Object-Oriented Programming

• 1 слайд

  
  Course by Mr. Erkki Mattila, M.Sc.
  Rovaniemi University of Applied Sciences
  Introduction to Object-Oriented Programming
  Part II: OOP Concepts
  

• 2 слайд

  Course Contents
  Part I: Object-Oriented Concepts and Principles
  Part II: Object-oriented Programming
  Closer look at the concepts of OOP
  
  OOP, Rovaniemi University of Applied Sciences
  2
  

• 3 слайд

  Member Variables
  Variables defined inside a class are called member variables
  Member variables fall into two categories: instance variables and class variables (static variables)
  OOP, Rovaniemi University of Applied Sciences
  3
  

• 4 слайд

  Instance Variables
  All objects have their own copies of instance variables
  System allocates memory for instance variables at object creation time
  Instance variables can be accessed only through an instance, for example
  Vector v = new Vector(); int c=v.elementCount;
  The state of an object is specified by the values of its instance variables
  OOP, Rovaniemi University of Applied Sciences
  4
  

• 5 слайд

  Class (Static) Variables
  The runtime system allocates a class variable once per class, regardless of the number of instances created of that class
  All instances of the same class share the same copy of class variables
  The system allocates memory for a class variable the first time it encounters the class (=loads it into memory)
  You can access class variables either through an instance or through the class itself
  The latter is better style, for example String s = Integer.toString(10);
  OOP, Rovaniemi University of Applied Sciences
  5
  

• 6 слайд

  Example of Instance and Class Variables in Java
  public abstract class Shape
  {
  public static int a_numberOfShapes=0;
  
  private Color a_color;
  
  public Shape(){
  Shape.a_numberOfShapes++;
  a_color = Color.BLACK;
  }
  
  public Shape(Color c){
  Shape.a_numberOfShapes++;
  a_color = c;
  }
  }
  
  OOP, Rovaniemi University of Applied Sciences
  6
  

• 7 слайд

  Methods
  The code that describes how to perform an operation on a specific object type is called a method
  In different OOP languages, methods can also be called as functions or operations, all meaning the same thing.
  Variables and methods can be called with a common name class members
  Methods fall into two categories: instance methods and class methods
  
  OOP, Rovaniemi University of Applied Sciences
  7
  

• 8 слайд

  Instance Methods
  Both instance and class members can be referenced from an instance method
  If no instance name is given when referring to an instance variable or method, the system uses the current instance (object self-reference)
  OOP, Rovaniemi University of Applied Sciences
  8
  

• 9 слайд

  Class (Static) Methods
  A class method is a static method in some class.
  It may not access the instance variables or methods of the class, but only the other static members of the class
  A static method can access an instance variable or method only if the instance (object) name is mentioned before the variable/method name
  OOP, Rovaniemi University of Applied Sciences
  9
  

• 10 слайд

  Method Overloading
  Method overloading means having two methods with the same name in the same class that differ in the amount and/or types of their arguments.
  It is not legal to have two methods of the same name that differ only in their return types
  Method overloading is not an O-O feature; it can exist in non-object languages as well.
  Do not mix with method overriding!
  OOP, Rovaniemi University of Applied Sciences
  10
  

• 11 слайд

  Example of Method Overloading
  public class Rectangle extends Shape
  {
  private int width;
  private int height;
  
  public Rectangle() {
  width=0;
  height=0;
  }
  public Rectangle(int w, int h) {
  width=w;
  height=h;
  }
  public Rectangle(int w, int h, Color c) {
  super(c);
  width=w;
  height=h;
  }
  }
  OOP, Rovaniemi University of Applied Sciences
  11
  

• 12 слайд

  Coercion
  Coercion and overloading often go hand in hand
  Coercion occurs when an argument of one type is converted into the expected type automatically
  Example:
  public float add( float a, float b );
  int x=1, y=2;
  float z = add(x,y);
  When you call add() with int arguments, the arguments are converted into floats by the compiler (type cast)
  To avoid coersion another overloaded method with int type of arguments could be added to the class:
  public int add( int a, int b );
  
