1 00:00:05,656 --> 00:00:07,537 [MUSIC]. Now that we've seen how basic data types 2 00:00:07,537 --> 00:00:11,584 are represented in Java, let's turn our attention to the differences between 3 00:00:11,584 --> 00:00:17,350 pointers and references be pointers in C and references in Java. 4 00:00:17,350 --> 00:00:20,840 We're going to take a look at that in much more detail than we have so far. 5 00:00:20,840 --> 00:00:25,500 And then we're also going to look at how we organized the code for our objects in 6 00:00:25,500 --> 00:00:28,790 Java. Where do we find the methods associated 7 00:00:28,790 --> 00:00:32,740 with each of the objects? Pointers to fields in C we've seen 8 00:00:32,740 --> 00:00:40,020 before, just in the last video actually. and we've seen that the common construct 9 00:00:40,020 --> 00:00:45,016 is to have a pointer to a struct de-reference it add an offset to a 10 00:00:45,016 --> 00:00:49,480 particular field. And this is such a common situation of 11 00:00:49,480 --> 00:00:55,100 referencing a field in an object to, to which we have a pointer to a struct to 12 00:00:55,100 --> 00:00:57,740 which we have a pointer. That we have a shorthand in C, r->a. 13 00:00:57,740 --> 00:01:03,500 Okay? Which encapsulates that de-reference 14 00:01:03,500 --> 00:01:06,080 there. In Java, since all variables are 15 00:01:06,080 --> 00:01:11,760 references to objects we don't have to worry about that explicit de-reference. 16 00:01:11,760 --> 00:01:16,200 Instead we always de-reference by default, so we can use a simpler 17 00:01:16,200 --> 00:01:21,850 notation, just r.a to refer to a field within an object. 18 00:01:21,850 --> 00:01:23,680 Alright? But the thing to remember is that what's 19 00:01:23,680 --> 00:01:29,470 actually going on here, is we are taking the reference or pointer to the object 20 00:01:29,470 --> 00:01:36,700 going to to that location, adding the offset, and finding that field that way. 21 00:01:38,440 --> 00:01:43,650 Let's take a look at how we do casting in C, and contrast that to what we do in 22 00:01:43,650 --> 00:01:49,280 what we can do in Java. this is an example of code from a memory 23 00:01:49,280 --> 00:01:53,219 allocation program similar to the, to the things that we've been talking about in 24 00:01:53,219 --> 00:01:56,900 the previous section. And uh, [COUGH] what we're going to do 25 00:01:56,900 --> 00:02:01,374 here is take a pointer to a block of memory. 26 00:02:01,374 --> 00:02:06,137 we're going to cast it as a simple pointer a byte pointer. 27 00:02:06,137 --> 00:02:12,330 add in offset of number of bytes. this will point us to a new location in 28 00:02:12,330 --> 00:02:16,675 memory. And then recast that as, of, a, a 29 00:02:16,675 --> 00:02:23,560 structive type block info. Now the reason we do this, is this would 30 00:02:23,560 --> 00:02:27,000 be a case, for example, we have a block of memory. 31 00:02:27,000 --> 00:02:31,680 And we want to break it into two blocks, because we want to allocate one to 32 00:02:31,680 --> 00:02:34,435 something, and have the remaining block left over. 33 00:02:34,435 --> 00:02:40,010 so remember that our block definitions included a couple of a few variables. 34 00:02:40,010 --> 00:02:44,750 the first one in integer to represent the size and the tags of the block. 35 00:02:44,750 --> 00:02:49,540 And then two pointers to the next and previous blocks in memory. 36 00:02:49,540 --> 00:02:55,280 So the way this ends up looking in in our memory, is we started off with a pointer 37 00:02:55,280 --> 00:02:58,625 tool block that had those three fields at the front. 38 00:02:58,625 --> 00:03:02,310 Alright? The two pointers and the size and tag. 39 00:03:02,310 --> 00:03:12,330 And we've now offset that by some number of bytes x and created a new area in 40 00:03:12,330 --> 00:03:18,078 memory. That we're going to interpret as a as a 41 00:03:18,078 --> 00:03:23,380 BlockInfo struct. this is equi-, the casting is equivalent 42 00:03:23,380 --> 00:03:27,555 to putting a lens over the memory, and saying, look at this memory at this 43 00:03:27,555 --> 00:03:31,340 location. We are going to view that as a BlockInfo. 