1 00:00:01,050 --> 00:00:03,440 Hi there this is they don't want to lose you. 2 00:00:03,500 --> 00:00:07,360 I know let's look at what an unbounded wild card is. 3 00:00:07,380 --> 00:00:10,880 Item number three which we saw earlier with regards to Roth pipes. 4 00:00:10,890 --> 00:00:14,500 Also discuss us about unbonded Walcott's. 5 00:00:15,360 --> 00:00:22,110 We know that a genetic bite through its type parameter indicates that it can be instantiated with an 6 00:00:22,230 --> 00:00:23,330 unknown type. 7 00:00:23,370 --> 00:00:31,260 Right here is a genetic type and it can be instantiated by using parameterized types like store of string 8 00:00:31,350 --> 00:00:39,450 or sort of integer but a parameterized type like a generic type can also indicate that each type argument 9 00:00:39,540 --> 00:00:44,660 can be an unknown type and that is done using a wildcard. 10 00:00:44,790 --> 00:00:51,870 And here is an example question mark symbol here is a white card and it can be read as store off some 11 00:00:51,870 --> 00:00:53,080 pipe. 12 00:00:53,100 --> 00:00:59,900 What this means is the met that girl doesn't know or doesn't care whether you will get with the story 13 00:00:59,900 --> 00:01:04,260 of straying or sort of double or start of integer Archstone or whatever. 14 00:01:04,800 --> 00:01:09,230 Basically parameterize stripe is being gentrified here. 15 00:01:09,240 --> 00:01:15,270 Also if you're referring to this VEIDT cart as unbonded Valcartier which means that there is also something 16 00:01:15,270 --> 00:01:19,900 called bounded by cart and we will discuss it later in a separate lesson. 17 00:01:20,790 --> 00:01:28,240 No dark wild card can be used only as a type argument so it cannot be used as a barometer. 18 00:01:28,680 --> 00:01:35,010 For instance if you use it as a pipe barometer like in this example then it would be ambiguous to veto 19 00:01:35,060 --> 00:01:39,770 the two type parameters that detached type of variable is referencing. 20 00:01:40,140 --> 00:01:48,330 So we cannot use wildcard for it by parameter not that commonly it is used just part off type of method 21 00:01:48,330 --> 00:01:55,160 parameters like in the case of method go here although it can be used in assignment statements. 22 00:01:55,170 --> 00:01:58,310 It is not common to see them being used in this way. 23 00:01:58,770 --> 00:02:04,920 Here we have stored a String instance which is assigned to a variable whose reference byte uses a wildcard 24 00:02:04,930 --> 00:02:05,420 . 25 00:02:05,940 --> 00:02:11,340 Just recall the store is a class we used in one of the earlier lessons and there's also part of the 26 00:02:11,460 --> 00:02:14,670 genetics demo class. 27 00:02:14,670 --> 00:02:20,970 One question we may have is if wildcard Jannette if I use a parameterized type then why not simply use 28 00:02:21,090 --> 00:02:28,380 object of type argument as object as the most generic class as it is a superclass of all process and 29 00:02:28,380 --> 00:02:31,710 hence can be assigned to any object. 30 00:02:31,710 --> 00:02:38,490 The limitation with using object of type argument would be that you can only assign an instance that 31 00:02:38,520 --> 00:02:41,580 also has object type argument. 32 00:02:42,200 --> 00:02:47,790 And this has to be a property called invariants which we will discuss later but it applies to not just 33 00:02:47,850 --> 00:02:53,970 object type but any type like string or integer if it is stored of string. 34 00:02:54,000 --> 00:03:02,490 You can only assign is stored of string but with white card we can assign an instance of any type that 35 00:03:02,490 --> 00:03:07,430 is storing all string Ostrov integer or anything else. 36 00:03:07,560 --> 00:03:10,970 And here is the example we saw in the previous lessons Jim on there. 37 00:03:10,970 --> 00:03:14,090 Rock pipes were used as parameters. 38 00:03:14,430 --> 00:03:19,560 You may recall that this method simply returns the number of elements that are common between the two 39 00:03:19,560 --> 00:03:21,120 in coatless. 40 00:03:21,120 --> 00:03:28,140 Only difference here and this method is that raw types are now replaced by wildcards so that this be 41 00:03:28,170 --> 00:03:32,910 once again how compile time type safety. 42 00:03:32,910 --> 00:03:38,220 Now here is an important pipe see if the restriction from the compiler when using unbonded Willcocks 43 00:03:38,940 --> 00:03:42,880 we just cannot invoke any methods on the part on the great stype. 44 00:03:43,020 --> 00:03:48,630 Were those methods use the parameter of the class in the method parameter. 