EE219 - Semester /2009 Solutions Page 1 of 10

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1 EE219 Object Oriented Programming I (2008/2009) SEMESTER 1 SOLUTIONS Q1(a) Corrected code is: #include<iostream.h> class Account protected: float balance; int actnum; ; class SavingsAccount:public Account private: float interestrate; ; public: SavingsAccount(float, int, float); virtual void display(); SavingsAccount::SavingsAccount(float abalance, int anaccnum, float irate) balance = abalance; actnum = anaccnum; interestrate = irate; void SavingsAccount::display() cout << "Balance is " << balance << endl; cout << "Act Num is " << actnum << endl; cout << "Interest Rate is " << interestrate << endl; int main(void) SavingsAccount a(10.0f, 12345, 5.0); a.display(); return 0; Errors: Line 01: #import should be #include Line 01: Missing use of standard namespace Line 05: States cannot be private in Account Line 06: Cannot initialise states in C++ Line 08: Class definition missing closing ; Line 16: Constructor cannot be virtual EE219 - Semester /2009 Solutions Page 1 of 10

2 Line 27: return type void missing from display() method Line 36: Wrong number of parameters passed to SavingsAccount constructor Line 37: Cannot access private state of SavingsAccount directly from outside class [2 marks for each point] [18 marks] Q1(b) int main(void) SavingsAccount myaccountarray[3] = SavingsAccount(100.0f,1,5.0f), SavingsAccount(200.0f,2,5.0f), SavingsAccount(300.0f,3,5.0f); SavingsAccount *ptr = &myaccountarray[0]; for(int i=0; i<3; i++) (ptr++)->display(); return 0; Alternative notation: Ptr->display() is equivalent to (*ptr).display(); [5 marks] [2 marks] [7 marks] Q2(a) #include<iostream.h> #include<string.h> class Person string name; string address; ; public: Person(string, string); virtual void display(); class Student: public Person string student_id; string programme; int year; EE219 - Semester /2009 Solutions Page 2 of 10

3 ; public: Student(Person, string, string, int); Student(string, string, string, string, int); virtual int getyear(); virtual void setyear(int); virtual void display(); Person::Person(string aname, string anaddress): name(aname), address(anaddress) void Person::display() cout << "Person Details" << endl; cout << "Name: " << name << endl; cout << "Address: " << address << endl; Student::Student(Person p, string anid, string aprogramme, int ayear): Person(p), student_id(anid), programme(aprogramme), year(ayear) Student::Student(string aname, string anaddress, string anid, string aprogramme, int ayear): Person(aName, anaddress), student_id(anid), programme(aprogramme), year(ayear) int Student::getYear() return year; void Student::setYear(int newyear) year = newyear; void Student::display() Person::display(); cout << "Student ID: " << student_id << endl; cout << "Programme: " << programme << endl; cout << "Year: " << year << endl; int main() Student s(person("john Doe", "DCU"), "12345", "Engineering", 2); s.display(); Student s2("jane Doe", "DCU", "23456", "Engineering", 3); s2.display(); s.setyear(10); cout << s.getyear() << endl; system("pause"); return 0; [2 marks for each constructor] [1 marks for each accessor/mutator] [5 marks for correct display methods] [5 marks for main] [18 marks total] EE219 - Semester /2009 Solutions Page 3 of 10

4 Q2(b) An abstract class is a class that is incomplete, in that it describes a set of operations, but is missing the actual implementation of these operations. Abstract classes: Cannot be instantiated. So, can only be used through inheritance. To make the Person class abstract we could write: virtual void display() = 0; (any method would do, even adding one) The constructor taking a Person object as a parameter would no longer be valid. You would then have to write a full implementation for display() in the child class. e.g. void Student::display() cout << Person Details << endl; cout << "Name: " << name << endl; cout << "Address: " << address << endl; cout << "Student ID: " << student_id << endl; cout << "Programme: " << programme << endl; cout << "Year: " << year << endl; Importantly, name and address would have to be specified as protected in the Person class. [7 marks] Q3(a) Polymorphism means "multiple forms". In OOP these multiple forms refer to multiple forms of the same method, where the exact same method name can be used in different classes, or the same method name can be used in the same class with slightly different parameters. There are two forms of polymorphism, over-riding and overloading. A derived class inherits its methods from the base class. It may be necessary to redefine an inherited method to provide specific behavior for a derived class - and so alter the implementation. So, over-riding is the term used to describe the situation where the same method name is called on two different objects and each object responds differently. Over-riding allows different kinds of objects that share a common behavior to be used in code that only requires that common behavior. EE219 - Semester /2009 Solutions Page 4 of 10

5 e.g. class ExampleBase virtual void display(); ; class ExampleDerived: public ExampleBase virtual void display(); ; Over-Loading is the second form of polymorphism. The same method name can be used, but the number of parameters or the types of parameters can differ, allowing the correct method to be chosen by the compiler. class Example virtual int add (int x, int y); virtual string add (String x, String y) ; [6 marks for description] [6 marks for code] [12 marks] Q3(b) Explain pass-by-value and pass-by-reference. Write a short segment of code to demonstrate the difference? 8 float addinterest(float val, float rate) 9 10 return val + (val * (rate/100)); void main() float balance = 5000; 16 float irate = 5.0; balance = addinterest(balance, irate); cout << "After interest your balance is " << balance << " Euro." << endl; 21 The previous code segment passed values to the function using pass by value. In this case you are really passing the value of the variables balance and irate, so in this case the numbers 5000 and 5.0. It is only possible to have one return value when passing by value. If we wish to have multiple return values, or wish to modify the source then we can pass by reference. EE219 - Semester /2009 Solutions Page 5 of 10