  OOP, Rovaniemi University of Applied Sciences
  12
  

• 13 слайд

  Exercise
  Look through the Java API. Find an example of overloading and explain it.
  OOP, Rovaniemi University of Applied Sciences
  13
  

• 14 слайд

  Variable Shadowing
  public class Circle extends Shape
  {
  private int radius;
  
  // Parameter radius shadows member variable radius
  public Circle(int radius) {
  this.radius=radius;
  }
  
  public Circle(int r, Color circleColor) {
  super(circleColor);
  radius=r;
  }
  }
  Shadowing means using the same names for member variables and method parameters/local variables
  Sometimes used, but better to be avoided
  OOP, Rovaniemi University of Applied Sciences
  14
  

• 15 слайд

  Naming the Member Variables
  A common way to avoid variable shadowing is to follow a naming convention for member variables
  For instance add a prefix to each member variable:
  m_ as member, or
  a_ as attribute
  The latter one might be better, because letter m can also be incorrectly associated with the word method
  Example: private int a_size;
  For local variables and parameters it is then possible to use the same name without the prefix
  How could you fix the example on the previous slide to avoid variable shadowing?
  OOP, Rovaniemi University of Applied Sciences
  15
  

• 16 слайд

  Variable Shadowing Example Fixed
  Rename member variable radius to a_radius
  Parameter radius no longer shadows the member variable
  
  public class Circle extends Shape
  {
  private int a_radius;
  
  public Circle(int radius) {
  a_radius=radius;
  }
  }
  
  OOP, Rovaniemi University of Applied Sciences
  16
  

• 17 слайд

  Object Self-reference
  In OOP languages, a special keyword is used to refer to the current instance of an object
  this in Java, C++ and C#
  Self-reference contains the object type and the value of the current object instance
  Self-reference is used to access the object whenever a reference to it is needed inside the object
  to access a member variable when shadowed by local variables
  to pass the object itself as a function argument
  to add the object itself to a collection
  to call another constructor method in the same class
  OOP, Rovaniemi University of Applied Sciences
  17
  

• 18 слайд

  Constructor
  A specialized method used to instantiate an object
  The constructor function has the same name as the class. It is never called directly, but the run-time system calls it when a new object is created
  Constructors are usually overloaded; a class contains multiple constructors, which have a different set of parameters
  OOP, Rovaniemi University of Applied Sciences
  18
  

• 19 слайд

  Destructor
  Destructor is a specialized method, which is called when an object is deleted
  Its purpose is to release the resources the object may have allocated
  Destructor cannot be overloaded and it cannot have any arguments
  Constructor and destructor methods can’t be inherited
  When a class is instantiated, the system first calls the constructor of its superclass and the superclass’s superclass until it reaches the root of the inheritance hierarchy
  OOP, Rovaniemi University of Applied Sciences
  19
  

• 20 слайд

  Constructor Example 1
  public class Rectangle extends Shape
  {
  private int width;
  private int height;
  
  public Rectangle() {
  width=0;
  height=0;
  }
  
  public Rectangle(int w, int h)
  {
  width=w;
  height=h;
  }
  
  public Rectangle(int w, int h, Color c)
  {
  super(c);
  width=w;
  height=h;
  }
  }
  OOP, Rovaniemi University of Applied Sciences
  20
  

• 21 слайд

  Constructor Example 2
  public class TestShape
  {
  public void someMethod()
  {
  Color myColor = new Color(100, 170, 15);
  
  // Call parameterless consructor
  Rectangle r1 = new Rectangle ();
  
  // Call constructor with two int type of parameters
  Rectangle r2 = new Rectangle(5,8);
  
  // Call constructor with two int and one Color type of parameters
  Rectangle r3 = new Rectangle(7, 15, myColor);
  }
  }
  
  OOP, Rovaniemi University of Applied Sciences
  21
  

• 22 слайд

  Method overriding
  Method overriding means providing a replacement method in a new class for a method inherited from the base class; i.e. we give a new implementation for an inherited superclass method in the subclass
  The replacement method must have exactly the same name, argument list and return type as the inherited method
  Overriding occurs when attributes and methods are inherited in the normal manner, but are then modified to the specific needs of the new class
  
