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【转】接口 与 抽象类

本文转自:http://blog.dic123.com/article.asp?id=89
介绍:
   在本文中,我将借一个DEMO讨论下接口和抽象类。抽象类和接口的的概念对初学面向对象编程的人来说,总容易迷惑。所以,我试着讨论下两者的
理论并比较他们的使用。最后我将演示下怎么用C#使用它们。

背景:

一个抽象类在没有执行时就象一个接口一样。但在它们间有很多不同点,下面就解释它们的概念,比较他们的相似点和不同点。

什么是抽象类?

一个抽象类是一种特殊的类,它无法实例化。所以,我们会问,那我们为什么还需要一个无法实例化的类呢?一个抽象类仅用来被继承的,即它只
允许其他类继承它,但自己不能实例化。对继承的子集,抽象类的优势是增强了某些层次结构。简而言之,它是一种契约,使所有继承它的子类都带有
一样的层次结构或标准。

什么是接口?

接口不是一个类。一个接口没有执行机制。它只能有一个信号,换句话说,它只能够定义方法名字,方法怎么实现的一无所有。有一点是和抽象类
相似的,它也具有一种契约,接口被用来定义所有子类的结构,或者说是定义一套子类方法。它们间的主要区别就是,一个类能实现或执行多个接口,
而只能从一个抽象类继承。在C#中,不支持向C++那样一个类可以多重继承,但接口的运用,从另外的角度看,以用来实现了多继承。


接口和抽象类:

   当我们创建一个接口,相当于我们基本上创建了一套没有执行的方法,需要我们在子类中过载(Overriden)实现。这样做有一个非常显著的优势,
它提供了一种方法,使一个类可以成为两个类的一部分.
   当我们创建一个抽象类,相当于我们创建了一个基类,它拥有一个或着多个完整的方法,  
   但至少有一个方法没有实现,即被声明为抽象方法。若所有的方法都没实现,那它和接口的功能是一样的,只是它还遵守无法被继承的规则。
   抽象类的目的是提供了一个基本类,定义了子类将如何设计,允许程序员在子类中实现这些方法。





特点
接 口
        抽象类

多继承
一个类能从几个接口继承
一个类只能继承于一个抽象类            

实现方式
一个接口没有任何实现的代码只显示方法名
抽象类可以提供完整的方法实现的代码

核心与外围
接口用来定义类的外围功能.
比如人类和交通工具两个类都能从IMOVABLE接口继承。  
抽象类多用来定义一个类的核心层

使用场合
如果很多实现都共享一些方法,则      

用接口比较
如果很多实现使用同一系列方法,使用一样的属性,则用抽象类较好

速度  
要求更多的时间,去找到实际的子类实现的方法
比较快

功能扩展性
如果我们要给接口添加一个方法,我们要捕捉所有使用该接口的实现子类,并分别添加一个新的方法,并实现它.
如果我们要给抽象类添加一个新的方法我们可以在抽象实现,也可以在子类实现




代码实例:

用代码来实现会更简单些。这里有一个Employee抽象类,和一个IEmployee接口。我会用实现的对象分别继承于Employee和IEmployee,Emp_Fulltime继承于Employee,Emp_Fulltime2继承于IEmployee。

在测试中,我创建两者的实例,并设置他们的属性,调用一个calculateWage的方法。

Abstract Class Employee
1 using System;
2
3 namespace AbstractsANDInterfaces
4 {
5     ///
6
7     /// Summary description for Employee.
8
9     ///
10
11    
12     public abstract class Employee
13     {
14         //we can have fields and properties
15
16         //in the Abstract class
17
18         protected String id;
19         protected String lname;
20         protected String fname;
21
22         //properties
23
24         public abstract String ID
25         {
26             get;
27             set;
28         }
29
30         public abstract String FirstName
31         {
32             get;
33             set;
34         }
35        
36         public abstract String LastName
37         {
38             get;
39             set;
40         }
41         //completed methods
42
43         public String Update()
44         {
45             return "Employee " + id + " " +
46                       lname + " " + fname +
47                       " updated";
48         }
49         //completed methods
50
51         public String Add()
52         {
53             return "Employee " + id + " " +
54                       lname + " " + fname +
55                       " added";
56         }
57         //completed methods
58
59         public String Delete()
60         {
61             return "Employee " + id + " " +
62                       lname + " " + fname +
63                       " deleted";
64         }
65         //completed methods
66
67         public String Search()
68         {
69             return "Employee " + id + " " +
70                       lname + " " + fname +
71                       " found";
72         }
73
74         //abstract method that is different
75
76         //from Fulltime and Contractor
77
78         //therefore i keep it uncompleted and
79
80         //let each implementation
81
82         //complete it the way they calculate the wage.
83
84         public abstract String CalculateWage();
85        
86     }
87 }
88




Interface Employee  1 using System;
2
3
4 namespace AbstractsANDInterfaces
5 {
6     /// <summary>
7
8     /// Summary description for IEmployee.
9
10     /// </summary>
11
12     public interface IEmployee
13     {
14         //cannot have fields. uncommenting
15
16         //will raise error!
17
18 //        protected String id;
19
20 //        protected String lname;
21
22 //        protected String fname;
23
24
25         //just signature of the properties
26
27         //and methods.
28
29         //setting a rule or contract to be
30
31         //followed by implementations.
32
33         String ID
34         {
35             get;
36             set;
37         }
38
39         String FirstName
40         {
41             get;
42             set;
43         }
44        
45         String LastName
46         {
47             get;
48             set;
49         }
50        
51         // cannot have implementation
52
53         // cannot have modifiers public
54
55         // etc all are assumed public
56
57         // cannot have virtual
58
59
60         String Update();
61
62         String Add();
63
64         String Delete();
65
66         String Search();
67
68         String CalculateWage();
69     }
70 }
71






