1 00:00:02,160 --> 00:00:04,470 Hi there this is they don't want to lose you. 2 00:00:04,470 --> 00:00:05,600 And welcome back. 3 00:00:05,940 --> 00:00:08,340 Earlier we looked at unbonded Walcott's. 4 00:00:08,340 --> 00:00:10,730 Now let's look at bonded Walcott's. 5 00:00:11,040 --> 00:00:16,320 In this lesson will look at the motivation for bondable cuts and then the follow up lesson we will get 6 00:00:16,320 --> 00:00:18,240 into the details. 7 00:00:18,240 --> 00:00:24,420 Now if you're developing a library for your own organization or let's say for public in general then 8 00:00:24,420 --> 00:00:31,920 using of brony cuts can make your API very flexible and uming that your model parameters deal with generic 9 00:00:31,920 --> 00:00:33,210 types. 10 00:00:33,210 --> 00:00:35,990 You can notice this in the method from the utility class. 11 00:00:36,030 --> 00:00:41,760 Joe Lockhart you can deduct collections just note that this whole topic of wild cards on generic methods 12 00:00:41,790 --> 00:00:47,490 can get pretty confusing but a lot of effort has been put into explaining these topics in a very intuitive 13 00:00:47,490 --> 00:00:47,980 way. 14 00:00:48,480 --> 00:00:53,280 But still you would have to do quite a bit of brainstorming on your own and to really internalize these 15 00:00:53,280 --> 00:00:54,540 concepts. 16 00:00:54,720 --> 00:00:58,380 So nothing to be worried if you're having heart Bangert in these concepts. 17 00:00:58,530 --> 00:01:04,230 They're pretty simple if you get a good conceptual understanding of these topics which is what we aim 18 00:01:04,230 --> 00:01:05,260 to do here. 19 00:01:05,790 --> 00:01:12,100 But some extra effort unpracticed might be needed from your and also not the terminology wise. 20 00:01:12,120 --> 00:01:19,140 We can also refer to Behrami Christ types that use wild cards as simply white card types just makes 21 00:01:19,140 --> 00:01:21,490 it more clear. 22 00:01:21,850 --> 00:01:26,510 We know that genetics is invariant which helps with compile time type system. 23 00:01:26,640 --> 00:01:33,870 That is if book is a type of bookmark then list of book is not a subtype off list of bookmark just a 24 00:01:33,880 --> 00:01:35,070 fish or a memory. 25 00:01:35,070 --> 00:01:38,850 This was the example we looked at while discussing invariance. 26 00:01:39,300 --> 00:01:41,470 If genetics are not invariant. 27 00:01:41,640 --> 00:01:47,820 The list of book can be passed as argument here and within the method movie object can be added to it 28 00:01:47,830 --> 00:01:48,300 . 29 00:01:48,780 --> 00:01:54,370 We know that this can very likely result in a class Guste exception at runtime in the calling environment 30 00:01:54,900 --> 00:01:59,000 as it would be expecting the list to contain only objects. 31 00:01:59,210 --> 00:02:00,930 So type safety is broken. 32 00:02:01,070 --> 00:02:05,140 Why did generics use this invariant property. 33 00:02:05,670 --> 00:02:09,600 The invariant property also helps with type safety at compile time. 34 00:02:09,810 --> 00:02:15,370 It is also on a strict built in certain harmless scenarios where type safety is not a concern. 35 00:02:15,420 --> 00:02:20,820 For instance we may have a method called display that picks a list of bookmarks and simply displays 36 00:02:20,820 --> 00:02:21,630 them. 37 00:02:21,720 --> 00:02:24,560 The method is not going to add anything to the list. 38 00:02:24,900 --> 00:02:30,930 So you can pass a list of book are not worried that a movie object will be added to the list type safety 39 00:02:30,930 --> 00:02:32,170 is not a concern. 40 00:02:32,170 --> 00:02:34,760 And so the method is completely harmless. 41 00:02:35,100 --> 00:02:36,590 Hello digital invariance. 42 00:02:36,660 --> 00:02:41,880 We know that we cannot pass a list of book auto industry off movie as the metric parameter. 