1
00:00:01,590 --> 00:00:06,240
‫In this lesson, you will learn the critical difference between abstract classes and interfaces.

2
00:00:06,330 --> 00:00:11,400
‫When designing applications, it is essential to know when to use an abstract class and when to use

3
00:00:11,400 --> 00:00:14,100
‫an interface and when to use neither of them.

4
00:00:14,580 --> 00:00:20,790
‫Abstract classes and interfaces seem similar in some ways, but there are key differences which will

5
00:00:20,790 --> 00:00:25,080
‫determine which is the best choice for what you are trying to achieve.

6
00:00:25,470 --> 00:00:30,840
‫Understanding the difference between an abstract class and an interface is very important, so we can

7
00:00:30,840 --> 00:00:37,200
‫be able to design loosely coupled and extensible applications, so applications that we can modify and

8
00:00:37,200 --> 00:00:39,570
‫change easily without breaking everything.

9
00:00:39,780 --> 00:00:44,790
‫We will discuss those differences and how to decide when to use which in this video.

10
00:00:44,850 --> 00:00:50,250
‫But before we do that, first, let's refresh our memory about abstract classes and interfaces.

11
00:00:50,520 --> 00:00:52,080
‫So first, abstract classes.

12
00:00:52,080 --> 00:00:58,320
‫An abstract class is an incomplete class or a class that can't be instantiated, which means we cannot

13
00:00:58,320 --> 00:00:59,610
‫create objects from it.

14
00:00:59,910 --> 00:01:07,620
‫The purpose of an abstract class is to provide a blueprint for derived classes and to set some rules

15
00:01:07,620 --> 00:01:10,680
‫like what the derived classes must implement.

16
00:01:11,010 --> 00:01:15,420
‫But why would we need a class to be marked as an abstract class to begin with?

17
00:01:15,600 --> 00:01:17,730
‫What's wrong with regular classes?

18
00:01:18,030 --> 00:01:23,730
‫Imagine we have a class motorcycle and a class car and they share some functionality to reduce code

19
00:01:23,730 --> 00:01:24,630
‫redundancy.

20
00:01:24,810 --> 00:01:33,060
‫We create a basic class called vehicle to prevent creating instances of type vehicle and to enforce

21
00:01:33,060 --> 00:01:36,960
‫every method to implement their version of the vehicle methods.

22
00:01:36,960 --> 00:01:40,050
‫We have to mark it as an abstract class.

23
00:01:40,320 --> 00:01:46,830
‫We can take advantage of abstract classes to design components and specify some level of common functionality

24
00:01:46,830 --> 00:01:49,620
‫that must be implemented by derived classes.

25
00:01:50,130 --> 00:01:53,970
‫So let's make this vehicle abstract and this will be fine.

26
00:01:54,120 --> 00:01:56,280
‫Now let's look at interfaces once again.

27
00:01:56,280 --> 00:02:01,740
‫So an interface is a contract and it can contain only method declarations.

28
00:02:01,800 --> 00:02:04,980
‫It cannot contain method definitions.

29
00:02:05,280 --> 00:02:08,160
‫They define an ability that a class has.

30
00:02:08,190 --> 00:02:14,460
‫Let's imagine we have three classes, so we have the class, bicycle, motorcycle and car.

31
00:02:14,460 --> 00:02:20,790
‫And then we want to make sure that all of these classes inherit from this class vehicle.

32
00:02:20,970 --> 00:02:26,310
‫And if you want to add self driving functionality to our car, for example, and we want you to create

33
00:02:26,310 --> 00:02:31,860
‫other classes, for example, that might create something like an airplane, it doesn't make sense to

34
00:02:31,860 --> 00:02:36,960
‫add the self driving functionality to the base class vehicle because not all of our subclasses will

35
00:02:36,960 --> 00:02:37,950
‫have this feature.

36
00:02:38,580 --> 00:02:44,640
‫It would be more convenient to add it in the form of an interface called I self driving.

37
00:02:44,760 --> 00:02:50,850
‫Then these classes that implement this interface can provide the logic to achieve it.

38
00:02:52,050 --> 00:02:55,850
‫Now let's talk about similarities between interfaces and abstract classes.

39
00:02:55,860 --> 00:02:59,770
‫Both interfaces and abstract classes can be instantiated.

40
00:02:59,790 --> 00:03:06,390
‫It means we can't create objects from them, but they both support polymorphism, meaning that we can

41
00:03:06,390 --> 00:03:10,830
‫store an object of type cat in a variable of type abstract class animal.

42
00:03:11,070 --> 00:03:18,420
‫The same thing goes for interfaces where we can store any collection like a list or a stack in a variable

43
00:03:18,420 --> 00:03:20,100
‫of type i enumerable.

44
00:03:21,000 --> 00:03:23,640
‫So now let's look at the key differences.

45
00:03:23,670 --> 00:03:31,810
‫An abstract class is designed to be inherited by subclasses that either implement or override its methods.

46
00:03:31,830 --> 00:03:36,900
‫In other words, abstract classes are either partially implemented or not implemented at all.