Inherited Objects
Emp_Fulltime:

  1 using System;
  2
  3 namespace AbstractsANDInterfaces
  4 {
  5     ///
  6
  7     /// Summary description for Emp_Fulltime.
  8
  9     ///
10
11      
12     //Inheriting from the Abstract class
13
14     public class Emp_Fulltime : Employee
15     {
16         //uses all the properties of the
17
18         //Abstract class therefore no
19
20         //properties or fields here!
21
22
23         public Emp_Fulltime()
24         {
25         }
26
27
28         public override String ID
29         {
30             get
31             {
32                 return id;
33             }
34             set
35             {
36                 id = value;
37             }
38         }
39        
40         public override String FirstName
41         {
42             get
43             {
44                 return fname;
45             }
46             set
47             {
48                 fname = value;
49             }
50         }
51
52         public override String LastName
53         {
54             get
55             {
56                 return lname;
57             }
58             set
59             {
60                 lname = value;
61             }
62         }
63
64         //common methods that are
65
66         //implemented in the abstract class
67
68         public new String Add()
69         {
70             return base.Add();
71         }
72         //common methods that are implemented
73
74         //in the abstract class
75
76         public new String Delete()
77         {
78             return base.Delete();
79         }
80         //common methods that are implemented
81
82         //in the abstract class
83
84         public new String Search()
85         {
86             return base.Search();
87         }
88         //common methods that are implemented
89
90         //in the abstract class
91
92         public new String Update()
93         {
94             return base.Update();
95         }
96        
97         //abstract method that is different
98
99         //from Fulltime and Contractor
100
101         //therefore I override it here.
102
103         public override String CalculateWage()
104         {
105             return "Full time employee " +
106                   base.fname + " is calculated " +
107                   "using the Abstract class";
108         }
109     }
110 }
111

Emp_Fulltime2

  1 using System;
  2
  3 namespace AbstractsANDInterfaces
  4 {
  5     ///
  6
  7     /// Summary description for Emp_fulltime2.
  8
  9     ///
10
11    
12     //Implementing the interface
13
14     public class Emp_fulltime2 : IEmployee
15     {
16         //All the properties and
17
18         //fields are defined here!
19
20         protected String id;
21         protected String lname;
22         protected String fname;
23
24         public Emp_fulltime2()
25         {
26             //
27
28             // TODO: Add constructor logic here
29
30             //
31
32         }
33
34         public String ID
35         {
36             get
37             {
38                 return id;
39             }
40             set
41             {
42                 id = value;
43             }
44         }
45        
46         public String FirstName
47         {
48             get
49             {
50                 return fname;
51             }
52             set
53             {
54                 fname = value;
55             }
56         }
57
58         public String LastName
59         {
60             get
61             {
62                 return lname;
63             }
64             set
65             {
66                 lname = value;
67             }
68         }
69
70         //all the manipulations including Add,Delete,
71
72         //Search, Update, Calculate are done
73
74         //within the object as there are not
75
76         //implementation in the Interface entity.
77
78         public String Add()
79         {
80             return "Fulltime Employee " +
81                           fname + " added.";
82         }
83
84         public String Delete()
85         {
86             return "Fulltime Employee " +
87                         fname + " deleted.";
88         }
89
90         public String Search()
91         {
92             return "Fulltime Employee " +
93                        fname + " searched.";
94         }
95
96         public String Update()
97         {
98             return "Fulltime Employee " +
99                         fname + " updated.";
100         }
101        
102         //if you change to Calculatewage().
103
104         //Just small 'w' it will raise
105
106         //error as in interface
107
108         //it is CalculateWage() with capital 'W'.
109
110         public String CalculateWage()
111         {
112             return "Full time employee " +
113                   fname + " caluculated using " +
114                   "Interface.";
115         }
116     }
117 }
118


Code for testing
1 private void InterfaceExample_Click(object sender,
2                                 System.EventArgs e)
3 {
4     try
5     {
6
7         IEmployee emp;
8
9         Emp_fulltime2 emp1 = new Emp_fulltime2();
10         //has to be casted because of the interface!
11
12         emp = (IEmployee) emp1;
13         emp.ID = "2234";
14         emp.FirstName= "Rahman" ;
15         emp.LastName = "Mahmoodi" ;
16         //call add method od the object
17
18         MessageBox.Show(emp.Add().ToString());
19        
20         //call the CalculateWage method
21
22         MessageBox.Show(emp.CalculateWage().ToString());
23
24
25     }
26     catch(Exception ex)
27     {
28         MessageBox.Show(ex.Message);
29     }
30
31 }
32
33 private void cmdAbstractExample_Click(object sender,
34                                    System.EventArgs e)
35 {
36
37     Employee emp;
38     //no casting is requird!
39
40     emp = new Emp_Fulltime();
41    
42
43     emp.ID = "2244";
44     emp.FirstName= "Maria" ;
45     emp.LastName = "Robinlius" ;
46     MessageBox.Show(emp.Add().ToString());
47
48     //call the CalculateWage method
49
50     MessageBox.Show(emp.CalculateWage().ToString());
51
52 }
53

结论:

我已经解释了接口和抽象类的不同点,并用一个Demo Project 讲解了他们实现的不同点.

posted on 2008-04-25 13:49  LeeXiaoLiang  阅读(179)  评论(0编辑  收藏  举报