📅  最后修改于: 2020-11-19 08:56:13             🧑  作者: Mango
定义类时,将为数据类型定义一个蓝图。这实际上并没有定义任何数据,但是确实定义了类名的含义,即,类的对象将由什么组成,以及可以对该对象执行什么操作。
对象是类的实例。构成类的方法和变量称为该类的成员。
类定义以关键字Class开头,后跟类名称。和类主体,以End Class语句结束。以下是类定义的一般形式-
[ ] [ accessmodifier ] [ Shadows ] [ MustInherit | NotInheritable ] [ Partial ] _
Class name [ ( Of typelist ) ]
[ Inherits classname ]
[ Implements interfacenames ]
[ statements ]
End Class
哪里,
attributelist是适用于该类的属性的列表。可选的。
accessmodifier定义类的访问级别,其值为-公共,受保护,朋友,受保护的朋友和私人。可选的。
阴影表示变量在基类中重新声明并隐藏了同名元素或一组重载元素。可选的。
MustInherit指定该类只能用作基类,并且不能直接从该对象创建对象,即抽象类。可选的。
NotInheritable指定该类不能用作基类。
部分表示类的部分定义。
Inherits指定要继承的基类。
实现指定类要从其继承的接口。
以下示例演示了Box类,其中包含三个数据成员,长度,宽度和高度-
Module mybox
Class Box
Public length As Double ' Length of a box
Public breadth As Double ' Breadth of a box
Public height As Double ' Height of a box
End Class
Sub Main()
Dim Box1 As Box = New Box() ' Declare Box1 of type Box
Dim Box2 As Box = New Box() ' Declare Box2 of type Box
Dim volume As Double = 0.0 ' Store the volume of a box here
' box 1 specification
Box1.height = 5.0
Box1.length = 6.0
Box1.breadth = 7.0
' box 2 specification
Box2.height = 10.0
Box2.length = 12.0
Box2.breadth = 13.0
'volume of box 1
volume = Box1.height * Box1.length * Box1.breadth
Console.WriteLine("Volume of Box1 : {0}", volume)
'volume of box 2
volume = Box2.height * Box2.length * Box2.breadth
Console.WriteLine("Volume of Box2 : {0}", volume)
Console.ReadKey()
End Sub
End Module
编译并执行上述代码后,将产生以下结果-
Volume of Box1 : 210
Volume of Box2 : 1560
类的成员函数是一个在类定义中具有其定义或原型的函数,就像其他任何变量一样。它的工作,其中它是一个成员,并且可以访问该对象的类的所有成员的类的任何对象上。
成员变量是对象的属性(从设计的角度来看),并且对它们进行私有化以实现封装。这些变量只能使用公共成员函数访问。
让我们把以上概念放在一个类中,以设置和获取不同类成员的价值-
Module mybox
Class Box
Public length As Double ' Length of a box
Public breadth As Double ' Breadth of a box
Public height As Double ' Height of a box
Public Sub setLength(ByVal len As Double)
length = len
End Sub
Public Sub setBreadth(ByVal bre As Double)
breadth = bre
End Sub
Public Sub setHeight(ByVal hei As Double)
height = hei
End Sub
Public Function getVolume() As Double
Return length * breadth * height
End Function
End Class
Sub Main()
Dim Box1 As Box = New Box() ' Declare Box1 of type Box
Dim Box2 As Box = New Box() ' Declare Box2 of type Box
Dim volume As Double = 0.0 ' Store the volume of a box here
' box 1 specification
Box1.setLength(6.0)
Box1.setBreadth(7.0)
Box1.setHeight(5.0)
'box 2 specification
Box2.setLength(12.0)
Box2.setBreadth(13.0)
Box2.setHeight(10.0)
' volume of box 1
volume = Box1.getVolume()
Console.WriteLine("Volume of Box1 : {0}", volume)
'volume of box 2
volume = Box2.getVolume()
Console.WriteLine("Volume of Box2 : {0}", volume)
Console.ReadKey()
End Sub
End Module
编译并执行上述代码后,将产生以下结果-
Volume of Box1 : 210
Volume of Box2 : 1560
类构造函数是类的特殊成员Sub,只要我们创建该类的新对象,该类便会执行。构造函数的名称为New ,并且没有任何返回类型。
以下程序解释了构造函数的概念-
Class Line
Private length As Double ' Length of a line
Public Sub New() 'constructor
Console.WriteLine("Object is being created")
End Sub
Public Sub setLength(ByVal len As Double)
length = len
End Sub
Public Function getLength() As Double
Return length
End Function
Shared Sub Main()
Dim line As Line = New Line()
'set line length
line.setLength(6.0)
Console.WriteLine("Length of line : {0}", line.getLength())
Console.ReadKey()
End Sub
End Class
编译并执行上述代码后,将产生以下结果-
Object is being created
Length of line : 6
默认构造函数没有任何参数,但是如果需要,构造函数可以具有参数。这种构造函数称为参数化构造函数。此技术可帮助您在创建对象时为对象分配初始值,如以下示例所示-
Class Line
Private length As Double ' Length of a line
Public Sub New(ByVal len As Double) 'parameterised constructor
Console.WriteLine("Object is being created, length = {0}", len)
length = len
End Sub
Public Sub setLength(ByVal len As Double)
length = len
End Sub
Public Function getLength() As Double
Return length
End Function
Shared Sub Main()
Dim line As Line = New Line(10.0)