45 00:03:48,720 --> 00:03:54,090 The only way we can invoke those methods is by passing a non-value as argument. 46 00:03:54,090 --> 00:03:57,170 So we cannot involve them by passing any other argument. 47 00:03:57,330 --> 00:04:01,100 And if you can't do that you get a compiler not that. 48 00:04:01,090 --> 00:04:04,200 Here we are seeing classes parameter. 49 00:04:04,470 --> 00:04:09,600 Later we will look at generic methods which can have their own parameters. 50 00:04:09,600 --> 00:04:15,330 That is methods can have pyper amateurs that are completely independent of the type that armiger declared 51 00:04:15,330 --> 00:04:16,670 at the class level. 52 00:04:17,190 --> 00:04:24,270 So this is the restriction be how it is to ensure pipes if B for instance it is the same example we 53 00:04:24,270 --> 00:04:25,180 saw earlier. 54 00:04:25,230 --> 00:04:30,930 Only difference here is the statement where we are in marking the odd matter of the list. 55 00:04:30,930 --> 00:04:37,530 This really was a compulsion at her as the add method uses sic class level type barometer for the method 56 00:04:37,550 --> 00:04:39,010 parameters parameters did. 57 00:04:40,060 --> 00:04:46,540 Just imagine compiler allowing us to quantify which is an Which I guess it would also allow us to add 58 00:04:46,660 --> 00:04:51,360 a double value with the simplest Just like in the case of a rock. 59 00:04:51,880 --> 00:04:55,320 And we once again would have the same safety issues. 60 00:04:55,610 --> 00:05:02,170 So compiler ensures that we cannot invoke such methods but it would allow us to access methods that 61 00:05:02,170 --> 00:05:09,580 did not use classloader parameters as method parameters and the condensed method in this class. 62 00:05:10,150 --> 00:05:16,930 But if you do want to access methods that method parameters that how Glass-Stegall by parameters then 63 00:05:16,930 --> 00:05:23,460 you can use generic methods or bonded wild card types both of which will be discussed later on. 64 00:05:23,470 --> 00:05:29,120 One other thing to note here is that apart from not being able to add elements to list one or list do 65 00:05:29,560 --> 00:05:35,900 we cannot also make any assumptions about the Bible objects that we can get out of them. 66 00:05:35,980 --> 00:05:40,220 We do not know whether it is a string or integer or some other object. 67 00:05:40,900 --> 00:05:46,930 If we need to know the object type then once again we need to use generic methods are bounded by the 68 00:05:46,930 --> 00:05:48,320 courts. 69 00:05:48,460 --> 00:05:50,040 We can also see this in the demo. 70 00:05:50,320 --> 00:05:53,870 So let's head over to clefs for a short demo. 71 00:05:55,240 --> 00:06:00,430 OK here we are in the genetics demo class and we are going to look at this method called just get common 72 00:06:00,430 --> 00:06:02,600 elements called wildcard. 73 00:06:02,920 --> 00:06:09,030 So in the previous animal we'll get this method get common elements caught little sticking to less list 74 00:06:09,030 --> 00:06:09,990 of strengths. 75 00:06:10,090 --> 00:06:15,310 The first one had these elements ABC and the second NBCC and all this method does as a reader and a 76 00:06:15,310 --> 00:06:17,990 number of elements that are common between the tulis. 77 00:06:18,040 --> 00:06:19,150 So it would be two. 78 00:06:19,270 --> 00:06:25,850 So this meant that I was using rock types and in this method we are using wildcards unbonded wild card 79 00:06:25,860 --> 00:06:27,780 stuff the only difference. 80 00:06:27,820 --> 00:06:28,780 So here we are. 81 00:06:28,870 --> 00:06:32,230 So we looked at the signature and the lecture to see it use it. 82 00:06:32,250 --> 00:06:35,080 It uses an unbonded wild card here. 83 00:06:35,170 --> 00:06:36,410 Everything else is same. 84 00:06:36,540 --> 00:06:39,920 And if we run it it prints the value too. 85 00:06:40,140 --> 00:06:40,640 OK. 86 00:06:40,900 --> 00:06:46,900 So when you are tempted to use rock types just because you do not know or you do not care about the 87 00:06:47,050 --> 00:06:50,930 objects and the less then you would want to use unbonded right. 88 00:06:50,920 --> 00:06:54,870 COX So that would give you the compile time type safety guarantee. 89 00:06:55,050 --> 00:06:55,790 OK. 90 00:06:56,380 --> 00:06:57,360 So let's go back. 91 00:06:57,370 --> 00:07:01,930 So that's the common way in which wild cards are used and method parameters are that are used in method 92 00:07:01,930 --> 00:07:06,840 parameters and they are not commonly used like this in assignment statements. 