47
00:03:37,020 --> 00:03:40,410
‫On the other hand, interfaces are not implemented at all.

48
00:03:40,950 --> 00:03:45,180
‫Then an abstract class can't have a constructor opposite to that.

49
00:03:45,180 --> 00:03:51,570
‫Abstract classes can have constructors and fields as well as method definitions, and the implementation

50
00:03:51,570 --> 00:03:56,070
‫will be the responsibility of the class that implemented the interface.

51
00:03:57,000 --> 00:04:04,470
‫On the other hand, an abstract class may contain method, definitions and fields, and since interfaces

52
00:04:04,470 --> 00:04:09,540
‫don't have any constructors, as we mentioned, they also cannot have fields.

53
00:04:10,320 --> 00:04:16,680
‫So now classes can implement many interfaces, which is a way to achieve some sort of multi inheritance,

54
00:04:16,680 --> 00:04:20,730
‫since C-sharp is a single inheritance language and doesn't allow that by default.

55
00:04:20,940 --> 00:04:24,450
‫So a class can only inherit extent one class.

56
00:04:25,950 --> 00:04:32,430
‫So if a class implemented an interface, it must implement all of its members, such as properties,

57
00:04:32,430 --> 00:04:34,110
‫methods and so forth.

58
00:04:34,140 --> 00:04:39,960
‫On the other hand, when a class extends an abstract class, it must implement its abstract members

59
00:04:39,960 --> 00:04:40,560
‫only.

60
00:04:40,650 --> 00:04:45,090
‫So the non implemented part of the abstract class.

61
00:04:45,420 --> 00:04:47,340
‫Let's look at an analogy here.

62
00:04:47,460 --> 00:04:54,150
‫Imagine I see sharp instructor at tutorials EU where the instructor can't achieve anything unless he's

63
00:04:54,150 --> 00:05:00,630
‫an expert in a topic or she's an expert in a topic because we can't have instances of abstract classes.

64
00:05:00,750 --> 00:05:06,060
‫So an instructor would need to become an expert in a topic to teach it.

65
00:05:06,060 --> 00:05:11,730
‫So the instructor became a sharp expert, and now he or she can teach C-sharp.

66
00:05:12,270 --> 00:05:15,700
‫At some point the C shop instructor had to edit his videos.

67
00:05:15,720 --> 00:05:19,290
‫Although not all instructors need to do the same thing.

68
00:05:19,620 --> 00:05:29,190
‫So we implemented the interface I video editor for this to work, so not all instructors need to be

69
00:05:29,190 --> 00:05:34,800
‫able to edit, but the ones that implement this video editor will have to do so.

70
00:05:35,910 --> 00:05:36,330
‫All right.

71
00:05:36,330 --> 00:05:42,090
‫One should you choose inheritance over an interface when designing shop class libraries?

72
00:05:42,210 --> 00:05:47,850
‫Well, use an abstract class if you have some functionality that you want its subclasses to have.

73
00:05:47,880 --> 00:05:52,950
‫For instance, if you have a set of functions, you want subclasses to have put them in an abstract

74
00:05:52,950 --> 00:05:53,530
‫class.

75
00:05:53,550 --> 00:06:00,300
‫The abstract class is used when the derived class shares its core properties and behavior of the abstract

76
00:06:00,300 --> 00:06:03,420
‫class, the kind of behavior that actually defines the class.

77
00:06:03,690 --> 00:06:08,640
‫Use an interface if you want a general contract only behavior or functionality.

78
00:06:08,760 --> 00:06:13,260
‫Interfaces are typically loose compared to abstract classes.

79
00:06:13,290 --> 00:06:19,920
‫You wouldn't want to use interfaces in a situation where you're constantly writing the same code for

80
00:06:19,920 --> 00:06:21,870
‫all of the interface methods.

81
00:06:22,290 --> 00:06:26,370
‫Use an abstract class and define each method once.

82
00:06:26,910 --> 00:06:28,350
‫So a quick summary.

83
00:06:28,350 --> 00:06:34,650
‫When we talk about abstract classes, we defined the characteristics of an object type specifying what

84
00:06:34,650 --> 00:06:35,700
‫an object is.

85
00:06:35,940 --> 00:06:42,090
‫When we talk about an interface and define capabilities that we promise to provide, we are talking

86
00:06:42,090 --> 00:06:46,170
‫about establishing a contract about what the object can do.

87
00:06:46,380 --> 00:06:49,860
‫So what the object is abstract class.

88
00:06:49,860 --> 00:06:51,210
‫What the object can do.

89
00:06:52,230 --> 00:06:53,250
‫An interface.

90
00:06:54,090 --> 00:06:54,510
‫All right.

91
00:06:54,510 --> 00:07:01,750
‫So I hope you enjoyed and understood this summary and this overview of the two different types of approaches.

92
00:07:01,770 --> 00:07:08,550
‫And now let's go ahead and use those concepts in different examples throughout the next lectures and

93
00:07:08,550 --> 00:07:09,870
‫throughout the rest of the course.