle9() { this(1.0); System.out.print("C"); } /** Construct circle with a specified radius */ public Circle9(double radius) { this(radius, "white", false); System.out.print("D"); } /** Construct a circle with specified radius, filled, and color */ public Circle9(double radius, String color, boolean filled) { super(color, filled); System.out.print("E"); } }

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9. What happens when you attempt to compile and run these two files in the same directory?

A. Both compile and P2 outputs "What a fancy method" when run. B. Neither will compile. C. Both compile but P2 has an error at run time. D. P1 compiles cleanly but P2 has an error at compile time.

D 10. If you want to draw a red circle inside of a green square in a JPanel where

the paintComponent method is passed a Graphics object called page, which of the following sets of commands might you use?

A. B. page.setColor(Color.green); page.setColor(Color.red); page.fillRect(50, 50, 100, 100); page.fillOval(60, 60, 80, 80); page.setColor(Color.red); page.setColor(Color.green); page.fillOval(60, 60, 80, 80); page.fillRect(50, 50, 100, 100);

C. D.

page.setColor(Color.green); page.setColor(Color.red); page.fillRect(60, 60, 80, 80); page.fillOval(50, 50, 100, 100); page.setColor(Color.red); page.setColor(Color.green); page.fillOval(50, 50, 100, 100); page.fillRect(60, 60, 80, 80);

E. any of the above would accomplish this.

A

//File P1.java package MyPackage; class P1{ void afancymethod(){ System.out.println("What a fancy method"); } } //File P2.java public class P2 extends P1{ public static void main(String argv[]){ P2 p2 = new P2(); p2.afancymethod(); } }

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Part III Programming Questions 60 marksALL JAVA CODE WRITTEN IN PART III SHOULD BE COMMENTED. Question 1: 13 marks The following program saved in the file Phone.java creates a GUI that resembles a phone with functionality and logical error(s).

import java.awt.*; import java.awt.event.*; import javax.swing.*; public class Phone extends JFrame {

private JButton keys[]; private JPanel keyPad, lcd; private JTextArea lcdContent; private String lcdOutput = ""; private int count = 0;

// constructor sets up GUI public Phone() {

super( "Phone" ); // set lcd layout to flow layout lcd = new JPanel(); lcd.setLayout(new FlowLayout()); // initialize lcdContent and add it to lcd panel lcdContent = new JTextArea( 4, 15 );

lcdContent.setEditable( false ); lcd.add( lcdContent ); keys = new JButton[ 15 ]; // initialize all digit key Buttons for ( int i = 3; i

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keys[ 0 ].addActionListener(new ActionListener() { public void actionPerformed (ActionEvent e) { lcdContent.setText( " " ); lcdOutput = " Calling...\n\n " + lcdOutput; lcdContent.append( lcdOutput );

} // end method actionPerformed } // end new ActionListener ); // end addActionListener call keys[ 1 ].addActionListener(new ActionListener() { public void actionPerformed( ActionEvent e ) { if ( lcdOutput.length() == 0 || lcdOutput.substring( 0, 1 ).equals( "C" ) ) return; else { lcdOutput = lcdOutput.substring( 0, ( lcdOutput.length() - 1 ) ); lcdContent.setText( " " ); lcdContent.append( lcdOutput ); } // end else } // end method actionPerformed } // end object ActionLstener ); // end addActionListener call

keys[ 2 ].addActionListener(new ActionListener() { public void actionPerformed( ActionEvent e ) { lcdContent.setText( " " ); lcdOutput = ""; } // end method actionPerformed } // end new ActionListener

); // end ActionListener call for ( int i = 3; i

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// set keyPad layout to grid layout keyPad = new JPanel(); keyPad.setLayout( new GridLayout( 5, 3 ) ); // add buttons to keyPad panel for ( int i = 0; i

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The above program compiles and runs. Answer each of the following questions. a) If you are using a command line terminal window on SOL to develop your java programs, write down the command to compile the above program. (1 mark) Your answer: b) Draw diagrams to show how the program works by following the sequence of commands/actions issued by user. (6 marks, 2 marks each)

(i) The user first types java Phone on the command line. Your answer:

(ii) After the command in (i), the user clicks the phone no. 2533000, then clicks send. Your answer:

(iii) Following the actions in (ii), the user clicks the Clr button four times. Your answer:

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c) The program in this question contains a logical error. When user dials a number, say, 2533000 and clicks send, the phone is supposed to call the number 2533000 and this number is shown in the phone display. However, at this point, if user continues to click another digit, say 9, the phone display shows the number 25330009. This means that during a phone call to 2533000, the phone user can still change the number. This is a logical error. Modify this program by changing ONE single existing statement in the event handler for the Send button so that this logical error can be eliminated. Show your change and explain why. Remember that you are allowed to change only one SINGLE existing statement. (6 marks) Your answer: Change the statement

lcdOutput = " Calling...\n\n " + lcdOutput; to

lcdOutput = "Calling...\n\n " + lcdOutput;

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Question 2: 15 marks Consider the classes MyClass, MyExtendedClass and MyFinalClass.

public class MyClass{ public int v1; public int v2; public MyClass(){ v1 = 1; v2 = 2; } public void method1(){ System.out.println("method1 in MyClass "+v1); } public void method2(){ System.out.println("method2 in MyClass "+v2); } public void method3(){ System.out.println("method3 in MyClass "+v2); this.method2(); } }

public class MyExtendedClass extends MyClass{ public int v2; public MyExtendedClass(){ v1=11; v2=22; } public void method2(){ System.out.println("method2 in MyExtendedClass "+v2); this.method1(); } }

public final class MyFinalClass extends MyExtendedClass{ public int v1; public MyFinalClass(){ v1 = 111; } public void method1(){ System.out.println("method1 in MyFinalClass "+v1); super.method1(); } }

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These classes, as well as the class Test given below, do not contain errors. What is the output produced after running the application Test?

public class Test{ public static void main(String args[]){ System.out.println("FIRST CASE"); MyClass obj1 = new MyClass(); obj1.method1(); obj1.method2(); obj1.method3(); System.out.println("SECOND CASE"); MyClass obj2 = new MyExtendedClass(); obj2.method1(); obj2.method2(); obj2.method3(); System.out.println("THIRD CASE"); MyClass obj3 = new MyFinalClass(); obj3.method1(); obj3.method2(); obj3.method3(); System.out.println("FOURTH CASE"); MyExtendedClass obj4 = new MyFinalClass(); obj4.method2(); } }

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Your answer: FIRST CASE method1 in MyClass 1 ->0.5 method2 in MyClass 2 -> 0.5 method3 in MyClass 2 ->0.5 method2 in MyClass 2 ->0.5 SECOND CASE method1 in MyClass 11 -> 0.5 mark method2 in MyExtendedClass 22 -> 1 marks method1 in MyClass 11 -> 1 mark method3 in MyClass 2 -> 0.5 mark method2 in MyExtendedClass 22 -> 0.5 mark method1 in MyClass 11 -> 0.5 mark THIRD CASE method1 in MyFinalClass 111 -> 1 marks method1 in MyClass 11 -> 1 marks method2 in MyExtendedClass 22 -> 0.5 mark method1 in MyFinalClass 111 -> 0.5 mark method1 in MyClass 11 -> 0.5 mark method3 in MyClass 2 -> 0.5 mark method2 in MyExtendedClass 22 -> 0.5 mark method1 in MyFinalClass 111 -> 0.5 mark method1 in MyClass 11 -> 1 mark FOURTH CASE method2 in MyExtendedClass 22 -> 1 mark method1 in MyFinalClass 111 -> 1 mark method1 in MyClass 11 -> 1 marks

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Question 3: 12 marks File ParseInts.java below contains a program template that should do the following:

z Prompts for and reads in a line of input z Takes the input line one token at a time and parses an integer from

each token as it is extracted. z Sums the integers. z Prints the sum.

For example, if you give it the input 10 20 30 40

it should print The sum of the integers on the line is 100.

However, if you try a line that contains both integers and other words, e.g., We have 2 dogs and 1 cat.

You will probably get a NumberFormatException when it tries to call Integer.parseInt on "We" if your program does not handle exceptions. Use exception handling, complete this program so that it always produces correct answer. For example, for the line of input We have 2 dogs and 1 cat., your program should print

The sum of the integers on the line is 3.

//what do you want to import

public class ParseInts {

public static void main(String[] args) {

// Prompts for and reads in a line of input // Takes the input line one token at a time and parses an integer from // each token as it is extracted. // Sums the integers. // Prints the sum.

