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Chapter 11: Classes and Objects

Chapter 11: Classes and Objects. Previewing the Woods Manufacturing Application. Calculates a salary for either an hourly worker or a salaried worker. Figure 11-1 Interface showing Charika’s gross pay and information. Figure 11-2 Interface showing Chris’s gross pay and information.

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Chapter 11: Classes and Objects

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  1. Chapter 11: Classes and Objects

  2. Previewing the Woods Manufacturing Application • Calculates a salary for either an hourly worker or a salaried worker Figure 11-1 Interface showing Charika’s gross pay and information Figure 11-2 Interface showing Chris’s gross pay and information Programming with Microsoft Visual Basic 2012

  3. Lesson A Objectives After studying Lesson A, you should be able to: • Explain the terminology used in object-oriented programming • Create a class • Instantiate an object • Add Property procedures to a class • Include data validation in a class • Create a default constructor • Create a parameterized constructor • Include methods other than constructors in a class Programming with Microsoft Visual Basic 2012

  4. Object-Oriented Programming Terminology • Object-oriented programming language • Uses objects to accomplish a program’s goal • Class • A pattern or blueprint for an object • Instance • An object created from a class • Instantiated • The process of creating an object from a class • Attributes • Characteristics that describe an object Programming with Microsoft Visual Basic 2012

  5. Object-Oriented Programming Terminology (cont.) • Behaviors • Methods and events that define how the object will act or react • Methods • Operations (actions) that an object is capable of performing • Events • Actions to which an object can respond • Encapsulates • To enclose in a capsule • A class encapsulates all attributes and behaviors of an object it instantiates Programming with Microsoft Visual Basic 2012

  6. Creating a Class • Two types of classes used in VB applications: • Built-in classes, such as the TextBox class • Programmer-defined classes • Class statement • Used to define a class • Defines attributes and behaviors of objects created from the class • After a class has been defined, it can be used to instantiate objects Programming with Microsoft Visual Basic 2012

  7. Creating a Class(cont.) Figure 11-3 Syntax of the Class statement Figure 11-4 Class statement entered in the TimeCard.vb class file Programming with Microsoft Visual Basic 2012

  8. Creating a Class(cont.) Figure 11-5 Syntax and examples of instantiating an object Programming with Microsoft Visual Basic 2012

  9. Example 1—A Class that Contains Public Variables Only • Norbert Pool & Spa Depot application from Chapter 10 • Input: Length, width, and depth of a pool • Calculates the volume of water required • This program was coded with a structure in Chapter 10 • The structure will be replaced with a class Programming with Microsoft Visual Basic 2012

  10. Example 1—A Class that Contains Public Variables Only (cont.) Figure 11-6 Code for the Norbert Pool & Spa Depot application (with a structure) Programming with Microsoft Visual Basic 2012

  11. Example 1—A Class that Contains Public Variables Only (cont.) Figure 11-7 Comments and Option statements entered in the class file Figure 11-8 Public variables included in the IntelliSense list Programming with Microsoft Visual Basic 2012

  12. Example 1—A Class that Contains Public Variables Only (cont.) Figure 11-9 Class statement, GetGallons function, and btnCalc_Click procedure Programming with Microsoft Visual Basic 2012

  13. Example 1—A Class that Contains Public Variables Only (cont.) Figure 11-10 Interface showing the number of gallons Programming with Microsoft Visual Basic 2012

  14. Example 2—A Class that Contains Private Variables, Public Properties, and Methods • Disadvantages of using Public variables in a class: • A class cannot control the values assigned to its Public variables • This violates the concept of encapsulation, in which class behaviors control a class’s data Programming with Microsoft Visual Basic 2012

  15. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Private Variables and Property Procedures • Private class variables • Can only be used within the class • Not visible to the rest of the application • Public property • Used to refer to (expose) the Private variable for use by other parts of the application • Property procedure • Creates a Public property Programming with Microsoft Visual Basic 2012