43 00:02:41,880 --> 00:02:45,230 Here is NStar bookmark. 44 00:02:45,300 --> 00:02:50,500 So one thing to do would be is to maintain separate methods for each of the parameter types. 45 00:02:50,980 --> 00:02:51,420 Hello. 46 00:02:51,420 --> 00:02:57,300 This will not compile as this matter would be considered as duplicates and that's because we know that 47 00:02:57,300 --> 00:03:03,390 at runtime data type it is for all genetic Soulik and notations will be addressed. 48 00:03:03,390 --> 00:03:10,050 So point to note here is that with generics such kind of method overloading will not work even if it 49 00:03:10,050 --> 00:03:15,870 was possible it would be very bad as we will have duplicate code which we know leads to a maintenance 50 00:03:15,870 --> 00:03:17,150 nightmare. 51 00:03:17,970 --> 00:03:20,070 So there are two solutions for this. 52 00:03:20,100 --> 00:03:24,540 One is to use an upper bound wild card which we will discuss in the next lesson. 53 00:03:24,630 --> 00:03:28,870 Basically the wild card here extends bookmark which acts as a bong. 54 00:03:29,190 --> 00:03:36,240 And with this we can invoke the display method with a list of bookmark or it list of any subtype of 55 00:03:36,240 --> 00:03:41,220 bookmark like list of book or a list of more the artist of the eyeblink. 56 00:03:41,960 --> 00:03:49,270 So not only do is to use a genetic method that bonded by parameter unbid this also we can with this 57 00:03:49,380 --> 00:03:54,760 primitive midlist of bookmark our last book or list of movie orderless tolerably. 58 00:03:55,140 --> 00:04:02,940 So book approaches are workarounds for invariance the are code much more flexible by allowing additional 59 00:04:02,940 --> 00:04:06,480 types of arguments to be passed us input. 60 00:04:06,480 --> 00:04:12,510 However Additionally they also ensure that compile time type safety is not compromised as we will see 61 00:04:12,510 --> 00:04:13,920 later. 62 00:04:13,920 --> 00:04:17,820 So you might wonder why not just use one of them all the time. 63 00:04:18,180 --> 00:04:19,370 That is all together. 64 00:04:19,400 --> 00:04:23,860 Avoid using get parametrized type like list off bookmark. 65 00:04:23,910 --> 00:04:29,580 The answer is there are certain situations where you would want to use these approaches unanswered in 66 00:04:29,580 --> 00:04:30,450 other situations. 67 00:04:30,450 --> 00:04:34,670 You cannot use them and that we will look at in the next lesson. 68 00:04:34,830 --> 00:04:39,450 Also at the end of this lesson there will be a demo of the second approach. 69 00:04:40,720 --> 00:04:45,500 Similarly let's look at another kind of restriction which is also due to invariants. 70 00:04:45,520 --> 00:04:51,190 Let's see if you have this method agregate which has a list of bookmark some not a parameter and the 71 00:04:51,190 --> 00:04:58,270 method can add things like bookmark book and movie objects to the input list for this discussion let's 72 00:04:58,270 --> 00:05:04,480 just assume that bookmark is not an abstract class and hence can be instantiated unordered as shown 73 00:05:04,480 --> 00:05:11,290 here period in rooking the matter with an other list of bookmark on the under there simply aggregates 74 00:05:11,710 --> 00:05:17,080 by adding different objects as you can see there is no type safety concern here. 75 00:05:17,140 --> 00:05:24,220 As you can pass only a list of bookmark you do invariance property nové may also want to invoke this 76 00:05:24,220 --> 00:05:31,570 method with a list of object that object is the supertype of book back on the bookmark book and movie 77 00:05:31,570 --> 00:05:33,990 objects can still be added here. 78 00:05:34,280 --> 00:05:35,580 How it'll do that in millions. 79 00:05:35,700 --> 00:05:41,820 Even this will not be possible as a list of objects cannot be assigned to a list or bookmark. 