EE219 Object Oriented Programming I (2006/2007) SEMESTER 1 SOLUTIONS

Transcription

1 Q1(a) Corrected code is: #include <iostream> using namespace std; class Shape protected: int x, y; public: Shape(int, int); ; EE219 Object Oriented Programming I (2006/2007) SEMESTER 1 SOLUTIONS Shape::Shape(int a, int b): x(a), y(b) class Circle: public Shape int rad; public: Circle(int, int, int); virtual void display(); ; Circle::Circle(int x, int y, int r): Shape(x,y), rad(r) void Circle::display() cout << "A circle with centre (" << x <<","<< y << ")"; cout << " and radius: " << rad << endl; int main(void) Circle c(10,10,5); c.display(); return 0; 1. rad should not have =0 as this is not allowed 2. a(x) and b(y) should be x(a), y(b) in the shape constructor 3. the Shape states should be protected, not private 4. d(12,12) is invalid as there is no such constructor 5. the word void is missing from the display implementation 6. the display method always displays x,y and not the values 7. missing using from the namespace statement 8. import should be include 9. There is no display method for the parent. Remove or write a method. [18 marks] Q1(b) DCU, student, staff, technician, lecturer, name, address, id, classroom, modules, desks, blackboard, phone, university EE219 - Semester /2007 Solutions Page 1 of 8

2 This answer is only one of hundreds available. Computer has software, hard-disk, CPU, DVD drive, colour, keyboard Laptop ISA Computer Desktop ISA computer Has monitor, mouse PC ISA computer DVD ISA disk Hard-disk ISA disk (questionable) CD-ROM ISA disk Dell ISA computer MacOS ISA software WindowsXP ISA software Linux ISA software -> can create an operating systems class LCD ISA monitor -> can create a CRT class etc. Students should draw a diagram of these relationships. Arrows must be in the correct direction, inheritance should be clear i.e. clearly distinguish ISA from IS A PART OF. [7 marks] Q2(a) #include <iostream> #include <string> using namespace std; class Vehicle int numberdoors, numberwheels; string color; public: Vehicle(int, int, string); virtual void display(); ; class Car: public Vehicle int numberseats; string modelname; public: Car(int, int, string, int, string); Car(Vehicle, int, string); virtual void display(); ; Vehicle::Vehicle(int numdoors, int numwheels, string col): numberdoors(numdoors), numberwheels(numwheels), color(col) void Vehicle::display() EE219 - Semester /2007 Solutions Page 2 of 8

3 cout << "The Vehicle has " << numberdoors << " doors, " // << numberwheels << " wheels, and is " << color << " in colour." << endl; Car::Car(int numdoors, int numwheels, string col, int numseats, string model): Vehicle(numDoors, numwheels, col), numberseats(numseats), modelname(model) Car::Car(Vehicle v, int numseats, string model): Vehicle(v), numberseats(numseats), modelname(model) void Car::display() Vehicle::display(); cout << " It is a Car with " << numberseats << " seats and " // << modelname << " as the model name" << endl; int main(void) Car c(5, 4, "blue", 5, "Toyota"); Car d(vehicle(3, 4, "red"), 2, "Ferrari"); c.display(); d.display(); return 0; Q2(b) [3 marks for each method approx] [18 marks] int main(void) Vehicle vehicles[3] = Vehicle(3, 4, "red"), Vehicle(5, 4, "blue"), Vehicle(4, 5, "green"); Vehicle *ptr = &vehicles[0]; for(int i=0; i<3; i++) ptr++->display(); return 0; a->display(); is equivalent to (*a).display(); [5 marks] [2 marks] [7 marks] 3(a) Pointers There are generally few problems with this fact as Java provides suitable replacement through the use of well-formatted references. Memory Management EE219 - Semester /2007 Solutions Page 3 of 8