System.out.println("The sum of the integers on this line is " + sum);

} }

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Your answer: // **************************************************************** // ParseInts.java // // Reads a line of text and prints the integers in the line. // // **************************************************************** import java.util.Scanner; public class ParseInts { public static void main(String[] args) { int val, sum=0; Scanner scan = new Scanner(System.in); System.out.println("Enter a line of text"); Scanner scanLine = new Scanner(scan.nextLine()); while (scanLine.hasNext()) { try { val = Integer.parseInt(scanLine.next()); sum += val; } catch (NumberFormatException exception) { } } System.out.println("The sum of the integers on this line is " + sum); } } // Prompts for and reads in a line of input 2 marks

// Takes the input line one token at a time and parses an integer from // each token as it is extracted, using proper exception handling // Sums the integers. exception handling 4 marks,

the overall structure for handling each token using some kind of loop structure 4 marks

// Prints the sum ->2 marks

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Question 4: 10 marks File IntList.java contains definitions for a linked list of integers. The class contains an inner class IntNode, which holds information for a single node in the list.

public class IntList { private IntNode front; //first node in list public IntList(){ front = null; } public void addToFront(int val){ front = new IntNode(val,front); } public void addToEnd(int val){ IntNode newnode = new IntNode(val,null); if (front == null) front = newnode; else { IntNode temp = front; while (temp.next != null) temp = temp.next; temp.next = newnode; } } public void removeFirst(){ if (front != null) front = front.next; } public void print(){ System.out.println("---------------------"); System.out.print("List elements: "); IntNode temp = front; while (temp != null){ System.out.print(temp.val + " "); temp = temp.next; } System.out.println("\n---------------------\n"); }

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Add the following two public methods to the IntList class. 1. public void printRec()prints the list from first to last using recursion. 2. public void printRecBackwards()prints the list from last to first using

recursion. Note you can not change the headers of these two methods although you may want to write other methods which facilitate the writing of the above two methods. Your answer: 5 marks for each method each //Prints the list recursively. public void printRec() { System.out.println ("\nList Elements..."); printRec(front); } //---------------------------------------------------- // Recursively prints the part of the list starting // starting at the node pointed to by first. //---------------------------------------------------- private void printRec (IntNode first) { if (first == null) System.out.println(); else { System.out.print (first.val + " "); printRec(first.next);

private class IntNode{ public int val; //value stored in node public IntNode next; //link to next node in list public IntNode(int val, IntNode next){ this.val = val; this.next = next; } } }

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} } //---------------------------------------------------- // Prints the list backwards using recursion. //---------------------------------------------------- public void printRecBackwards() { System.out.println ("\nThe list backwards..."); printRecBackwards(front); } //--------------------------------------------------- // Recursively prints the part of the list starting // at the node pointed to by first backwards. //--------------------------------------------------- private void printRecBackwards(IntNode first) { if (first != null) { printRecBackwards(first.next); System.out.print(first.val + " "); } }

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Question 5: 10 marks Write a Java program that can display the windows shown below. The size of the frame is 200 by 150.

When user presses the button "Register", your program should display a dialog window showing the contents of the name and email JTextFields. When user presses the button "Clear", your program should clear the two JTextFields for name and email. You only need to handle the events for buttons. For the button "Register", you should use inner class to handle the event. For the button "Clear", you should use anonymous class to handle the event. Sample Answers: import javax.swing.*; import java.awt.event.*; import java.awt.*; import javax.swing.*; public class Register extends JFrame implements ActionListener { JLabel name, email; JTextField nameArea, emailArea; JButton okButton, clearButton; public Register() { super("register me!"); // 0.5 mark for calling super. Container c = getContentPane(); //optional statement if you are using J2SE 5.0 c.setLayout(new FlowLayout()); // 0.5 mark for setting up layout manager name = new JLabel( "Your Name:" ); c.add( name ); // 0.5 mark for setting up label 1 // add(name) if J2SE 5.0

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nameArea = new JTextField( 10 ); c.add( nameArea ); // 0.5 mark for setting up JTextfield 1 email = new JLabel( "Your email:" ); c.add(email ); //// 0.5 mark for setting up label 2 emailArea = new JTextField( 10 ); c.add( emailArea ); // 0.5 mark for setting up JTextfield 2 okButton = new JButton("Register"); okButton.addActionListener(new registerButtonHandler()); c.add( okButton ); clearButton = new JButton("Clear"); clearButton.addActionListener( new ActionListener() { public void actionPerformed( ActionEvent event) { nameArea.setText(""); emailArea.setText(""); } } ); // 3.5 marks for registering and writing correct anonymous class event handler. c.add(clearButton ); setSize(200, 150); show(); } // 3 marks for registering and writing correct inner class event handler. class registerButtonHandler implements ActionListener{ public void actionPerformed( ActionEvent event) { JOptionPane.showMessageDialog(null, "You clicked OK==>"+nameArea.getText()+" and "+emailArea.getText()); } } public static void main(String args[]) // 0.5 mark for proper main method and frame setup (setSize, show, etc). { Register AFrame = new Register(); } }

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