EE219 Object Oriented Programming I Repeat Solutions for 2005/2006

Transcription

1 EE219 Object Oriented Programming I Repeat Solutions for 2005/2006 Q1(a) Corrected Code: #include <iostream> using namespace std; class Question1 int a,b; public: Question1(); Question1(int, int); virtual void increment(int); virtual void display(); virtual ~Question1(); ; Question1::Question1(): a(1), b(2) Question1::Question1(int x,int y): a(x), b(y) Question1::~Question1() cout << "(" << a << "," << b << ") destroyed" << endl; void Question1::display() cout << "a has value:" << a << endl; cout << "b has value:" << b << endl; void Question1::increment(int amt = 1) a+=amt; b+=amt; int main() Question1 q1; q1.increment(); q1.display(); q1 = Question1(); q1.increment(5); q1.display(); return 0; No void on line 13 as destructor cannot return anything Line 19 cout missing a << Line 01 should include iostream not string Lines 9 and 10 constructors should not be virtual Line 39 main should return an int, not true (may not give compiler error) Lines 16 and 17 missing empty constructor implementation Line 4 should have class keyword instead Line 2 should be using namespace std; [16 marks 2 each]

2 The output from the code should be: a has value:2 b has value:3 (1,2) destroyed a has value:6 b has value:7 (6,7) destroyed [9 marks] Need to understand when destructors are called 2 marks each Need to understand output due to constructor 1 mark each Need to understand assignment of new object 2 marks [10 possible] [9 marks total] Q2(a) #include<iostream> #include<string> using namespace std; class Person string name; string address; public: Person(string, string); virtual void display(); ; class Staff: public Person string id; int phonenumber; public: Staff(Person, string, int); virtual string getid(); virtual int getphonenumber(); virtual void display(); ; Person::Person(string aname, string anaddress): name(aname), address (anaddress) void Person::display() cout << "Person has name: " << name << endl; cout << "And address: " << address << endl; Staff::Staff(Person p, string anid, int aphone): Person(p), id(anid), phonenumber(aphone) string Staff::getID() return id;

3 int Staff::getPhoneNumber() return phonenumber; void Staff::display() Person::display(); cout << "And ID: " << id << endl; cout << "And Phone Number: " << phonenumber << endl; int main() Staff s(person("derek", "DCU"), "12345", 5355); s.display(); system("pause"); return 0; [19 marks total] [2-3 for each method and main, except 4 for staff display and 4 for main] 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) You would then have to write a full implementation for display() in the child class. e.g. void Staff::display() cout << "Person has name: " << name << endl; cout << "And address: " << address << endl; cout << "And ID: " << id << endl; cout << "And Phone Number: " << phonenumber << endl; Importantly, name and address would have to be specified as protected. You would make a class abstract to impose a behaviour on the child classes. You may also wish to prevent a class from being instantiated. [6 marks] Q3(a) Since the responsibility for the initialisation of the class is in the hands of the programmer of the class, the responsibility for the destruction of the class should also be the responsibility of the programmer. In C++ we can write a destructor for a class that performs some clean-up operations and even notifications. To define a destructor for a class the ~ symbol is used in front of the class name, so for the Account class you would use:

4 class Account // state definition public: Account(int thenumber, float thebalance); virtual ~Account(); //destructor ; // other member methods here. Account::~Account() cout << "Account object being destroyed!" << endl; Note: A similar destructor is also written for the CurrentAccount class. The destructor must take no parameters and cannot return a result. A constructor cannot be virtual, but a destructor can (and should) be virtual (we will discuss this later). When is a destructor called? It is not invoked explicitly. It is called when an object goes out of scope. int main() CurrentAccount b = CurrentAccount(50.0, 12345, 200.0); b.display(); What would happen? Since CurrentAccount is a child of the Account class then destruction of an object of the CurrentAccount object will result in the Account destructor being called. The CurrentAccount destructor followed by the Account destructor will be called, just after the main() method ended, and just before the application ran to completion. If you don't declare a destructor, C++ generates a default destructor, that frees up the memory of the object. This is suitable for most classes; however, a destructor may be required for specific actions: Q3(b) Notifying other objects about an imminent destruction. Closing open files, database connections, sockets etc. Freeing dynamically allocated data. If an object does not free data that is dynamically allocated, then that memory will remain locked until the application terminates. [12 marks] So to write a simple method that returns a single value we can use:

5 1 2 // Using Methods 3 4 #include<iostream> 5 using namespace std; 6 7 float addinterest(float val, float rate) 8 9 return val + (val * (rate/100)); int main() float balance = 5000; 15 float irate = 5.0; balance = addinterest(balance, irate); cout << "After interest your balance is " 20 << balance << " Euro." << endl; The source code for this is in Functions1.cpp The method is defined to return a float value, so it is required to do so. In this case the return value will be the new balance. The return keyword defines the value to return. Since the return type has been specified as float the return value must also be of float type. The return value of the method is assigned to the left-hand side of the =. In this case the balance value has been modified to update the balance in the main() method. This will result in the output: After interest your balance is 5250 Euro. Some points about methods: If a method has a return type then it must return a value! The void keyword implies that no return value is expected. void can also mean "no type" in the case of pointers (discussed later). int is the default return type in C++. char* is the string type (pointer to characters). In C if we specified a function with no parameters, e.g. void somefunction() it actually meant that there was an undeterminded number of parameters, thus disabling type checking. In C++ this means that there are zero parameters. The previous code segment passed values to the method 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.

6 This example is the same as the previous example except this time we are passing by reference: 1 2 // Using Methods (with Pass by reference) 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; The source code for this is in Functions2.cpp The method is defined not to return a value, so the return type is set as void. This is to prove that the pass by reference actually works. You will notice that the first parameter val has been changed so that it has an & in front of it. This is notation to signify that the val parameter is to be passed by reference, not by value. You will notice the return keyword is not used, as the return type is set to void. The value is not returned, rather it is modified directly. So when the value of val is modified in the addinterest() this modifies the value of balance directly. The balance is passed in the same way, but the method receives the reference in this case, not the value. The balance variable has been updated by the method and the new value is displayed as below This will result in the output: After interest your balance is 5250 Euro. So the output is exactly the same. Passing by reference is like passing a copy of the name of the variable to the method. [13 marks] [not same level of detail required any example will suffice]

7 Q4(a) 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++. Q4(b) public class Reverser private String reversedstring; [10 marks]

8 public Reverser(String s) int len = s.length(); char[] revtext = new char[len]; for (int i=0; i<len; i++) revtext[i] = s.charat(len-i-1); reversedstring = new String(revText); public String getreversedstring() return reversedstring; [10 marks] [5 for structure, 5 for reverse code] Q4(c) 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. [5 marks] Q5(a) // A Button Events Application import java.applet.*; import java.awt.*; import java.awt.event.*; public class Question5 extends Applet implements ActionListener private Button button1, button2; private TextField status; public void init()

9 status = new TextField(20); this.button1 = new Button("Upper case"); this.button2 = new Button("Lower case"); this.button1.addactionlistener(this); this.button2.addactionlistener(this); this.add(button1); this.add(status); this.add(button2); public void actionperformed(actionevent e) if (e.getactioncommand().equals("upper case")) status.settext( status.gettext().touppercase() ); else if (e.getactioncommand().equals("lower case")) status.settext( status.gettext().tolowercase() ); HTML page: <title> Question 5 Page </title> <hr> <applet code=question5.class width=200 height=200> </applet> <hr> Q5(b) [20 marks] [2 marks for HTML page, 5 marks for layout, 5 marks for events, 5 marks for functionality, 3 mark for states etc] 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. [5 marks]