44 00:03:31,340 --> 00:03:37,660 so that means we will interpret the bits in memory in the first four bytes as the 45 00:03:37,660 --> 00:03:42,580 size in tag, and the next four bytes as the next pointer, and the next four bytes 46 00:03:42,580 --> 00:03:47,620 as the previous pointer. It's purely a reinterpretation of whats 47 00:03:47,620 --> 00:03:49,720 is in memory. And then of course we have to make sure 48 00:03:49,720 --> 00:03:54,540 to put the right values there, the right bits there to really be those pointers in 49 00:03:54,540 --> 00:03:57,990 that size. Okay, but we can do this to any region of 50 00:03:57,990 --> 00:04:01,720 memory, and cast it into any struct we want. 51 00:04:01,720 --> 00:04:07,220 All we're doing is instructing the system to view that memory in that way. 52 00:04:07,220 --> 00:04:13,440 To look at it through that lens. In Java on the other hand we can only 53 00:04:13,440 --> 00:04:18,850 have pointers or references to objects. We cannot just point to an arbitrary part 54 00:04:18,850 --> 00:04:22,110 of memory. So in, in this case I want to show an 55 00:04:22,110 --> 00:04:27,100 example of a, of a, object hierarchy. We started off with our main object. 56 00:04:27,100 --> 00:04:32,610 We defined the parent object, and then two sub-classes of that called sister and 57 00:04:32,610 --> 00:04:34,840 brother. Okay? 58 00:04:34,840 --> 00:04:40,959 this code fragment here shows things that are legal and illegal in Java. 59 00:04:40,959 --> 00:04:46,400 the ones that are bad are shown in red. but let's take a look at the beginning of 60 00:04:46,400 --> 00:04:49,259 this code. We've just defined three objects here, 61 00:04:49,259 --> 00:04:54,060 one of type parent, one of type sister, and one of type brother using the new 62 00:04:54,060 --> 00:04:58,260 construct, and assigned them to a variable of that type. 63 00:04:58,260 --> 00:05:02,190 Pretty straight forward, okay. Now let's take a look at our first cast. 64 00:05:02,190 --> 00:05:08,910 We've created a new object called sister, but put it into a pointer or a reference 65 00:05:08,910 --> 00:05:12,600 to an object of type parent. And that's okay. 66 00:05:12,600 --> 00:05:16,390 Why is that okay? Well, because sister inherits everything 67 00:05:16,390 --> 00:05:19,430 from parent. So it has all of parents feels and 68 00:05:19,430 --> 00:05:28,770 methods so we can still use all those things if we refer to the object as a 69 00:05:28,770 --> 00:05:32,702 parent. So it can, we can get away with that. 70 00:05:32,702 --> 00:05:38,570 as an example if we went in the other direction, if we had created a new 71 00:05:38,570 --> 00:05:43,850 parent, and try to put it into a variable of type sister, we have a problem. 72 00:05:43,850 --> 00:05:47,870 Because sister has things that parent doesn't have. 73 00:05:47,870 --> 00:05:51,990 So when we try to cast that parent as sister, we're missing some things. 74 00:05:51,990 --> 00:05:56,800 What are we missing? Well, at the very least we're missing 75 00:05:56,800 --> 00:05:59,930 this variable hers that is expected to be in a sister object. 76 00:05:59,930 --> 00:06:02,740 Parent didn't have that. Parent only had address. 77 00:06:04,280 --> 00:06:08,590 also there might be some methods that go along with the, with the object sister, 78 00:06:08,590 --> 00:06:13,850 that we wouldn't be inheriting either. So, that is the wrong direction to be 79 00:06:13,850 --> 00:06:19,740 able to do a cast. let's take a look at another example, 80 00:06:19,740 --> 00:06:23,790 just above this one. If we create a new object called brother, 81 00:06:23,790 --> 00:06:26,540 try to cast that as a sister, that doesn't work either. 82 00:06:26,540 --> 00:06:30,160 And the reason in this case is, of course, that brother also has a different 83 00:06:30,160 --> 00:06:36,030 variable called his, that we don't know where to put if we move it to the type 84 00:06:36,030 --> 00:06:39,900 sister. so not only is it missing hers but, it 85 00:06:39,900 --> 00:06:43,630 has this other thing his, that sister doesn't know what to deal with what to do 86 00:06:43,630 --> 00:06:44,214 with. Okay. 87 00:06:44,214 --> 00:06:51,789 let's take a look at yet another one. here you'll notice that we've taken our, 88 00:06:51,789 --> 00:06:57,220 our Parent object tried to cast it as Brother, and assigned it to a Brother 89 00:06:57,220 --> 00:07:00,365 variable. well that's not going to work either 90 00:07:00,365 --> 00:07:06,330 because Brother that Parent object does not contain all the elements of Brother. 91 00:07:06,330 --> 00:07:09,390 It's missing the variable his in this case. 92 00:07:10,710 --> 00:07:15,540 Here's one that does work. If we take a Parent object, cast it as 93 00:07:15,540 --> 00:07:20,630 Sister, and then assign it to another variable of type Sister. 