1 00:00:01,130 --> 00:00:08,850 Not look at polymorphism it is very likely that you must have heard of book inheritance as well as polymorphism 2 00:00:08,970 --> 00:00:11,610 even before you enrolled in this course. 3 00:00:11,610 --> 00:00:18,390 We already a report inheritance and polymorphism is another awesome feature of object oriented programming 4 00:00:19,110 --> 00:00:21,430 and it exists Jadoo inheritance. 5 00:00:21,600 --> 00:00:23,270 That is because we have inheritance. 6 00:00:23,430 --> 00:00:26,640 We are able to take advantage of polymorphism. 7 00:00:26,640 --> 00:00:31,340 So let's look at what polymorphism is and what benefits we can have from it. 8 00:00:32,070 --> 00:00:35,840 Every class defines a contract through its methods. 9 00:00:36,390 --> 00:00:40,310 That is it and says that I have these kinds of methods. 10 00:00:40,310 --> 00:00:46,870 Now if the class is public and if it has public methods then the contract is the API. 11 00:00:47,670 --> 00:00:54,510 Now if class is a supertype then its contract is also defining a common protocol for all its subtypes 12 00:00:54,510 --> 00:00:55,700 . 13 00:00:56,070 --> 00:01:03,430 That is it is announcing that book myself and all my suppliers have these kinds of methods and Vira 14 00:01:03,450 --> 00:01:10,170 that inheritance permits this are subtypes will inherit all the methods defined in the supertype. 15 00:01:10,350 --> 00:01:16,950 Nobody finding such a common protocol for subtypes supercop is also providing the benefit of polymorphism 16 00:01:17,450 --> 00:01:25,500 and lets see what it means this polymorphism is supertype can be assigned any of its subtypes. 17 00:01:25,770 --> 00:01:30,960 For example lets consider this method abrade profile which Abdias the profile information of a user 18 00:01:30,960 --> 00:01:31,710 . 19 00:01:31,710 --> 00:01:34,830 So the method but ometer is of pipe user. 20 00:01:34,830 --> 00:01:42,330 So obviously we can pass a user object to this method but due to polymorphism we can pass an instance 21 00:01:42,540 --> 00:01:45,570 of any subclass of the User class. 22 00:01:45,630 --> 00:01:51,390 This is possible supertype is defining a common protocol for all its suppliers. 23 00:01:51,780 --> 00:01:55,620 That is anything you can do with a supertype object within this method. 24 00:01:55,680 --> 00:02:02,070 You can also do the same with any of the objects of subtypes that is within this method. 25 00:02:02,160 --> 00:02:07,980 If you are in working a method on the user object then that same method will still be invoked even if 26 00:02:07,980 --> 00:02:11,020 an object off subtype is passed. 27 00:02:11,040 --> 00:02:13,450 So what is the benefit of this. 28 00:02:13,860 --> 00:02:19,850 One is that code is much more flexible that its code is less susceptible to change. 29 00:02:19,920 --> 00:02:21,340 And what does that mean. 30 00:02:21,570 --> 00:02:25,720 For example user class can acquire new cell process in future. 31 00:02:26,340 --> 00:02:30,650 But this method of profile doesnt have to change at all. 32 00:02:30,720 --> 00:02:32,340 That is it would remain the same. 33 00:02:32,730 --> 00:02:40,580 So fewer changes imply fewer bucks and the court is also locked in are no better polymorphism. 34 00:02:40,640 --> 00:02:45,750 We would have maintained four different versions of a blade profile method for the four different types 35 00:02:45,750 --> 00:02:46,920 of users. 36 00:02:47,070 --> 00:02:52,410 That is we would have one update pro-helmet method with the user's parameter on one with staffer's parameter 37 00:02:52,460 --> 00:02:53,570 and so on. 38 00:02:53,790 --> 00:03:00,050 Also with every newly added user type and you update profile method would have to be created but now 39 00:03:00,060 --> 00:03:05,210 we have only a single market that is less good and also plinker. 