  OOP, Rovaniemi University of Applied Sciences
  22
  

• 23 слайд

  Example of Method Overriding 1
  public abstract class Shape
  {
  public static int a_numberOfShapes=0;
  protected Color a_color;
  
  public Shape(Color c) {
  Shape.a_numberOfShapes++;
  a_color = c;
  }
  
  public long getArea() { return 0; };
  
  public abstract long getCircumference();
  
  }
  
  OOP, Rovaniemi University of Applied Sciences
  23
  

• 24 слайд

  Example of Method Overriding 2
  public class Circle extends Shape
  {
  private int a_radius;
  
  public Circle(int radius, Color circleColor) {
  super(circleColor);
  a_radius=radius;
  }
  public long getCircumference() {
  return Math.round(2* Math.PI*m_radius);
  }
  public long getArea() {
  return Math.round(Math.PI*m_radius*m_radius);
  }
  }
  
  OOP, Rovaniemi University of Applied Sciences
  24
  

• 25 слайд

  Redefinition
  Occurs when a subclass defines a method using the same name as a method in the superclass but with a different type signature
  The change in type signature (parameter list) is what differentiates redefinition from overriding
  Two different techniques to resolve the redefined name: merge model (Java, C#) and hierarchical model (C++)
  Merge model: all the currently active scopes are examined to find the best match
  Hierarchical model: scopes are examined one by one, starting from the subclass scope. If the name is defined there, the closest match in that scope will be the one selected
  Redefinition is bad programming style: easy to cause confusion and errors
  OOP, Rovaniemi University of Applied Sciences
  25
  

• 26 слайд

  Example of Redefinition
  class Parent {
  public void example(int a) {…}
  }
  class Child extends Parent {
  public void example(int a, int b) {}
  Let’s create an instance of the child and execute the method with one argument:
  Child c = new Child();
  c.example(7);
  In Java and C#, the method from the parent class will be selected
  In C++ the code will produce a compilation error. Why?
  
  
  OOP, Rovaniemi University of Applied Sciences
  26
  

• 27 слайд

  Variable Hiding
  Within a class, a member variable that has the same name as a member variable in the superclass hides the superclass's member variable
  Within the subclass, the member variable in the superclass can no longer be referenced by its simple name
  Instead, the superclass member variable must be accessed through the super operator (Java, C++)
  Variable hiding is bad programming style
  Easy to cause confusion and errors
  Just because a programming language allows you to do something, it does not always mean that it is a desirable thing to do!
  OOP, Rovaniemi University of Applied Sciences
  27
  

• 28 слайд

  Example of Variable Hiding
  class Employee {
  protected int salary;
  protected int hours;
  }
  class PartTimeEmployee extends Employee {
  protected int salary;
  public void setSalary(int sal) {
  super.salary = sal; // Sets the superclass salary
  }
  public int calculateMonthlySalary() {
  // Uses superclass hours, but subclass salary
  return hours * salary;
  }
  }
  OOP, Rovaniemi University of Applied Sciences
  28
  

• 29 слайд

  Exercise
  Consider the Java API class java.io.Writer
  What kind of examples of method overriding can you find in the subclasses of class Writer?
  Find another example of method overriding in the Java API
  OOP, Rovaniemi University of Applied Sciences
  29
  

• 30 слайд

  Access Specifiers
  (Visibility Identifiers)
  There are three parts in a class, which have different protection level:
  public part
  protected part
  private part
  An access specier is given to a class and to all members of the class
  Public variables and methods are visible to all classes
  Protected members are only visible inside the class itself and its subclasses
  OOP, Rovaniemi University of Applied Sciences
  30
  

• 31 слайд

  Access Specifiers
  Private members are only visible inside the class itself
  Public and protected parts define the class’s interface to other classes
  Constructor methods must be declared as public
  Otherwise the class cannot be instantiated as constructors are always called outside the class itself
  With the rare exception of singleton pattern
  OOP, Rovaniemi University of Applied Sciences
  31
  