Console.WriteLine("Length of line set by constructor : {0}", line.getLength())
'set line length
line.setLength(6.0)
Console.WriteLine("Length of line set by setLength : {0}", line.getLength())
Console.ReadKey()
End Sub
End Class
编译并执行上述代码后,将产生以下结果-
Object is being created, length = 10
Length of line set by constructor : 10
Length of line set by setLength : 6
析构函数是类的特殊成员Sub,只要其类的对象超出范围,该析构函数就会执行。
析构函数的名称为Finalize ,并且既不能返回值,也不能采用任何参数。析构函数对于在程序退出之前释放资源非常有用,例如关闭文件,释放内存等。
析构函数不能被继承或重载。
以下示例解释了析构函数的概念-
Class Line
Private length As Double ' Length of a line
Public Sub New() 'parameterised constructor
Console.WriteLine("Object is being created")
End Sub
Protected Overrides Sub Finalize() ' destructor
Console.WriteLine("Object is being deleted")
End Sub
Public Sub setLength(ByVal len As Double)
length = len
End Sub
Public Function getLength() As Double
Return length
End Function
Shared Sub Main()
Dim line As Line = New Line()
'set line length
line.setLength(6.0)
Console.WriteLine("Length of line : {0}", line.getLength())
Console.ReadKey()
End Sub
End Class
编译并执行上述代码后,将产生以下结果-
Object is being created
Length of line : 6
Object is being deleted
我们可以使用Shared关键字将类成员定义为静态成员。当我们将某个类的成员声明为Shared时,这意味着无论创建了多少个该类的对象,该成员只有一个副本。
关键字Shared表示该类仅存在成员的一个实例。共享变量用于定义常量,因为可以通过调用类而不创建实例来检索其值。
共享变量可以在成员函数或类定义之外初始化。您也可以在类定义中初始化Shared变量。
您还可以将成员函数声明为Shared。此类函数只能访问共享变量。共享功能甚至在创建对象之前就已存在。
以下示例演示共享成员的用法-
Class StaticVar
Public Shared num As Integer
Public Sub count()
num = num + 1
End Sub
Public Shared Function getNum() As Integer
Return num
End Function
Shared Sub Main()
Dim s As StaticVar = New StaticVar()
s.count()
s.count()
s.count()
Console.WriteLine("Value of variable num: {0}", StaticVar.getNum())
Console.ReadKey()
End Sub
End Class
编译并执行上述代码后,将产生以下结果-
Value of variable num: 3
面向对象编程中最重要的概念之一就是继承。继承允许我们用另一个类来定义一个类,这使得创建和维护应用程序变得更加容易。这也提供了重用代码功能和快速实现时间的机会。
创建类时,程序员可以指定新类继承现有类的成员,而不必编写全新的数据成员和成员函数。此现有类称为基类,而新类称为派生类。
一个类可以从一个以上的类或接口派生,这意味着它可以从多个基类或接口继承数据和函数。
VB.Net中用于创建派生类的语法如下-
Class
...
End Class
Class : Inherits
...
End Class
考虑一个基类Shape及其派生类Rectangle-
' Base class
Class Shape
Protected width As Integer
Protected height As Integer
Public Sub setWidth(ByVal w As Integer)
width = w
End Sub
Public Sub setHeight(ByVal h As Integer)
height = h
End Sub
End Class
' Derived class
Class Rectangle : Inherits Shape
Public Function getArea() As Integer
Return (width * height)
End Function
End Class
Class RectangleTester
Shared Sub Main()
Dim rect As Rectangle = New Rectangle()
rect.setWidth(5)
rect.setHeight(7)
' Print the area of the object.
Console.WriteLine("Total area: {0}", rect.getArea())
Console.ReadKey()
End Sub
End Class
编译并执行上述代码后,将产生以下结果-
Total area: 35
派生类继承基类的成员变量和成员方法。因此,应在创建子类之前创建超类对象。在VB.Net中,超类或基类被隐式称为MyBase 。
以下程序演示了这一点-
' Base class
Class Rectangle
Protected width As Double
Protected length As Double
Public Sub New(ByVal l As Double, ByVal w As Double)
length = l
width = w
End Sub
Public Function GetArea() As Double
Return (width * length)
End Function
Public Overridable Sub Display()
Console.WriteLine("Length: {0}", length)
Console.WriteLine("Width: {0}", width)
Console.WriteLine("Area: {0}", GetArea())
End Sub
'end class Rectangle
End Class
'Derived class
Class Tabletop : Inherits Rectangle
Private cost As Double
Public Sub New(ByVal l As Double, ByVal w As Double)
MyBase.New(l, w)
End Sub
Public Function GetCost() As Double
Dim cost As Double
cost = GetArea() * 70
Return cost
End Function
Public Overrides Sub Display()
MyBase.Display()
Console.WriteLine("Cost: {0}", GetCost())
End Sub
'end class Tabletop
End Class
Class RectangleTester
Shared Sub Main()
Dim t As Tabletop = New Tabletop(4.5, 7.5)
t.Display()
Console.ReadKey()
End Sub
End Class
编译并执行上述代码后,将产生以下结果-
Length: 4.5
Width: 7.5
Area: 33.75
Cost: 2362.5
VB.Net支持多重继承。