0 1 00:00:01,290 --> 00:00:08,460 Let's begin the chapter by looking at inheritance. 1 2 00:00:08,550 --> 00:00:11,620 And let’s begin by looking at motivation for the need for inheritance. 2 3 00:00:11,620 --> 00:00:12,200 Now, 3 4 00:00:12,210 --> 00:00:17,210 in our case study, we looked at these different users and their capabilities 4 5 00:00:17,370 --> 00:00:20,650 So, each user is basically a class. 5 6 00:00:20,820 --> 00:00:25,290 So, in this case, can you see any issue with this design? 6 7 00:00:26,040 --> 00:00:31,860 You may notice that the methods emphasized in red are exactly identical in what they do 7 8 00:00:31,890 --> 00:00:37,800 , i.e., all users can bookmark any item and then can also rate any bookmark. 8 9 00:00:38,370 --> 00:00:40,560 So, what does that mean. 9 10 00:00:40,620 --> 00:00:48,110 It means duplicate code and duplicate code implies maintenance nightmare, i.e., change in one method 10 11 00:00:48,280 --> 00:00:54,670 will need to be updated in all four of the classes. And what is the solution for this. 11 12 00:00:55,160 --> 00:00:57,480 As you must have guessed, it is inheritance! 12 13 00:00:57,810 --> 00:00:59,730 So, let's see what it means. 13 14 00:00:59,750 --> 00:01:05,790 Also, just note that postAReview method is not common to all of the classes as a review written by 14 15 00:01:05,790 --> 00:01:11,620 a regular user needs approval from a content editor in order to be published on the Website. 15 16 00:01:12,080 --> 00:01:16,930 We know that it is not required for other users who are staff members 16 17 00:01:18,300 --> 00:01:24,460 Inheritance is one of the fundamental features of object-oriented programming. 17 18 00:01:25,600 --> 00:01:30,300 And here is how the new design looks like after applying inheritance. 18 19 00:01:30,330 --> 00:01:37,260 Basically, what we do is we abstract out all the common features and put them only in User class, 19 20 00:01:38,150 --> 00:01:42,350 i.e., we remove all the duplicate code and move it into User class. 20 21 00:01:42,390 --> 00:01:49,050 You can see that all the common methods are now in User. User class will also include any methods that 21 22 00:01:49,050 --> 00:01:50,330 are unique to itself 22 23 00:01:50,520 --> 00:01:56,940 like the postAReview method. The remaining classes are now linked to User class via inheritance. 23 24 00:01:56,940 --> 00:01:57,740 . 24 25 00:01:57,840 --> 00:02:04,380 So, it is a special type of relation and we now call the User class as a super class and the three classes 25 26 00:02:04,500 --> 00:02:05,600 linking to it 26 27 00:02:05,640 --> 00:02:13,100 via inheritance as subclasses. A general phrase used to refer to such a design with 27 28 00:02:13,170 --> 00:02:21,210 inheritance relationship is inheritance tree or inheritance hierarchy. As the name inheritance suggests, subclasses 28 29 00:02:21,480 --> 00:02:28,350 inherit features from super classes; and features are basically members, i.e., variables and methods 29 30 00:02:28,410 --> 00:02:33,390 defined in superclass are now automatically part of subclasses. 30 31 00:02:33,600 --> 00:02:40,370 So, it is as if those methods and variables are declared in the subclasses itself. In fact, subclasses 31 32 00:02:40,530 --> 00:02:43,240 can access superclass members directly. 32 33 00:02:43,380 --> 00:02:50,370 For example, EmailAdmin can directly invoke saveWebLink method defined in User without any 33 34 00:02:50,370 --> 00:02:58,110 special operator. Note that though subclass can access superclass members, a superclass can never access members 34 35 00:02:58,110 --> 00:03:00,270 of a subclass. 35 36 00:03:00,270 --> 00:03:07,020 With this new design, we can now say that EmailAdmin inherits from User, Editor inherits from User 36 37 00:03:07,530 --> 00:03:10,960 and also ChiefEditor inherits from User. 37 38 00:03:10,970 --> 00:03:16,660 Note that this inheriting of members applies to only non-private members of superclass, 38 39 00:03:16,980 --> 00:03:21,860 but let’s not worry about accessibility for now and it will be discussed later. 39 40 00:03:21,960 --> 00:03:28,070 So, basically we don’t have duplicate code anymore as it is moved nicely into one single location 40 41 00:03:28,110 --> 00:03:34,150 called a superclass and it is automatically available to all its subclasses. 41 42 00:03:34,170 --> 00:03:39,990 In other words, we get the benefit of code re-use due to inheritance. 42 43 00:03:39,990 --> 00:03:46,520 Now if we have to make a code change, we do it only at one place, which is the superclass. 43 44 00:03:46,530 --> 00:03:52,410 Also, if superclass acquires new methods, then you can simply compile the modified 44 45 00:03:52,410 --> 00:03:58,140 superclass and replace the old .class file with this newer one and with that all 45 46 00:03:58,140 --> 00:04:01,300 all subclasses would have those new methods too, 46 47 00:04:01,590 --> 00:04:09,210 i.e., you don’t even have to touch your subclasses. Subclasses are basically specialized versions of 47 48 00:04:09,210 --> 00:04:10,630 superclasses. 48 49 00:04:10,950 --> 00:04:16,440 i.e., they can do whatever their super classes do and in addition can also do their own unique stuff 49 50 00:04:16,440 --> 00:04:17,280 . 