  16. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Private Variables and Property Procedures (cont.) • ReadOnly keyword • Indicates an application can read the property’s value • Cannot set the value • WriteOnly keyword • Indicates an application can set the property’s value • But it cannot retrieve the value • Property procedures contain blocks of code: • Get block: Retrieves the contents of the Private variable • Set block: Used to assign a value to the Private variable • Blocks may be used individually or together Programming with Microsoft Visual Basic 2012

  17. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Private Variables and Property Procedures (cont.) Figure 11-13 Syntax and examples of a Property procedure Programming with Microsoft Visual Basic 2012

  18. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Private Variables and Property Procedures (cont.) Figure 11-14 Length Property procedure entered in the class Programming with Microsoft Visual Basic 2012

  19. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Constructors • Constructor • A class method whose purpose is to initialize the class’s Private variables • Processed each time an object is created • Must be coded as a Sub procedure named New • A class can have more than one constructor • The names are the same, but the parameterLists must differ • Default constructor • A constructor without parameters Programming with Microsoft Visual Basic 2012

  20. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Constructors (cont.) • Parameterized constructor • A constructor containing one or more parameters • Method’s signature • A method name combined with an optional parameterList Programming with Microsoft Visual Basic 2012

  21. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Constructors (cont.) Figure 11-16 Statements that invoke the constructors shown in Figure 11-15 Figure 11-15 Syntax and examples of a constructor Programming with Microsoft Visual Basic 2012

  22. Methods Other than Constructors May be either Sub or Function procedures Functions return a value; Sub procedures do not Rules for naming methods: The name should be entered using Pascal case The first word in a name should be a verb Subsequent words should be nouns and adjectives Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Programming with Microsoft Visual Basic 2012

  23. Methods Other than Constructors (cont.) Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Figure 11-17 Syntax and examples of a method that is not a constructor Programming with Microsoft Visual Basic 2012

  24. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Coding the Carpets Galore Application Figure 11-18 Pseudocode for the Calculate button’s Click event procedure Figure 11-19 TryParse methods entered in the procedure Programming with Microsoft Visual Basic 2012

  25. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Coding the Carpets Galore Application (cont.) Figure 11-20 Rectangle class definition and btnCalc_Click procedure (continues) Programming with Microsoft Visual Basic 2012

  26. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Coding the Carpets Galore Application (cont.) Figure 11-20 Rectangle class definition and btnCalc_Click procedure Programming with Microsoft Visual Basic 2012

  27. Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.) Coding the Carpets Galore Application (cont.) Figure 11-21 Interface showing the square yards and cost Programming with Microsoft Visual Basic 2012

  28. Example 3—A Class that Contains a Parameterized Constructor • A parameterized constructor is simply a constructor that has parameters • When a Rectangle object is created, a parameterized constructor allows an application to specify the object’s initial values Figure 11-22 Default and parameterized constructors Programming with Microsoft Visual Basic 2012

  29. Example 3—A Class that Contains a Parameterized Constructor (cont.) Figure 11-23 Modified Rectangle class definition and btnCalc_Click procedure (continues) Programming with Microsoft Visual Basic 2012

  30. Example 3—A Class that Contains a Parameterized Constructor (cont.) Figure 11-24 Square yards and cost shown in the interface Figure 11-23 Modified Rectangle class definition and btnCalc_Click procedure Programming with Microsoft Visual Basic 2012

  31. Example 4—Reusing a Class • Rectangle class from Examples 2 and 3: • Reused here to represent a square pizza • A square is a rectangle with four equal sides • Using an object for more than one purpose saves programming time and money • An advantage of object-oriented programming Figure 11-25 Interface for the Pete’s Pizzeria application Programming with Microsoft Visual Basic 2012

  32. Example 4—Reusing a Class (cont.) Figure 11-26 Pseudocode for the Calculate button’s Click event procedure Programming with Microsoft Visual Basic 2012

  33. Example 4—Reusing a Class (cont.) Figure 11-27 btnCalc_Click procedure Figure 11-28 Number of pizza slices shown in the interface Programming with Microsoft Visual Basic 2012

  34. Lesson A Summary • The Class statement is used to define a class • Defined classes are added to a PROJECT with a .vb extension • Objects are instantiated from a defined class • The Get block allows an application to retrieve the contents of the Private variable associated with the Property procedure • The Set block allows an application to assign a value to the Private variable associated with the Property procedure Programming with Microsoft Visual Basic 2012

