接口 VS 抽象类
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. |
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
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
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
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
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
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.
注：本文是我花了半天时间，翻译自Rahman Mahmoodi的<<Abstract class versus Interface>>.
原文链接：http://www.cnblogs.com/jerrymu/archive/2007/12/14/995541.html
