Object-Oriented Programming: Inheritance and Polymorphism

1. Object-Oriented Programming: Inheritance and Polymorphism

2. Introduction • Inheritance-, a form of software reuse in which a new class is created quickly and easily by absorbing an existing class’s members and customizing them with new or modified capabilities. • When creating a class, rather than declaring completely new members, you can designate that the new class inherits the members of an existing class. • The existing class is called the base class, and the new class is the derived class. • A derived class can add its own instance variables, Shared variables, properties and methods, and it can customize methods and properties it inherits.

3. Base Classes and Derived Classes • Inheritance enables an is-a relationship. • In an is-a relationship, an object of a derived class also can be treated as an object of its base class.

4. Base Classes and Derived Classes • A direct base class is the class from which a derived class explicitly inherits. • An indirect base class is inherited from two or more levels up in the class hierarchy.

5. Inheritance • Ability to create a new class from an existing class • Purpose of inheritance is reusability. • For example, each form created is inherited from the existing Form class. • New class can: • Be based on another class (base class) • Inherit the properties and methods (but not constructors) of the base class, which can be • Use the Inherits statement following the class header and prior to any comments

6. Inheritance (2 of 2) • Examine first line of code for a form in the Editor. Inherited Class: Subclass, Derived Class,Child Class Partial Public Class Form1 Inherits System.Windows.Forms.Form Original Class: Base Class, Superclass, Parent Class

7. Polymorphism • Methods having identical names, but different implementations • Radio button, check boxes, and list boxes all have a Select method—the Select method operates appropriately for its class. • Overloading — Several argument lists for calling the method • Example: MessageBox.Show method • Overriding — Refers to a method that has the same name as its base class • Method in subclass takes precedence. • A base-class method must be declared Overridable if a derived class should be allowed to override the method with a version more appropriate for that class.

8. Inheriting Properties and Methods • Public and protected data members and methods of the base class are inherited in the derived class. • Must write the method in the derived class if wanting to override the base-class method. • Can declare elements with the Protected keyword which specifies that the element is accessible only within its own class or any class derived from that class.

9. Constructors in Inheritance • There is one exception for a derived class inheriting all public and protected methods: • A subclass cannot inherit constructors from the base class. • Each class must have its own constructors . • Exception is if the only constructor needed is an empty constructor — VB automatically creates an empty constructor for all classes.

10. Constructors in Inheritance • Creating a derived-class object begins a chain of constructor calls in which the derived-class constructor, before performing its own tasks, invokes its direct base class’s constructor either explicitly (via the MyBase reference) • The original derived-class constructor’s body finishes executing last. • Each base class’s constructor initializes the base-class instance variables that are part of the derived-class object.

11. Accessing Properties • Derived class can set and retrieve base class properties by using the property accessor methods. • Call the base class constructor from the derived class constructor to use the property values from the base class. • If the constructor requires arguments, the values can be passed when the constructor is called.

12. Protected Members • A base class’s Protected members can be accessed only by members of that base class and by members of its derived classes. • In inheritance, Public members of the base class become Public members of the derived class, and Protected members of the base class become Protected members of the derived class. • A base class’s Private members are inherited but not directly accessible to its derived classes.

13. Protected Members • Derived-class methods can refer to Public and Protected members inherited from the base class simply by using the member names. • Derived-class methods cannot directly access Private members of their base class. • A derived class can change the state of Private base-class instance variables only through Public and Protected methods provided in the base class and inherited by the derived class. • In most cases, it’s better to use Private instance variables to encourage proper software engineering.

14. Protected Members • Using Protected instance variables creates several potential problems • Problems with Protected instance variables. • The derived-class object can set an inherited variable’s value directly without using a Set accessor. • Therefore, a derived-class object can assign an invalid value to the variable. • Derived-class methods are more likely to be written so that they depend on the base class’s data implementation.

15. Overriding Methods • Methods with the same name and the same argument list as the base class • Derived class (subclass) will use the new method rather than the method in the base class. • To override a method • Declare the original method with the Overridable keyword. • Declare the new method with the Overrides keyword. • The access modifier for the base-class procedure can be Private or Protected (not Public).

16. Abstract Classes and Methods • When we think of a class type, we assume that programs will create objects of that type. • In some cases, however, it’s useful to declare classes for which you never intend to instantiate objects. • Such classes are called abstract classes. • Because they’re typically used as base classes in inheritance hierarchies, we refer to them as abstract base classes. • These classes cannot be used to instantiate objects, because, as you’ll soon see, abstract classes are incomplete.

17. Abstract Classes and Methods • Abstract base classes are too general to create real objects—they specify only what’s common among derived classes. • We need to be more specific before we can create objects. • For example, if you send the Draw message to abstract class TwoDimensionalShape, it knows that two-dimensional shapes should be drawable, but it does not know what specific shape to draw, so it cannot implement a real Draw method. • Concrete classes provide the specifics that make it reasonable to instantiate objects.

18. Abstract Classes and Methods • The purpose of an abstract class is primarily to provide an appropriate base class from which other classes can inherit and thus share a common design. • In the Shape hierarchy below, derived classes inherit the notion of what it means to be a Shape—possibly including common properties such as Location, Color and Border-Thickness, and behaviors such as Draw, Move, Resize and ChangeColor.

19. Using Multiple Forms • Projects can appear more professional when using different windows for different types of information. • Often, the first form a project displays is a summary form. • Projects can contain as many forms as desired.

20. Creating New Forms • Select Add Windows Form from the Project menu and select from many installed templates.

21. Modal versus Modeless Forms • Show method displays a form as modeless — means that both forms are open and the user can navigate from one form to the other • ShowDialog method displays a new form as modal — the user must respond to the form in some way, usually by clicking a button. • No other program code can execute until the user responds to and hides or closes the modal form. • With a modeless form, the user may switch to another form in the project without responding to the form.

22. Show Method • General Form • Example • The Show method creates a form object from the specified class and displays it modelessly. The FormName is the name of the form to be displayed. FormName.Show () SummaryForm.Show ()

23. ShowDialog Method • General Form • Example • Use the ShowDialog method when you want the user to notice, respond to, and close the form before proceeding with the application. FormName.ShowDialog () SummaryForm.ShowDialog ()

24. Hiding or Closing a Form • The Close method behaves differently for a modeless form compared to a modal form. • Modeless — Close destroys the form instance and removes it from memory. • Modal — the form is only hidden. • Choosing a form’s Hide method sets the form’s Visible property to False and keeps the form instance in memory. • An example would be for a form with instructions or Help text