SEMESTER ONE EXAMINATIONS SOLUTIONS 2003/2004

Transcription

1 SEMESTER ONE EXAMINATIONS SOLUTIONS 2003/2004 MODULE: Object-Oriented Programming for Engineers EE219 COURSE: B.Eng. in Electronic Engineering B.Eng. in Telecommunications Engineering B.Eng. in Digital Media Engineering INSTRUCTIONS: Answer FOUR questions. All questions carry equal marks. Page 1 of 11

2 Question 1. (a) The corrected code is: #include<iostream.h> #include<string.h> class TestClass string somestring; int someint; public: ; TestClass(string, int); virtual void display(); TestClass::TestClass(string thestring, int theint) somestring = thestring; someint = theint; void TestClass::display() cout << "The string is " << somestring << endl; void main(void) TestClass t("hello", 21); t.display(); The errors are: 1. The ; is missing from the class definition. 2. t.someint is not allowed in the main method as the someint is private 3. the int is missing from the constructor implementation 4. the constructor should have no return type, not even void 5. the <string.h> header file has not been included 6. the display() method should not return a value 7. the return in main(), where there is no return type [2 points each = 14total] (b) End-of-line comments allow the compiler to ignore from // to the end of the line. Block comments are started by /* and ended by */ Page 2 of 11

3 End of line comments can be nested and block comments cannot. You should use end of line comments where possible as block comments are very useful for commenting out a section of code when debugging. x = x * 4.533; // explain what you are doing // end of line commenting /* An example of block commenting */ y = 5 * 3; /* TODO: This section needs to be fixed! Derek 25/12/99 j = j * ; k = k * j + 1; */ [5 marks] (c) char characters ( 'a', 'b', 'c', etc.) int integers ( -5, -1, 0, 1, 3 etc.) float floating point numbers ( , 1.001, etc.) double larger more accurate floating point numbers (i.e. to more decimal places or with a larger magnitude) long (long int) larger integer value range than int if using 16-bit ints. bool contains either the true or false value (i.e. a 1 or 0 respectively). short (short int) smaller integer value range than int unsigned int 0,1,2,3, etc. unsigned long 0,1,2,3,4, etc. [6 marks one for each type listed] Question 2. (a) #include<iostream.h> #include<string.h> class Person private: string name; string address; ; public: Person(string, string); virtual void display(); Page 3 of 11

4 class Student: public Person private: string id; string programme; int year; public: Student(Person, string, string, int); ; virtual string getprogramme(); virtual int getyear(); virtual void display(); Person::Person(string thename, string theaddress): name(thename), address(theaddress) void Person::display() cout << "Name: " << name << endl; cout << "Address: " << address << endl; Student::Student(Person p, string theid, string theprog, int theyear): Person(p), id(theid), programme(theprog), year(theyear) void Student::display() Person::display(); cout << "ID: " << id << endl; cout << "programme: " << programme << endl; cout << "Year: " << year << endl; bool Student::isDoingModule(string amodule) return true; int Student::getYear() return year; string Student::getProgramme() return programme; (b) [12 marks 2 per method] void main(void) Person p("derek Molloy", "Dublin."); Student s(p, " ", "EE", 3); s.display(); [5 marks] Page 4 of 11

5 (c) 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. public: Student(Person, string, string, int); Student(Person, string, string ); ; Student::Student(Person p, string theid, string theprog, int theyear): Person(p), id(theid), programme(theprog), year(theyear) Student::Student(Person p, string theid, string theprog): Person(p), id(theid), programme(theprog), year(1) [5 marks] (d) Default parameters can be added to the end of a parameter list. If a value is to be passed to a particular default parameter, then the preceding default parameters must also be passed. For example: void decrement(int &avalue, int amount=1) avalue = avalue - amount; [3 marks] Page 5 of 11

6 Question 3. (a) What are abstract classes? How does the notation differ from C++ to Java? 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. For example: In the Vehicle class example previously the draw() method may be defined as abstract as it is not really possible to draw a generic vehicle. By doing this we are forcing all derived classes to write a draw() method if they are to be instantiated. virtual string getaccounttype() = 0; The assignment = 0 allows the method to be defined as abstract. This means that no implementation will be present for that method, in this case the getaccounttype() method. While abstract classes are not much different to abstract classes in C++, there are some points to note. Once again, abstract methods are a place-holder that define a set of operations within the class that have not yet been implemented. In Java you can actually set a class to be abstract, even if it has no abstract methods. This would prevent this class from being instantiated. However, if one or more methods in a class are abstract then the class must be defined as abstract. [12 marks] [6 marks for description] [3 cpp notation] [3 java notation] Page 6 of 11

7 (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 " 21 << balance << " Euro." << endl; 22 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. This example is the same as the previous example except this time we are passing by reference: Passing by reference is like passing a copy of the name of the variable to the method. 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 " 21 << balance << " Euro." << endl; 22 [13 marks] [4 marks for each segment] [5 marks for discussion] Page 7 of 11

8 Question 4. (a) Describe the life-cycle of a Java application. In your description explain the following terms: bytecode, Java virtual machine, API, interpreter, compiler. Java programs exist in the form of compiled bytecode, that is similar to machine code, except that the platform target is the Java Virtual Machine (JVM). A JVM resides within every Java compatible WWW browser and indeed stand-alone with the Java Run-time Environment (JRE). A JVM is, in effect, a bytecode interpreting machine running on a hardware machine. This interpreting stage has an overhead and slows the program execution performance of Java applications. Java bytecode is extremely compact, allowing it to be easily delivered over a network. In theory, the JVM in each Web browser is built to the same specification, requiring that one version of source code should be compatible with many platforms, provided that a Java enabled Web browser exists for that platform. In reality, not all Web browsers implement the exact same JVM specification, introducing minor inconsistencies when the same Java applet is viewed using several different browsers, on different platforms. The Java compiler converts the.java source code to bytecode. At each stage the compiler and JVM interact with the Java library APIs (Application Programming Interface). The Java application life cycle can be illustrated as below: (b) Explain the Java terms super and this. [10 marks total] [5 marks for description and diagram] [1 mark for each point mentioned in the description] Page 8 of 11

9 super - allows us to refer to 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 - allows us to refer to the object of this class that we are 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. (c) Explain the Java classes Object class and Class class. [6 marks] [3 marks each] 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. (d) What is a Java interface? [6 marks] [3 marks each] 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. [3 marks] Question 5. (a) (a) Explain the terms FlowLayout, GridLayout and BorderLayout using diagrams. [13 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. Page 9 of 11

10 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) 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: Page 10 of 11

11 (c) Explain the following segment of code in detail: 1 public void paint(graphics g) 2 3 g.setcolor(color.red); 4 g.drawline(10,10,50,50); 5 [6 marks] [2 marks each] The paint method is publically accessible outside of this class. It accepts a Graphics object g. This object is passed to the method by the VM whenever the area of the applet needs to be repainted. The paint method does not return a value. The setcolor method is called on the graphics context object. This sets the paintbrush colour to the Color object that is passed. In this case the Colour objects is a static Color object obtained from the Color class. The drawline method is also called on the graphics context object, i.e. it is a method of the Graphics class. It accepts 4 int parameters, the x,y start point and the x,y end point. In this case a line will be drawn at 45 degrees. [9 marks] [3 marks for each line] Page 11 of 11