EE219 Object Oriented Programming I (2007/2008) REPEAT SOLUTIONS

Transcription

1 Q1(a) Corrected code is: EE219 Object Oriented Programming I (2007/2008) REPEAT SOLUTIONS 00 #include <iostream> 01 using namespace std; class Shape protected: 06 float posx, posy; 07 public: 08 Shape(float, float); 09 void move(float, float); 10 virtual void display(); 11 ; class Circle : public Shape float radius; 16 public: 17 Circle(float, float, float); 18 virtual void setradius(float); 19 virtual void display(); 20 ; Shape::Shape(float x, float y): posx(x), posy(y) void Shape::display() cout << "The Shape is at position (" << posx << "," << posy << ")\n"; Circle::Circle(float rad, float x, float y): Shape(x,y), radius(rad) void Circle::display() Shape::display(); 34 cout << " and a radius of " << radius << endl; int main() Circle c(10, 20, 30); 40 c.display(); 41 return 0; 42 Line 5 state cannot be private, must be protected Line 6 cannot initialise states in C++. Line 8 constructor cannot be virtual Line 22 variables are wrong way around Line 26 should be posx and posy, not x and y Line 33 should be Shape::display(), infinite loop here. Line 39 should be a third parameter EE219 - Semester /2008 Solutions Page 1 of 9

2 Line 34 should be radius, not rad [18 marks] Q1(b) Organise the following concepts into suitable class hierarchies: television, channel, volume, DVD player, video player, LCD television, Plasma television, dimensions, aerial. Concisely explain the rationale for your groupings and create appropriate additional classes if required. Create new class Media Device Television has-a channel, volume, aerial, dimensions LCD Television is-a Television Plasma Television is-a Television DVD Player is-a Media Device Video Player is-a Media Device Media Device has-a dimensions, channel Students should draw diagrams of these relationships. [7 marks] Q2(a) #include <iostream> #include <string> using namespace std; class Vehicle string color; string brandname; public: Vehicle(string, string); virtual void display(); ; class Car: public Vehicle int numberseats, numberdoors, numberwheels; ; public: Car(string, string, int, int, int); Car(Vehicle, int, int, int); virtual void display(); class Motorbike: public Vehicle int numberwheels; bool isoffroad; public: Motorbike(string, string, int, bool); Motorbike(Vehicle, int, bool); virtual void display(); ; EE219 - Semester /2008 Solutions Page 2 of 9

3 Vehicle::Vehicle(string brand, string col): color(col), brandname(brand) void Vehicle::display() cout << "The Vehicle is a " << color << " " << brandname << endl; Car::Car(string brand, string col, int numseats, int numdoors, int numwheels): Vehicle(brand, col), numberseats(numseats), numberdoors(numdoors), numberwheels(numwheels) Car::Car(Vehicle v, int numseats, int numdoors, int numwheels): Vehicle(v), numberseats(numseats), numberdoors(numdoors), numberwheels(numwheels) void Car::display() Vehicle::display(); cout << " It is a Car with " << numberseats << " seats, " << numberwheels << " wheels and " << numberdoors << " doors\n"; Motorbike::Motorbike(string brand, string col, int numwheels, bool offroad): Vehicle(brand, col), numberwheels(numwheels), isoffroad(offroad) Motorbike::Motorbike(Vehicle v, int numwheels, bool offroad): Vehicle(v), numberwheels(numwheels), isoffroad(offroad) void Motorbike::display() Vehicle::display(); cout << " It is a Motorbile with " << numberwheels << " wheels and "; cout << (isoffroad? "is off road\n" : "is not off road\n"); int main(void) Car c("ferrari", "red", 2, 2, 4); Car d(vehicle("ford", "blue"), 4, 5, 4); Motorbike m("ducatti", "red", 2, false); c.display(); d.display(); m.display(); system("pause"); return 0; [1-2 mark for each method] [4 marks for display] [5 marks for motorbike] [3 marks for main with both constructors] [22 marks total] Q2(b) Conditional operator?. It has the form conditionalexpression? trueexpression1 : falseexpression2, for example: EE219 - Semester /2008 Solutions Page 3 of 9

4 // Display the shorter of s and t cout << (s.length() < t.length()? s : t); Note that the expressions must have the same type. [3 marks] 3(a) The output will be exactly the same in all four cases: The value of x is 0 The value of x is 1 The value of x is 2 The value of x is 3 The value of x is 4 [5 marks] 3(b) Explain pass-by-value and pass-by-reference. Write a short segment of code to demonstrate the difference? 3 4 #include<iostream> 5 using namespace std; 6 7 void addinterest(float &val, float rate) 8 9 val = val + (val * (rate/100)); int main() float balance = 5000; 15 float irate = 5.0; addinterest(balance,irate); cout << "After interest your balance is " 20 << balance << " Euro." << endl; This will result in the output: After interest your balance is 5250 Euro. In this example on line 7, val is passed by reference and rate is passed by value. Pass by value passes a copy of the value contained in the variable irate. Pass by reference passes the address of the variable balance. This means that any adjustment to the val reference affects the balance variable as both of these reference refer to the same 4 byte area of memory. So the output is exactly the same. Passing by reference is like passing a copy of the name of the variable to the method. [10 marks] EE219 - Semester /2008 Solutions Page 4 of 9