• 32 слайд

  Access Specifiers
  OOP, Rovaniemi University of Applied Sciences
  32
  private method
  public method
  public variable
  private
  variable
  No access outside the class
  X
  X
  Access allowed outside the class
  Class members have access to all other members of the same class
  

• 33 слайд

  Private Data – Public Accessor methods
  Member variables are usually declared as private
  Public get and set methods are used to set and get the variable values
  Advantages of using set and get methods:
  We can give a read-only access to certain data by specifying only the get method
  We can check for the validity of the data in the set method, e.g. not allow negative values for person’s age or height
  We can ensure that required updates are done, e.g. a field value is updated on the screen as well
  We can change the data structures inside the class, yet keep its public interface untouched causing no side-effects outside the class
  OOP, Rovaniemi University of Applied Sciences
  33
  

• 34 слайд

  Checking Parameters for Validity
  Methods should be made as general as practical
  Do not set arbitrary restrictions on method parameters
  Yet most methods have restrictions on what values may be passed into their parameters
  E.g. no negative values, no null values
  Document clearly all such restrictions
  Check the validity of parameters before the execution of the method
  If the check fails, exit quickly and cleanly with an appropriate exception
  Public methods should throw an appropriate exception (Java: Null Pointer-, IllegalArgument-, IndexOutOfBounds-, etc.) For non-public methods use assertions, since you are the author of the package and therefore responsible for any illegal method calls
  
  OOP, Rovaniemi University of Applied Sciences
  34
  

• 35 слайд

  Example of Using Access Speciers with Set and Get Methods
  public class Circle extends Shape
  {
  private int radius;
  public Circle() {
  radius=0;
  }
  public int getRadius() {
  return radius;
  }
  public void setRadius(int r) {
  if (r >= 0) // Checking validity
  radius = r;
  else
  radius = 0;
  }
  }
  
  OOP, Rovaniemi University of Applied Sciences
  35
  

• 36 слайд

  Exercise: Access Specifiers - Set and Get Methods
  Modify the class Shape so that the colour will be represented with three int type of variables red, green and blue instead of a single Color type of a variable. Red, green and blue are integer values from 0 to 255.
  Make the required mapping between the new and old data structure in the setColor, getColor and constructor –methods of class Shape.
  Only change the method implementations, not the prototypes/signatures
  You can create a new Color type of an object by giving the integer values as parameters to Color class constructor
  For instance Color newColor = new Color(r, g, b);
  You can get the RGB –values from a Color type of an object using the methods getRed(), getGreen() and getBlue().
  For instance int green = newColor.getGreen();
  You may also add new set and get methods for setting and getting the attributes red, green and blue.
  What changes are needed outside of the class Shape?
  36
  OOP, Rovaniemi University of Applied Sciences
  

• 37 слайд

  
  
  Exercise: Checking Parameter Values for Validity
  Add set and get methods for the new int type of member variables red, green and blue in the modified Shape class
  Check in the set methods that the parameter value is in the range from 0 to 255. If it is not, leave the attribute value unchanged (and throw an exception or return an error code)
  OOP, Rovaniemi University of Applied Sciences
  37
  

• 38 слайд

  Using Protected Instance Variables
  Advantages
  Subclasses can modify values directly
  Slight increase in performance
  Avoid set/get function call overhead
  Disadvantages
  No validity checking
  subclass can assign illegal value
  Implementation dependent
  subclass methods more likely dependent on superclass implementation
  superclass implementation changes may result in subclass modifications
  Fragile (brittle) software
  
  38
  OOP, Rovaniemi University of Applied Sciences
  

• 39 слайд

  Abstract Class
  Abstract methods are methods that are declared but not yet implemented
  Abstract methods have name, return value and arguments, but no method body (implementation)
  Also called as deferred methods, or in C++ pure virtual methods
  Only abstract classes may have abstract methods; if a class has even one abstract method the class itself must be declared as abstract
  An abstract class cannot be instantiated, but it can be inherited
  OOP, Rovaniemi University of Applied Sciences
  39
  