93 00:07:07,000 --> 00:07:11,510 So here we see how a container of some type will get counted. 94 00:07:11,530 --> 00:07:17,680 If you recall it is an interface unbe or assigning a string store so String store was created and one 95 00:07:17,680 --> 00:07:21,310 of the earlier demos here does contain on all string. 96 00:07:21,370 --> 00:07:26,660 And we are instantiating it with a store stored implements container and we are setting the value Djala 97 00:07:26,730 --> 00:07:27,110 . 98 00:07:27,490 --> 00:07:29,340 And we can also move the get method. 99 00:07:29,590 --> 00:07:35,890 So we are taking this string store not all string and we are assigning it to a container of some type 100 00:07:35,910 --> 00:07:36,150 . 101 00:07:36,390 --> 00:07:37,200 OK. 102 00:07:37,210 --> 00:07:42,880 Now once this is there we really do not know what this particular object will help. 103 00:07:42,990 --> 00:07:45,200 Ok that's why it's named some store. 104 00:07:45,340 --> 00:07:52,280 And when we get the only thing we can do is we can assign it to object because this is the supertype 105 00:07:52,360 --> 00:07:53,690 all process. 106 00:07:53,740 --> 00:07:58,970 So we can assign it to this because we really do not know what this one has because it's been at off 107 00:07:58,990 --> 00:08:00,260 some time. 108 00:08:00,550 --> 00:08:06,070 But if you for sure know that it has an object of certain type like string in this case then you can 109 00:08:06,070 --> 00:08:08,840 help string but unique or type construct here. 110 00:08:08,890 --> 00:08:09,660 So that's the thing. 111 00:08:09,660 --> 00:08:11,730 But it's not commonly used in this way. 112 00:08:11,740 --> 00:08:14,440 So here is the stored element Java. 113 00:08:15,010 --> 00:08:17,760 Now let's look at the third one which is a compiler restriction. 114 00:08:17,770 --> 00:08:23,230 So here we have to list lots of integers and it has these elements 1 2 3 and 4. 115 00:08:23,560 --> 00:08:28,790 And what we want to do as we wander past is there's less to this method called aggregate we are named 116 00:08:28,790 --> 00:08:28,910 . 117 00:08:28,960 --> 00:08:32,320 It's named as invalid aggregate because it is really invalid. 118 00:08:32,410 --> 00:08:37,960 So we're passing this nice list on here and it is still here and we want to aggregate them into this 119 00:08:37,960 --> 00:08:38,640 list. 120 00:08:38,650 --> 00:08:40,680 So there will be added into this list. 121 00:08:40,690 --> 00:08:43,430 So let's look at this method. 122 00:08:43,900 --> 00:08:46,360 So we scroll down here. 123 00:08:46,510 --> 00:08:53,900 This is the method that sonor does so these are all panoramic Rice types lists one list to unnecessary 124 00:08:53,990 --> 00:08:54,250 . 125 00:08:54,490 --> 00:09:00,340 So here we are trying to invoke the argument that on list 3 on our list one and then the second statement 126 00:09:00,370 --> 00:09:01,600 and list two. 127 00:09:01,950 --> 00:09:08,830 But the compiler is giving an error here because we cannot move these methods such methods because these 128 00:09:08,860 --> 00:09:15,910 method this method uses a class level type parameter and we know that we cannot in those matters and 129 00:09:15,910 --> 00:09:22,120 that is how the compiler is ensuring that safety came back we can invoke other methods like campaigns 130 00:09:22,210 --> 00:09:24,490 or any other method which does not use. 131 00:09:24,670 --> 00:09:28,700 It does not use class level type parameter just that we can't be we in this. 132 00:09:28,750 --> 00:09:33,940 And if you really will only mean you can invoke is by using NULL value so if you are not here you can 133 00:09:33,940 --> 00:09:36,760 see that there is no compulsion at all anymore. 134 00:09:37,520 --> 00:09:44,050 And but if you're not ok with this restriction or if you're not ok with this restriction here like this 135 00:09:44,080 --> 00:09:51,400 you really want to know what element is stored like in such cases you have to use either bounded wildcards 136 00:09:51,460 --> 00:09:53,830 or you can use generic methods. 137 00:09:53,940 --> 00:09:54,450 OK. 138 00:09:54,490 --> 00:10:00,250 Our next move will look at generic methods and we would also look at this particular method with Widger 139 00:10:00,400 --> 00:10:05,940 with generic that's where we can agree to get things set up Saunders wisdom on. 140 00:10:06,100 --> 00:10:06,500 Thank you. 141 00:10:06,500 --> 00:10:09,100 And this code is also available in the resource section. 142 00:10:09,100 --> 00:10:10,240 Happy coding.