50 51 00:04:17,310 --> 00:04:24,030 In other words, in addition to inheriting superclass features, they can also add their own members like 51 52 00:04:24,040 --> 00:04:30,100 the approveReview method or the handleEmailCampaign method or even any new fields. 52 53 00:04:30,660 --> 00:04:35,950 In addition to adding their own members, they can also override superclass methods 53 54 00:04:36,120 --> 00:04:42,730 thus redefining behavior, i.e., they can change or extend the superclass’s behavior. 54 55 00:04:42,960 --> 00:04:49,800 For example, postAReview method from User class is also inherited by subclasses, but postAReview method 55 56 00:04:49,800 --> 00:04:52,640 in subclasses work differently, 56 57 00:04:52,740 --> 00:04:57,780 i.e., when a review is posted by EmailAdmin or an Editor or a ChiefEditor, 57 58 00:04:57,780 --> 00:05:01,560 then that review requires no special approval, which is not the case 58 59 00:05:01,590 --> 00:05:06,720 with postAReview method in User. So, we need to override that behavior 59 60 00:05:06,870 --> 00:05:14,700 and that’s precisely what postAReview method in subclasses are doing. So, subclass capability would 60 61 00:05:14,730 --> 00:05:21,370 would include superclass capabilities as well as the capabilities defined in its own class. 61 62 00:05:21,720 --> 00:05:25,990 Note that sometimes a superclass is also referred to as supertype. 62 63 00:05:26,430 --> 00:05:29,840 Sometimes it is also referred to as a base class. 63 64 00:05:29,850 --> 00:05:37,260 Similarly, a subclass is also referred to as a subtype or sometimes as a derived class as it is derived 64 65 00:05:37,260 --> 00:05:40,350 from a base class. 65 66 00:05:40,350 --> 00:05:46,830 So, we have this new design after incorporating inheritance. But, can you spot any more redundancies here 66 67 00:05:46,830 --> 00:05:47,340 , 67 68 00:05:47,460 --> 00:05:50,670 i.e., do you think some more inheritance is possible here? 68 69 00:05:50,820 --> 00:05:56,640 You may notice that postAReview method is identical across all subclasses. 69 70 00:05:56,670 --> 00:06:04,560 Moreover, approveReview & rejectReview methods are common to Editor and ChiefEditor classes. So, we can 70 71 00:06:04,560 --> 00:06:07,740 apply inheritance across subclasses 71 72 00:06:08,660 --> 00:06:16,020 And here is the new design. postAReview method has been moved into a new superclass called Staff, which will 72 73 00:06:16,020 --> 00:06:18,940 now be a subclass of User. 73 74 00:06:19,020 --> 00:06:25,260 We have also made ChiefEditor a subclass of Editor and the two common methods approveReview & rejectReview 74 75 00:06:25,260 --> 00:06:27,870 are moved into Editor. 75 76 00:06:28,020 --> 00:06:29,510 So that's the final design. 76 77 00:06:29,700 --> 00:06:33,430 and the inheritance tree is now three levels deep. 77 78 00:06:33,720 --> 00:06:40,530 Note that a class would inherit members from all its super classes in the hierarchy, i.e., inheriting 78 79 00:06:40,590 --> 00:06:47,940 members is not restricted to only its direct superclass. For example, ChiefEditor would inherit members 79 80 00:06:47,940 --> 00:06:48,980 from Editor, 80 81 00:06:48,990 --> 00:06:51,250 Staff, and also User. 81 82 00:06:51,710 --> 00:06:58,650 However, it doesn’t inherit from EmailAdmin as EmailAdmin is in a separate branch and so is not one 82 83 00:06:58,650 --> 00:07:00,710 of its superclasses. 83 84 00:07:00,930 --> 00:07:07,140 So, we have seen how inheritance has helped us to remove duplicate code. There are few more very 84 85 00:07:07,140 --> 00:07:11,750 exotic advantages of inheritance and we will look at them soon. 85 86 00:07:12,120 --> 00:07:14,900 So, we looked at inheritance from a design standpoint. 86 87 00:07:14,910 --> 00:07:19,650 Next, let's see how this can be translated into code. 87 88 00:07:19,810 --> 00:07:25,460 Now every object-oriented programming language has its own way of implementing inheritance. 88 89 00:07:25,650 --> 00:07:32,700 In java, keyword called “extends” is used in the class declaration to build an inheritance relationship 89 90 00:07:32,700 --> 00:07:33,180 , 90 91 00:07:33,600 --> 00:07:39,430 i.e., a subclass would use the keyword extend to inherit from a superclass. 91 92 00:07:39,470 --> 00:07:42,960 Here are the class declarations for our classes. 92 93 00:07:42,990 --> 00:07:50,040 User class is at the topmost level and hence does not use “extends” keyword. However, Staff is a subclass 93 94 00:07:50,040 --> 00:07:56,010 of User and hence its declaration includes the keyword “extends” followed by the superclass name, which 94 95 00:07:56,010 --> 00:07:57,300 is User. 95 96 00:07:57,300 --> 00:08:04,620 Similarly, EmailAdmin & Editor extend Staff while ChiefEditor extends Editor 96 97 00:08:05,130 --> 00:08:09,380 Note that a class can extend from only one class. 97 98 00:08:09,540 --> 00:08:11,070 So that's the rule!! 98 99 00:08:11,610 --> 00:08:12,820 So, that’s about it 99 100 00:08:12,860 --> 00:08:16,470 and we have seen how inheritance is implemented. 100 101 00:08:16,470 --> 00:08:22,550 Next, we will learn about some really important stuff related to inheritance and we will also be testing 101 102 00:08:22,650 --> 00:08:25,670 all of the inheritance stuff by writing some code. 102 103 00:08:25,980 --> 00:08:26,430 Thank you.