4 The Java garbage collector manages all memory in Java. It reclaims all memory for objects once there are no remaining references to that object. When the garbage collector runs, it searches for all objects that have no references and reclaims the memory. C++ developers are required to provide their own memory management routines, otherwise a complex C++ application, running for a long period would simply cause the computer to run out of memory. Implementing memory management correctly in C++ is complicated, and so memory leaks are a very frequent problem with C++ developed applications. Garbage collection was not added to the C++ language specification, as all developers would be required to deal with a standard garbage collector and every C++ application would incur the overhead. There are many 3rd party C++ garbage collectors available, such as the Boehm-Demers-Weiser conservative garbage collector, that replaces the C malloc() or C++ new calls (It can alternatively be used as a leak detector!). Access Specifiers Like C++, in Java we have public, private and protected access specifiers, but we also have another access specifier "package". This is the default access specifier and means that all states and methods are accessible to all classes within the same package. There is no package access specifier keyword, it is simply the default if public, private or protected are not used. Access specifiers in Java are: public - accessible everywhere, an interface method (same as C++) private - accessible only in the class where it was defined (same as C++) protected - accessible in the class where it is defined, the derived classes and in the same package (almost the same as C++ with the exception of the package) "package" - default (there is no package specifier keyword) and means that it is accessible by any class in the same package. You can equate this to the C++ friendly condition by saying that all of the classes in the same package (directory) are friendly. In Java there is no direct equivalent to protected in C++. In Java version 1.0 there was! it was called "private protected", but was deemed complex and unnecessary, and so was removed. Destructors There are no destructors in Java, even thought there is a new keyword, there is no corresponding delete keyword, as Java takes care of all of the memory managment. Java has a special finalize() method that you can add to any class. The strange thing about this method is that you can use it for tidying up (printing a paper record for Account etc.), but you do not know when it will be called. In C++ this was easier, as the destructor is called when the object goes out of scope, but in Java the finalize() method will be called when the garbage collector destroys the object. We do not know when this will be called! As discussed previously, we can request that the garbage collector should run, but we do not know when it will run! You can help the garbage collector slightly by setting all your references to null when you no longer need them, e.g. a = null; - will send a hint to the garbage collector that the object is no longer being used. [12 marks] 3(b) 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 EE219 - Semester /2007 Solutions Page 4 of 8

5 // 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++. [11 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? EE219 - Semester /2007 Solutions Page 5 of 8

6 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. [9 marks] 4(b) This is better than making the accountnumber state public, retaining encapsulation. Inline is ignored with virtual methods. We can provide full access to a state through methods, rather than directly to the state by making it public, by providing an accessor and a mutator. For example for the accountnumber state in the Account class, these would look like int getaccountnumber(); //accessor void setaccountnumber(int newnumber); //mutator The main advantage of this format is that access can be controlled to the accountnumber state, so, if additional functionality is required at a later stage, it can be achieved, without effecting the developers using the Account class. We can also declare standalone methods (global methods) to be inline, such as: inline int account::getaccountnumber() //etc 4(c) The output is: x has the value: 1 [5 marks] EE219 - Semester /2007 Solutions Page 6 of 8

7 y has the value: z has the value: 14 This is because x=z++ increments z after the assignment has taken place. Y is undefined as variables in c++ are not set to zero on initialisation and z has the value of 14 as it is assigned the last value of i in the loop (i.e. 9) then it is added to the previous value of i, which is 5, giving a total of 14. Q4(d) [6 marks] By default, every class extends the Object class, even if you don't extend any class. The Object class provides methods that are inherited by all Java classes, such as equals(), getclass(), tostring() and more. The Class class (called class descriptors) are automatically created and associated with the objects to which they refer. For example, the getname() and tostring() methods return the String containing the name of the class or interface. We could use this to compare the classes of objects. Q5(a) [5 marks] import java.awt.*; import java.awt.event.*; import java.applet.*; 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); EE219 - Semester /2007 Solutions Page 7 of 8

8 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) 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))); HTML page: <title> Question 5 Page </title> <hr> <applet code=question5.class width=200 height=200> </applet> <hr> [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 /2007 Solutions Page 8 of 8