80 00:05:42,250 --> 00:05:47,040 It is not on the list off some type of book Mont. cannot be passed. 81 00:05:47,170 --> 00:05:51,560 But even a list of supertype of book bookmarks cannot be passed. 82 00:05:51,640 --> 00:05:54,010 It's got to be exact match. 83 00:05:54,090 --> 00:05:59,470 You cannot even use an upper bounded by the cart for the matter barometer here as it applies to on the 84 00:05:59,540 --> 00:06:03,140 bookmark on which subtypes. 85 00:06:03,190 --> 00:06:06,940 So the solution here is to use a lot more nearby. 86 00:06:07,540 --> 00:06:11,680 And here the key word super is used instead of excellence. 87 00:06:11,690 --> 00:06:18,490 Under this we could not invoke the method with either a list of bookmark or a list of object. 88 00:06:18,520 --> 00:06:22,050 We will also look at Lord Bernard Willcocks in the next lesson. 89 00:06:22,330 --> 00:06:28,510 No let's go ahead and do a quick demo of how the genetic method with Bond a type parameter can be used 90 00:06:28,510 --> 00:06:31,130 as an invariance book on clay. 91 00:06:31,150 --> 00:06:33,360 And it also ensures compile time type safety. 92 00:06:33,670 --> 00:06:41,840 So let's head over to equites OK here we are in the genetics class and a new method called invariants 93 00:06:41,840 --> 00:06:47,750 work on has been added and it's a generic method and it uses it bounded by parameter. 94 00:06:47,770 --> 00:06:52,220 So it has been appropriately named as in where it's worked on in the lecture we call it does display 95 00:06:52,750 --> 00:06:57,160 and this method takes a motet parameter single method parameter list of D. 96 00:06:57,190 --> 00:07:01,780 So here is the type parameter and we know that the extents number. 97 00:07:01,780 --> 00:07:09,520 So because of that we cannot pass either a list of number or list of some subtype of number like lowest 98 00:07:09,600 --> 00:07:13,040 of integer or a list of double or a list of a lot and so on. 99 00:07:13,320 --> 00:07:13,760 OK. 100 00:07:13,900 --> 00:07:19,690 So it just gives us more flexibility and it is a look at on the invariant property. 101 00:07:20,080 --> 00:07:25,570 It is use for harmless scenarios where types if B is not a concern or in the lecture we say that we 102 00:07:25,570 --> 00:07:30,910 just want to display some items in the list which is a harmless scenario that is we are not adding anything 103 00:07:30,910 --> 00:07:32,290 to the list. 104 00:07:32,290 --> 00:07:35,990 Now let's go back to the calling context which is in the main matter. 105 00:07:36,370 --> 00:07:40,430 So here it is here. 106 00:07:40,480 --> 00:07:42,040 So we're invoking them a third. 107 00:07:42,090 --> 00:07:45,040 Let me uncomment this. 108 00:07:45,190 --> 00:07:50,860 So here we are in working with an analyst of string and it is giving us a compilation error because 109 00:07:50,920 --> 00:07:55,030 we can only pass a list of number or a list of some type of number. 110 00:07:55,300 --> 00:07:56,930 OK so we can get past this. 111 00:07:57,010 --> 00:08:02,390 So let me comment that out here we are passing an analyst off number. 112 00:08:02,560 --> 00:08:02,860 OK. 113 00:08:02,860 --> 00:08:04,250 It's just perfectly fine. 114 00:08:04,360 --> 00:08:10,990 Or we can also has an integer that's also fine and we don't see any completion or. 115 00:08:11,230 --> 00:08:12,330 OK. 116 00:08:13,030 --> 00:08:15,310 Now let's get back into the matter. 117 00:08:15,940 --> 00:08:20,330 So we also said that it gives us compile time typesafe beat. 118 00:08:20,470 --> 00:08:24,970 So we should not be able to do something like a double. 119 00:08:25,300 --> 00:08:30,530 Because if we are to double then there is a chance for getting a runtime exception. 