  35. Lesson A Summary (cont.) • A constructor initializes the variables of a class • The constructor method must be named New • The default constructor has no parameters • A class may have many parameterized constructors • A class can have methods other than constructors Programming with Microsoft Visual Basic 2012

  36. Lesson B Objectives After studying Lesson B, you should be able to: • Include a ReadOnly property in a class • Create an auto-implemented property • Overload a method in a class Programming with Microsoft Visual Basic 2012

  37. Example 5—A Class that Contains a ReadOnly Property • ReadOnlykeyword • Indicates that the property’s value can be retrieved (read) but not set (written) • The ReadOnly property gets its value from the class instead of from the application • Grade Calculator application • Returns a letter grade based on a numeric grade Figure 11-33 Interface for the Grade Calculator application Programming with Microsoft Visual Basic 2012

  38. Example 5—A Class that Contains a ReadOnly Property (cont.) Figure 11-34 ReadOnly property message Figure 11-35 CourseGrade class definition and btnDisplay_Click procedure (continues) Programming with Microsoft Visual Basic 2012

  39. Example 5—A Class that Contains a ReadOnly Property (cont.) (continued) Figure 11-36 Grade shown in the interface Figure 11-35 CourseGrade class definition and btnDisplay_Click procedure Programming with Microsoft Visual Basic 2012

  40. Example 6—A Class that Contains Auto-Implemented Properties • Auto-implemented properties feature • Enables you to specify the property of a class in one line of code • Visual Basic automatically creates a hidden Private variable that it associates with the property • It also automatically creates hidden Get and Set blocks • It provides a shorter syntax to use when creating a class • You don’t need to create the Private variable associated with a property, nor do you need to enter the property’s Get and Set blocks of code Programming with Microsoft Visual Basic 2012

  41. Example 6—A Class that Contains Auto-Implemented Properties (cont.) Figure 11-37 Syntax and examples of creating an auto-implemented property Programming with Microsoft Visual Basic 2012

  42. Example 6—A Class that Contains Auto-Implemented Properties(cont.) Figure 11-38 Modified CourseGrade class definition Programming with Microsoft Visual Basic 2012

  43. Example 7—A Class that Contains Overloaded Methods • Overloaded methods • Methods with the same name but different parameters Figure 11-39 Attributes and behaviors of an Employee object Programming with Microsoft Visual Basic 2012

  44. Example 7—A Class that Contains Overloaded Methods (cont.) Figure 11-40 Employee class definition Programming with Microsoft Visual Basic 2012

  45. Example 7—A Class that Contains Overloaded Methods (cont.) Figure 11-42 Interface for the Woods Manufacturing application Programming with Microsoft Visual Basic 2012

  46. Example 7—A Class that Contains Overloaded Methods (cont.) Figure 11-43 Pseudocode for the Calculate button’s Click event procedure Programming with Microsoft Visual Basic 2012

  47. Example 7—A Class that Contains Overloaded Methods (cont.) Figure 11-45 btnCalc_Click procedure Programming with Microsoft Visual Basic 2012

  48. Example 7—A Class that Contains Overloaded Methods (cont.) Figure 11-46 Jake’s gross pay and information shown in the interface Figure 11-47 Sherri’s gross pay and information shown in the interface Programming with Microsoft Visual Basic 2012

  49. Lesson B Summary • Use the ReadOnly keyword to create a property whose value an application can only retrieve • To specify the property of a class in one line: • Create an auto-implemented property using the syntax Public PropertypropertyNameAsdataType • To include a parameterized method in a class: • Enter the parameters between the parentheses that follow the method’s name • To create two or more methods that perform the same task but require different parameters: • Overload the methods by giving them the same name but different parameterLists Programming with Microsoft Visual Basic 2012

  50. Lesson C Objectives After studying Lesson C, you should be able to: • Create a derived class • Refer to the base class using the MyBase keyword • Override a method in the base class Programming with Microsoft Visual Basic 2012

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