6 This example is the same as the previous example except this time we are passing by eference: Passing by reference is like passing a copy of the name of the variable to the method. 8 void addinterest(float &val, float rate) 9 10 val = val + (val * (rate/100)); void main() float balance = 5000; 16 float irate = 5.0; addinterest(balance,irate); cout << "After interest your balance is " << balance << " Euro." << endl; 21 [4 marks for each segment] [5 marks for discussion] [13 marks] Q4(a) Java does not suffer from the memory leaks like C++, due to the addition of an embedded Garbage Collector (GC) and carefully structured reference counting. The GC can be run in three ways: The system calls the GC when memory is low. The system calls the GC when the CPU is idle. The user can call the GC directly, but since the GC runs as a thread in a threaded environment, there is no guarantee that it will run immediately. [7 marks] Q4(b) A Java Interface is a way of grouping methods that describe a particular behaviour. They allow developers to reuse design and they capture similarity, independent of the class hierarchy. We can share both methods and constants using Interfaces, but there can be no states in an interface. Methods in an interface are implicitly public and abstract. Constant states in an interface are implicitly public, static and final. Interfaces differ from abstract classes in that an interface cannot have an implementation for any method, i.e. all methods are abstract. Classes can implement many interfaces, all unconstrained by a class inheritance structure. interface Demo public static final String democonst = "hello"; // public static final can be omitted // as is implicit void go(); // these methods are public and abstract void stop(); // even if public abstract is omitted class SomeClass extends Object implements Demo EE219 - Semester /2009 Solutions Page 6 of 10

7 public void go() // write implementation here public void stop() // write implementation here public class TestApp public static void main(string args[]) SomeClass c = new SomeClass(); System.out.println(SomeClass.demoConst); // will print out "hello" Q4(c) [5 marks for description] [4 marks for code] [9 marks] The life cycle of the applet has the following stages: The appletviewer loads and creates the specified class. The applet initializes itself. The applet starts running. When finished the applet is destroyed. The applet always receives an init() method first and then a start() and paint() call (Note: This may happen asynchronously! i.e. at the same time). init() - This method 'sets-up' the applet, called once when the applet is first initialized. Use this method to set up the display of the applet, set colours etc. start() - Called whenever the applet is visited, i.e. the applet starts running again. You could use this method to start up an animation that stops when the applet is minimised. This method would be called when the web browser has been restored after being minimised. stop() - Called when the applet is no longer active (e.g. minimized) paint()- This method is called whenever the appearance of the applet is required to be updated, such as when the applet is being maximized from a minimized state. The appearance needs to be updated. It is also possible for the programmer to call this method. This method does nothing by default, you must write the code. destroy() - Called when the applet is discarded. This method should perform any closing down tasks that you would wish to perform, such as closing connections to databases, etc. If we do not write one of these methods then the applet will still compile as the method will be inherited from the parent Applet class. [7 marks for discussion of lifecycle] [2 marks for correct answer with correct explanation] [9 marks] EE219 - Semester /2009 Solutions Page 7 of 10

8 Q5(a) import java.awt.*; import java.awt.event.*; import java.applet.*; public class CircleClass extends Applet implements ActionListener TextField rad_str, area_str, circ_str; float rad_val, area_val, circ_val; public void init() this.add(new Label(" Enter radius of circle: ")); rad_str = new TextField("0.0",10); this.add(rad_str); Button b = new Button("Go"); b.addactionlistener(this); this.add(b); this.add(new Label("Area: ")); area_str = new TextField("0.0",10); this.add(area_str); this.add(new Label("Circumference: ")); circ_str = new TextField("0.0", 10); this.add(circ_str); this.setsize(600,40); public void actionperformed(actionevent e) try rad_val = Float.valueOf(rad_str.getText()).floatValue(); catch(exception ex) circ_val = 2*(float)Math.PI*rad_val; circ_str.settext(float.tostring(circ_val)); area_val = (float)math.pi*rad_val*rad_val; EE219 - Semester /2009 Solutions Page 8 of 10

9 area_str.settext(float.tostring(area_val)); <html><title>question5</title> <body> <center> <applet width=500 height=100 code="circleclass.class"></applet> </center> </html> Q5(b) [3 marks for HTML page, 3 marks for layout, 4 marks for events, 6 marks for functionality, 3 mark for states etc] [19 marks] A FlowLayout arranges components left-to-right top-to-bottom, much like the "centred text" button in Microsoft Word for Windows, where each line is filled and centered until the line is full. In a flow layout form each component takes its preferred size. A GridLayout lays out the components in a rectangular grid. The container that you apply the layout to, divides the area into equal sized squares and each component that you add is sized to fit that square. E.g. new GridLayout(3,2) EE219 - Semester /2009 Solutions Page 9 of 10

10 A BorderLayout arranges a container into five regions, North, South, East, West and Center. Each region can have either one or zero components. If you do not place a component in the region then it will not be displayed. When we are adding components to a container we could write something like: [2 marks each] [6 marks] EE219 - Semester /2009 Solutions Page 10 of 10