40 00:03:05,320 --> 00:03:13,560 So here a user can pick any form hence this feature has been named as Polly morphism in this example 41 00:03:13,580 --> 00:03:13,920 . 42 00:03:13,950 --> 00:03:19,250 The parameter is it polymorphic reference that is how it is referred to us. 43 00:03:19,410 --> 00:03:28,020 However even the method return type can be polymorphic reference so we set up with polymorphism as supertype 44 00:03:28,020 --> 00:03:31,110 can be assigned any of its subtypes. 45 00:03:31,110 --> 00:03:37,920 It means that the type of object reference and the actual object referenced can be different. 46 00:03:37,920 --> 00:03:44,760 In this case the type of object reference has to be a supertype of the actual object type that is being 47 00:03:44,760 --> 00:03:48,430 referenced in this example declaration. 48 00:03:48,460 --> 00:03:54,720 The reference type on the left is user while the actual object referenced is editor and we know that 49 00:03:54,740 --> 00:03:58,740 user is supertype of editor before polymorphism. 50 00:03:58,740 --> 00:04:04,010 All the declarations we Lubet have same type on either side of the assignment. 51 00:04:05,010 --> 00:04:08,230 Now that reference type an object type can be different. 52 00:04:08,390 --> 00:04:12,700 There is something important you need to know about method invocation. 53 00:04:13,500 --> 00:04:20,520 Compiler uses a reference pipe to decide on vedette a method can be invoked on an object reference or 54 00:04:20,520 --> 00:04:21,210 not. 55 00:04:21,600 --> 00:04:27,030 So it doesn't use the object type but it uses a reference type to make this decision. 56 00:04:27,030 --> 00:04:32,550 Here in this declaration a reference type is user an object type is editor and we get a compiler error 57 00:04:32,970 --> 00:04:37,220 when we try to invoke a prove review method on the reference variable. 58 00:04:37,620 --> 00:04:45,060 That's because compiler knows that User class which is a reference type does not have this method but 59 00:04:45,060 --> 00:04:53,670 at runtime JVM uses the actual object type to decide on which method is invoke and GBM in books. 60 00:04:53,670 --> 00:05:00,240 The more specific question of the method in the inheritance tree starting from the object type and moving 61 00:05:00,270 --> 00:05:01,730 up words in the tree. 62 00:05:02,070 --> 00:05:09,690 Let's actually look at an example it as a class diagram from orchestrally let's have we have this declaration 63 00:05:09,780 --> 00:05:11,880 that we saw earlier. 64 00:05:11,880 --> 00:05:16,230 Next we are invoking post a review method on use that variable. 65 00:05:16,320 --> 00:05:18,260 Now there are two implementations of this method. 66 00:05:18,330 --> 00:05:24,780 One in User class and Bunyan's stuff and GBM would invoke the wash and stuff. 67 00:05:24,780 --> 00:05:31,330 That's because JVM starts searching for this method in the inheritance tree starting from Ed. onwards 68 00:05:31,340 --> 00:05:31,510 . 69 00:05:31,870 --> 00:05:34,620 As editor is the object type in the declaration. 70 00:05:35,160 --> 00:05:41,300 So as it comes up the inheritance tree from Ed. It will find the method and stuff across. 71 00:05:41,500 --> 00:05:49,470 So JVM tries to find the more specific portion of them at their next offensive blink but is book then 72 00:05:49,480 --> 00:05:50,430 GBM in groups. 73 00:05:50,500 --> 00:05:57,630 The only question in the User class knowledge to assume that Syb bubbling muttered internally in gawks 74 00:05:57,810 --> 00:05:59,590 post-Arab you matter. 75 00:05:59,880 --> 00:06:03,600 Then can you guess which version of the method would be invoked. 76 00:06:03,610 --> 00:06:04,360 Would it work. 77 00:06:04,380 --> 00:06:06,240 Posted in User class. 78 00:06:06,250 --> 00:06:09,600 A sibling is also in use of class. 