120 00:08:30,600 --> 00:08:36,250 Cost exception at runtime and we would also see that later for this at this time we get a compiler error 121 00:08:36,280 --> 00:08:36,390 . 122 00:08:36,560 --> 00:08:36,920 OK. 123 00:08:36,970 --> 00:08:45,040 Is that we cannot I guess you into the augmented because the ADD methhead uses this parameter type D 124 00:08:45,520 --> 00:08:48,190 which is nothing but this on the metric parameter. 125 00:08:48,190 --> 00:08:56,080 So because of this the add method is expecting the element that we are to be off by deep cannot be double 126 00:08:56,290 --> 00:09:00,650 and B as the boundary is a bounded type parameter here. 127 00:09:00,880 --> 00:09:08,660 So we can do that but we can we can actually do something like this so we can typecast it could be. 128 00:09:08,740 --> 00:09:13,070 So that element type is the here and we are typecasting the double B. 129 00:09:13,090 --> 00:09:17,530 And then we can order that is fine but normally you wouldn't do that. 130 00:09:17,700 --> 00:09:18,000 OK. 131 00:09:18,010 --> 00:09:18,880 Because. 132 00:09:19,150 --> 00:09:22,690 Because it can lead to an exception at runtime. 133 00:09:22,980 --> 00:09:26,790 We are compromising that typesafe be here by doing something like this. 134 00:09:27,070 --> 00:09:33,310 OK so if we go back into the into the into the IN WORKING called here. 135 00:09:33,310 --> 00:09:39,540 So let's say we have this list in dejour list of integer and we're passing it to the invariance we look 136 00:09:39,530 --> 00:09:42,710 at on matter and within the matter we are writing a double. 137 00:09:42,970 --> 00:09:48,640 So this is supposed to contain only only integers but in the method we are writing a double hand here 138 00:09:48,640 --> 00:09:54,780 we are fetching the zero at the element and the compiler will insert an implicit cast here. 139 00:09:54,800 --> 00:09:55,720 You know that. 140 00:09:55,720 --> 00:09:58,370 So here it is x we are expecting an integer. 141 00:09:58,480 --> 00:10:00,810 But what we have is a double value. 142 00:10:01,000 --> 00:10:08,050 So if we exit this so we get a crosscuts exception because the element is actually a double which has 143 00:10:08,050 --> 00:10:11,380 been inserted in the method and it cannot be cast to integer. 144 00:10:11,710 --> 00:10:17,910 OK so you normally would not do that but if I don't you have to do something like that. 145 00:10:17,980 --> 00:10:23,920 Then the typical way would be to pass the element here and then that is fine. 146 00:10:24,070 --> 00:10:31,750 Now it will not be a problem because in the invoking code we will be passing a list of integer and we 147 00:10:31,750 --> 00:10:37,290 would also be passing an integer here because we have that the type is the same type. 148 00:10:38,020 --> 00:10:39,880 So we can actually pass 149 00:10:42,520 --> 00:10:50,620 your test in this example we are passing the same list of integer but we we're also passing an angry 150 00:10:50,620 --> 00:10:51,250 value here. 151 00:10:51,250 --> 00:10:56,060 So this is perfectly fine and we won't have any issues when we run it. 152 00:10:56,530 --> 00:11:01,310 So if at all we want to add something then we can do something like this. 153 00:11:01,330 --> 00:11:03,090 It's got to be the same. 154 00:11:03,730 --> 00:11:09,110 Otherwise we get a compiler compiler error like we got here or we may get a runtime exception. 155 00:11:09,610 --> 00:11:14,380 OK so that's the invariants work and the second approach on the first approach was to use a wildcard 156 00:11:14,420 --> 00:11:14,690 . 157 00:11:14,960 --> 00:11:19,000 I'm not a proponent wildcard and we can see that we will see that in the next lesson. 158 00:11:19,240 --> 00:11:19,670 Thank you. 159 00:11:19,670 --> 00:11:22,030 And the code is available in the resources section. 160 00:11:22,090 --> 00:11:23,410 Happy coding.