5 3(c) Namespaces allow the programmer to define a region that is limited in scope. When writing C++ applications we can make it explicit that we are using the standard namespace by a "using directive": using namespace std; // i.e. I want to use the declarations and // definitions in the "Standard Library" // namespace at the beginning of our code segments, however this can be considered poor practice in certain circumstances as the entire contents of the namespace are included. The alternative is to make it explicit that we were calling the standard cout output stream by typing: std::cout << "Hello World!" << std::endl; which states that the cout and endl that we wish to use are both defined in the std standard library. It is possible to include the exact namespace member to be included by a "using declaration":: using std::cout; would allow us to use cout without including all of the names in the std namespace. Packages are groups of similar classes, that can be, but are not necessarily related to each other through an inheritance structure. Packages are classes organised into directories on a file system. Packages are '.' separated words, where the package refers to the directory that the class files are in. For example java.awt (the directory /java/awt/ is a package that stores the classes for creating buttons, textfields, etc. A package can also contain more packages in a subfolder. For example, java.awt.event contains classes for events, within the awt package. Use the import keyword to load in packages. Multiple classes can be loaded using the * character. So, for example,import java.awt.*; imports all the classes in the awt package, but please note, it does not include classes in the sub-packages, so, you must explicitly import java.awt.event.*;. In a source file, if no package name is defined on an import e.g. import SomeClass; then it is assumed that the class SomeClass is in the same directory as the source file. Packages are very similar to a combination of C++ namespaces and includes, only a lot easier to use. Packages and Namespaces can be nested. There is a friendly default access specifier in java that shares across packages. There is no such access specifier in C++. [10 marks] 4(a) Java applets generally do not have a main() method. What is the lifecycle of a Java applet and what standard methods do we have to write? If we do not write one of these methods will the Applet compile? The life cycle of the applet has the following stages: EE219 - Semester /2008 Solutions Page 5 of 9

6 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. [10 marks] 4(b) The value of x[0] is 3 The value of x[1] is 4 The value of x[2] is 5 The value of x[3] is 6 The value of x[4] is 7 [5 marks] 4(c) super - allows us to refer to states and methods of the parent class directly. So, super.display() would call the display() method, written or inherited by the parent class. If the parent class of the parent class with a display() method also had a display() method, we do not have access to that method. This super predefined variable can also be used in an unusual way to call the parent constructor from within the child constructor, provided it is used as the first line of the child constructor (just like member initialisation lists in C++). EE219 - Semester /2008 Solutions Page 6 of 9

7 this - allows us to refer to "this object", i.e. to access states or methods for the class that the code is currently within. It allows us to pass a reference to the current object and it also allows us to access the states of the class that are currently out of scope. So, an example of super and this would be: public class TestParent protected int y=2; // example parent state public TestParent(int a) // example parent constructor y=a; public class Test extends TestParent // inheritance private int x; public Test(int a) super(a); // calls the parent constructor x = 5; public void somemethod(int x) this.x = x; // this.x refers to the state of the class super.y = 5; // sets the y state of the parent class to a value of 5. someothermethod(this); // passes the current object to the // someothermethod() method [6 marks for desc (2 x 3). 4 marks for code] [10 marks] Q5(a) import java.awt.*; import java.awt.event.*; import java.applet.*; EE219 - Semester /2008 Solutions Page 7 of 9

8 public class VectorApplet extends Applet implements ActionListener TextField u1str, u2str, v1str, v2str, thetastr; float u1, u2, v1, v2, theta; public void init() this.add(new Label(" u = (")); u1str = new TextField("0.0"); this.add(u1str); this.add(new Label(",")); u2str = new TextField("0.0"); this.add(u2str); this.add(new Label(") and v = (")); v1str = new TextField("0.0"); this.add(v1str); this.add(new Label(",")); v2str = new TextField("0.0"); this.add(v2str); this.add(new Label("), gives theta = ")); thetastr = new TextField("0", 10); this.add(thetastr); Button b = new Button("go"); b.addactionlistener(this); this.add(b); this.setsize(500,40); public void actionperformed(actionevent e) try u1 = Float.valueOf(u1str.getText()).floatValue(); u2 = Float.valueOf(u2str.getText()).floatValue(); v1 = Float.valueOf(v1str.getText()).floatValue(); v2 = Float.valueOf(v2str.getText()).floatValue(); catch(exception ex) HTML page: float magu = (float) Math.sqrt((u1*u1) + (u2*u2)); float magv = (float) Math.sqrt((v1*v1) + (v2*v2)); float dotuv = (u1*v1) + (u2*v2); theta = (float) Math.acos(dotuv/(magu*magv)); thetastr.settext(float.tostring((float)math.todegrees(theta))); <title> Question 5 Page </title> <hr> <applet code=question5.class width=200 height=200> </applet> <hr> EE219 - Semester /2008 Solutions Page 8 of 9

9 [25 marks] [3 marks for HTML page, 4 marks for layout, 5 marks for events, 10 marks for functionality, 3 mark for states etc] Note Maths must be perfect as API documentation was provided. EE219 - Semester /2008 Solutions Page 9 of 9