79 00:06:09,630 --> 00:06:16,270 No it would still invoke the poster if you mentored in the staff class and that's because JVM once again 80 00:06:16,360 --> 00:06:24,150 starts exploding from Ed. class onwards as editor is the object type that is being referenced and that's 81 00:06:24,140 --> 00:06:25,690 about polymorphism. 82 00:06:25,690 --> 00:06:30,820 The main takeaway is that polymorphism allows us to write code that is flexible. 83 00:06:31,290 --> 00:06:34,670 That is code that is less susceptible to change. 84 00:06:34,790 --> 00:06:41,680 And the second benefit is it allows us to write less code and consequently the code is cleaner. 85 00:06:41,680 --> 00:06:47,550 Let me not give a very quick more of what we learned in this demo. 86 00:06:47,660 --> 00:06:50,350 People does do things that we have learnt. 87 00:06:50,510 --> 00:06:56,810 Customer does the main polymorphism feature that allows subclass objects to be assigned to a superclass 88 00:06:56,820 --> 00:06:58,300 reference. 89 00:06:58,290 --> 00:07:02,300 Next rule does the method in location concept to save some time. 90 00:07:02,500 --> 00:07:07,450 I'm sure in the demo in my Eclipse soon I'll use switchover to eclipse too. 91 00:07:08,010 --> 00:07:14,770 So here we have three classes use and or DOS dots or Java an editor or Java just like the way we have 92 00:07:14,760 --> 00:07:18,330 in our history and user Dajjal out. 93 00:07:18,370 --> 00:07:20,520 That is a user class is the superclass. 94 00:07:20,520 --> 00:07:25,880 We know that and all three classes reside in this package called semantics here. 95 00:07:25,890 --> 00:07:34,500 Oop staff do our job in order to extenso user iron ed. class expense staff and Witan user we have this 96 00:07:34,500 --> 00:07:36,350 method called Print user type. 97 00:07:36,420 --> 00:07:44,640 All it does is simply prints the word user and stuff also has the same method but it prints stuff editor 98 00:07:44,670 --> 00:07:47,810 also has the same method and it prints editor. 99 00:07:47,820 --> 00:07:53,100 Now there is a fourth class at testor Class B which is qualis user test. 100 00:07:53,130 --> 00:07:59,590 It also has the same method print user type which has a parameter user. 101 00:07:59,580 --> 00:08:07,180 So that's a polymorphic reference type and all it does is it invokes the method print user type on the 102 00:08:07,620 --> 00:08:10,480 power of the object that is being passed here. 103 00:08:10,980 --> 00:08:16,650 And this is the main method here and user is also in the same package. 104 00:08:16,690 --> 00:08:17,900 So here. 105 00:08:17,940 --> 00:08:24,870 Initially we are declaring to be able to use our staff an editor and the reference type is user for 106 00:08:24,880 --> 00:08:32,680 all of them and for the user variable the object typist user and for the stuff variable the object is 107 00:08:32,680 --> 00:08:35,850 stuff and for editor it is editor. 108 00:08:36,000 --> 00:08:43,930 Now we are first creating an instance of user test and then we're invoking its own limited print user 109 00:08:43,920 --> 00:08:44,480 type. 110 00:08:44,740 --> 00:08:51,390 And we are passing user object directly and then in this statement We are passing the staff object and 111 00:08:51,380 --> 00:08:53,940 then the third one we are passing the editor object. 112 00:08:53,980 --> 00:09:00,320 So we are passing three different objects and the reference type is just a user. 113 00:09:00,370 --> 00:09:04,870 So we discussed this we said that that's the benefit of polymorphism. 114 00:09:04,870 --> 00:09:05,190 Right. 115 00:09:05,190 --> 00:09:11,740 We can just how one matter and in future if we add any other objects like email or admin or chief editor 116 00:09:11,980 --> 00:09:12,720 we can. 117 00:09:12,930 --> 00:09:14,980 We don't have to make any changes. 118 00:09:15,070 --> 00:09:18,450 We don't have to change this method and we can simply use the same method. 