• 40 слайд

  Purpose of Abstract Classes
  Allow a programmer to define common interface for a group of classes
  Allow a programmer to use polymorphic method calls (late binding)
  For example in a collection of classes representing geometric shapes, we can define a method to calculate the area of the shape in each of the subclasses Circle, Rectangle, Triangle
  Suppose a programmer defines a polymorphic variable of class Shape that will, at various times, contain instances of each different shape. Compiler will permit message getArea() to be used with this variable only if it can ensure that the message will be understood by any value that can be associated with the variable
  Solution is to declare an abstract method getArea() in the superclass Shape
  OOP, Rovaniemi University of Applied Sciences
  40
  

• 41 слайд

  Abstract Class vs. Interface
  Besides abstract methods, an abstract class can contain variable definitions and method implementations
  Can inherit other abstract classes and implement interfaces
  An interface can only contain abstract methods and constant definitions
  ”Pure abstract class”
  Can inherit other interfaces and only them
  Purpose of Interfaces
  Interfaces define and standardize the ways in which people and systems can interact with one another
  
  OOP, Rovaniemi University of Applied Sciences
  41
  

• 42 слайд

  Polymorphism
  Generally, the ability to appear in many forms
  In OOP polymorphism refers to a programming language's ability to process objects differently depending on their class
  Polymorphism is a characteristic that greatly reduces the effort required to extend an existing OO system
  
  OOP, Rovaniemi University of Applied Sciences
  42
  

• 43 слайд

  Polymorphism in OOP
  We can refer to a subclass type of an object with a superclass type of a variable
  The class of the referred object is not known at compile time
  Responds at run time according to the actual class of the referred object (late binding)
  Subclasses can override inherited superclass methods; we can have several implementations of the same method in the class hierarchy
  For example, given a base class Shape, polymorphism enables the programmer to define different getArea() method for any number of derived classes, such as Circle, Rectangle and Triangle
  No matter what shape an object is, applying the getArea() method to it will return the correct results (late binding)
  
  OOP, Rovaniemi University of Applied Sciences
  43
  

• 44 слайд

  Polymorphism Example 1
  OOP, Rovaniemi University of Applied Sciences
  Class: Shape
  getArea()
  Class: Triangle
  getArea()
  Class: Rectangle
  getArea()
  Class: Circle
  getArea()
  Base class
  Derived classes
  44
  

• 45 слайд

  Polymorphism Example 2
  A simple example of polymorphism is the method append in the Java class StringBuffer
  The argument to this method is declared as Object and thus can be any object type
  The method has the following definition:
  class StringBuffer {
  String append(object value) {
  return append( value.toString() );
  }
  }
  The method toString is defferred. It is defined in class Object and redefined in a large number of different subclasses
  Each of these definitions of toString will have slightly different effect: a Double will produce a textual representation of a numeric value; Color will generate a string that describes the red, green and blue values in the color, etc.
  OOP, Rovaniemi University of Applied Sciences
  45
  

• 46 слайд

  Exercises
  In your own words, explain polymorphism
  Consider again the Java API class java.io.Writer.
  (What kind of examples of method overriding can you find in the subclasses of class Writer?)
  When programming, you should write your objects and methods to act on instances of Writer (instead of subclass types). Why?
  
  OOP, Rovaniemi University of Applied Sciences
  46
  

• 47 слайд

  Late Binding 1
  Dynamic binding of messages (method calls) to method definitions
  Polymorphism ensures that the proper version of the method is called based on the type of the object
  Late binding only applies to instance methods; this sort of dynamic lookup does not happen for static/class methods. Why?
  OOP, Rovaniemi University of Applied Sciences
  47
  

• 48 слайд

  Late Binding 2
  Actual operations can be bound to messages as soon as the type of the object receiving the message is known
  If this is known during the compilation, an early binding occurs, otherwise the mapping is done on run-time, late binding occurs
  Early binding is typically more efficient in terms of hardware resource usage
  Late binding is a mechanism to implement polymorphism, which is one of the main principles of OOP
  Reminder: polymorphism - different type of objects invoke different methods in response to the same message (method call)
  OOP, Rovaniemi University of Applied Sciences
  48
  