119 00:09:18,690 --> 00:09:24,990 So awfully It gives us flexible code and it's also less that you don't have to maintain through different 120 00:09:25,000 --> 00:09:27,390 methods for each of the user objects. 121 00:09:27,390 --> 00:09:34,800 So let me just run this and it prints the user when the user object got passed when the stuff got passed 122 00:09:34,880 --> 00:09:41,370 a printed staff because it invoked the print user type method in the office staff object that got passed 123 00:09:41,390 --> 00:09:41,480 . 124 00:09:41,590 --> 00:09:43,520 And then for editor it entered it. 125 00:09:43,620 --> 00:09:47,340 So that's part one of our demo. 126 00:09:47,360 --> 00:09:51,590 Now let's do the part 2. 127 00:09:52,090 --> 00:09:59,380 So user or Java has these two amateur's save a blink and post a review on Silverlink. 128 00:09:59,380 --> 00:10:02,760 It just prints this message user Colonsay a blank. 129 00:10:03,000 --> 00:10:05,860 And it is also in working the Post review method. 130 00:10:06,070 --> 00:10:13,090 Unposted if you mattered also we have this text that's getting printed user it just means that we are 131 00:10:13,090 --> 00:10:19,900 inside user class and then we are printing the name of the method unstaffed or Java has just the Post 132 00:10:19,900 --> 00:10:20,670 review matter. 133 00:10:20,670 --> 00:10:27,490 Just like in the case study and editor just has this empty matter approve review. 134 00:10:27,630 --> 00:10:34,830 And so let's just go back to user test and let's just consider this declaration here. 135 00:10:35,390 --> 00:10:38,590 And because it helps us get past the method location. 136 00:10:38,590 --> 00:10:41,480 So let me just say editor dot. 137 00:10:41,620 --> 00:10:48,150 Now you can see this the benefit of using an ID it gives this job done with all the methods that the 138 00:10:48,150 --> 00:10:49,990 user class has. 139 00:10:50,460 --> 00:10:59,860 So let me just try to prove review of is inside the editor but compiler says that there is no appropriate 140 00:10:59,940 --> 00:11:00,510 method. 141 00:11:00,580 --> 00:11:04,360 That's because compiler is looking at the reference type. 142 00:11:04,360 --> 00:11:04,650 Right. 143 00:11:04,650 --> 00:11:05,770 We discuss this. 144 00:11:05,970 --> 00:11:07,780 So it's not going to look at the object right. 145 00:11:07,810 --> 00:11:09,330 So this is not going to work. 146 00:11:09,580 --> 00:11:12,120 So let me just comment this out. 147 00:11:12,660 --> 00:11:14,700 Now let's call Ed.. 148 00:11:14,740 --> 00:11:23,430 Dr. Boorstein review and let me just comment these three out. 149 00:11:23,850 --> 00:11:24,520 OK. 150 00:11:24,580 --> 00:11:27,580 Let me just your on this. 151 00:11:27,580 --> 00:11:32,650 Now it says staff posted if you're supposed to review in the staff class has been invoked. 152 00:11:33,000 --> 00:11:39,420 So JVM is actually searching the inheritance tree starting from Ed.. 153 00:11:39,490 --> 00:11:45,280 It is going up words the words user class and it could find the method and stuff even though there was 154 00:11:45,630 --> 00:11:47,770 the same method and user. 155 00:11:48,070 --> 00:11:50,820 So that's supposed to review next. 156 00:11:51,100 --> 00:11:53,160 Let me comment this out. 157 00:11:53,160 --> 00:11:59,980 So the more specific method was accessed called Save of a blink. 158 00:12:00,720 --> 00:12:07,630 And from this now it's just invoked saber rattling in the User class. 159 00:12:07,870 --> 00:12:15,250 So that got blended and we know that see a weblink is in Durnan rocking Post review and a post review 160 00:12:15,250 --> 00:12:22,510 it did not invoke the one end user but it invoked the run and stuff we already discussed this and we 161 00:12:22,500 --> 00:12:30,460 said that once again because we are GBM is going to start its search from Editor because editor is the 162 00:12:30,450 --> 00:12:31,990 object type. 163 00:12:32,380 --> 00:12:34,920 And that's about it that's about polymorphism