• 49 слайд

  Virtual Operation
  An operation which late binding can be applied to is called a virtual operation
  In Java, all methods are virtual by default Method overriding can be prohibited by defining the method as final
  In C++ all methods are non-virtual by default. Methods must by specified as virtual by using the virtual keyword
  OOP, Rovaniemi University of Applied Sciences
  49
  

• 50 слайд

  Group Work
  Write down the names of the Shape, Triangle, Rectangle and Circle classes (see previos slide)
  What kind of attributes the classes should have (to be able to calculate the area of the shape they represent)?
  Find common attributes, if any, and move them to the common base class Shape
  Add an abstract method getArea() to the base class
  Implement the getArea() method in each of the subclasses. It should return the area of that particular shape
  Area of a triangle = ½absinC, Area of a circle = ¶r2
  Similarly add method getCircumference() to the superclass and subclasses
  OOP, Rovaniemi University of Applied Sciences
  50
  

• 51 слайд

  Group Work
  Add a new subclass for the class Shape, for instance Parallelogram ( area = side1 * side2 * sin(angle) )
  Use the functions Math.sin(angle in radians) and Math.toRadians(angle in degrees)
  Create an instance of the new class in the Test class and call the printAreaAndCircumference for it
  OOP, Rovaniemi University of Applied Sciences
  51
  

• 52 слайд

  Other Forms of Polymorphism
  The type of polymorphism described earlier is called pure polymorphism or inclusion polymorphism
  Pure polymorphism occurs when a polymorphic variable is used as an argument in a method
  Another type of polymorphism is parametric polymorphism, which is implemented in Java as generic classes and in C++ with template classes
  Method overriding and overloading can also be considered as types of polymorphism
  OOP, Rovaniemi University of Applied Sciences
  52
  

• 53 слайд

  Multiple Inheritance
  Sometimes it is tempting to inherit some attributes and methods from one class and others from another class. This is called multiple inheritance
  Multiple inheritance means having more than one direct superclass
  Multiple inheritance complicates the class hierarchy and creates potential problems in configuration control (sequences of multiple inheritance are more difficult to trace)
  OOP, Rovaniemi University of Applied Sciences
  53
  

• 54 слайд

  Problems with Multiple Inheritance
  Name ambiguity
  Inherited, different features can have the same name
  Same feature may be inherited several times
  Impact on substitutability
  Overriding a method that has been inherited from several superclasses
  deck.draw(), rectangle.draw()
  Many types of tricks invented to overcome the problems
  Make the program complicated, is it worth it?
  OOP, Rovaniemi University of Applied Sciences
  54
  

• 55 слайд

  Reference and Value Semantics
  Reference semantics: variable values are references to objects (Java)
  Assignment x=y causes a pointer copy: both x and y refer to the same object after the assignment has been done.
  Value semantics: variable values are the objects (default in C++)
  Assignment x=y causes all the field values of object y to be copied to the fields of object x. Two separate, identical objects exist after the assigment has been performed
  OOP, Rovaniemi University of Applied Sciences
  55
  

• 56 слайд

  Reference and Value Semantics
  Pure object languages use reference semantics
  Value semantic type of copying can be done with a separate cloning method.
  Hybrid languages (such as C++) contain a special type for handling object references: a pointer type
  Assignment xPointer = yPointer causes a pointer copy: both xPointer and yPointer refer to the same object after the assigment has been done.
  OOP, Rovaniemi University of Applied Sciences
  56
  

• 57 слайд

  Copying Objects
  Pointer copy
  According to reference semantics, creates a new reference to the same object
  Occurs when assignment operator is applied to pointers in C++, or to object references in Java
  Shallow copy
  Creates a separate copy of the object
  Pointer or reference members refer to the same objects as in the original object
  Default behaviour in C++ when assignment operator is applied to object values
  In Java the default clone method produces a shallow copy
  Deep copy
  Creates a separate copy of the object
  The objects referred to by pointer or reference members are also copied
  In C++ you must define a copy constructor and overload the assignment operator to enable creating a deep copy of an object
  In Java serialization can be used to create a deep copy of an object
  OOP, Rovaniemi University of Applied Sciences
  57