94 00:07:20,630 --> 00:07:25,710 Why does that one work? Well, this one turns out to work because 95 00:07:25,710 --> 00:07:31,960 this original object, p2, started out as something of type Sister. 96 00:07:31,960 --> 00:07:40,440 You'll notice that p2 was assigned p1. And p1 was originally an object of type 97 00:07:40,440 --> 00:07:46,220 Sister, that was cast as a Parent. But it started it's life as an object of 98 00:07:46,220 --> 00:07:50,190 type Sister. So that it has everything a Sister needs, 99 00:07:50,190 --> 00:07:54,430 a Sister object needs. And so that for, therefore, that cast 100 00:07:54,430 --> 00:07:56,930 works out. So you can see things can get pretty 101 00:07:56,930 --> 00:08:02,350 elaborate in Java but the basic idea is that, to be able to do a cast the objects 102 00:08:02,350 --> 00:08:06,770 have to be compatible. They have to have the same variables in 103 00:08:06,770 --> 00:08:15,030 both types, and the same methods available so that they can be converted. 104 00:08:17,640 --> 00:08:20,760 Let's take a look at objects in Java in a little bit more detail. 105 00:08:20,760 --> 00:08:24,405 We're going to run through this example for a couple of slides here. 106 00:08:24,405 --> 00:08:32,120 we've defined the class called Point. It has two fields x and y coordinates. 107 00:08:32,120 --> 00:08:36,490 It has a constructor, a function that is called, or a method that is called 108 00:08:36,490 --> 00:08:39,800 whenever we create a new object of this type. 109 00:08:39,800 --> 00:08:45,370 And what this one does is just initialize that those coordinates at the 0. 110 00:08:45,370 --> 00:08:52,460 It also has a method called samePlace, which takes one argument another point. 111 00:08:52,460 --> 00:08:56,000 And compares the x-coordinates and y-coordinates, and sees that they're the 112 00:08:56,000 --> 00:09:01,950 same logically adding them together. When we create a new one of these Points, 113 00:09:01,950 --> 00:09:07,350 this is what it would look like in our code, we would say new Point object. 114 00:09:07,350 --> 00:09:13,140 And this would cause a memory allocation to find a place to create this object. 115 00:09:13,140 --> 00:09:17,390 Namely a place to store these two variables that it's going to need. 116 00:09:17,390 --> 00:09:24,990 and it's going to return a reference to this new location in memory, this new 117 00:09:24,990 --> 00:09:30,530 object. And then run the point constructor code 118 00:09:30,530 --> 00:09:34,020 on that object. that will cause those variables to be 119 00:09:34,020 --> 00:09:39,130 initialized, and then that reference will be stored in the variable newPoint, which 120 00:09:39,130 --> 00:09:42,390 is of type Point. So these are compatible. 121 00:09:44,070 --> 00:09:47,850 So, let's review that again. Whene, when we call new, what's happened 122 00:09:47,850 --> 00:09:51,770 is we've allocated space for the data field, in this case x and y. 123 00:09:51,770 --> 00:09:57,720 But we've also done one other thing. Here's x and y, those two doubles that we 124 00:09:57,720 --> 00:10:00,840 needed. But we've also allocated a pointer at the 125 00:10:00,840 --> 00:10:05,000 beginning of the object. To what's called a v table or virtual 126 00:10:05,000 --> 00:10:09,150 table. The v table is shared across the v table 127 00:10:09,150 --> 00:10:13,930 pointer is the same for all the objects of this class, all the points we might 128 00:10:13,930 --> 00:10:16,660 create. What it does is it points to another 129 00:10:16,660 --> 00:10:23,210 region in memory, where we've stored the pointers to the code we need for this 130 00:10:23,210 --> 00:10:26,290 type of object. In this case, there were two pieces of 131 00:10:26,290 --> 00:10:29,540 code, two methods. One was the constructor. 132 00:10:29,540 --> 00:10:34,670 So we need a pointer to the constructor code, and then samePlace. 133 00:10:34,670 --> 00:10:37,390 We need a pointer to the code for that method. 134 00:10:39,190 --> 00:10:43,070 You'll notice that this is using a level of indirection, actually two levels. 135 00:10:43,070 --> 00:10:48,580 inside of the object that we've allocated, we've left a pointer so that 136 00:10:48,580 --> 00:10:52,550 we can get to the table of pointers to the code. 137 00:10:52,550 --> 00:10:57,250 That's one level of indirection. Then when we go to that location in 138 00:10:57,250 --> 00:11:02,070 memory, we'll find another pointer another level of indirection, to the code 139 00:11:02,070 --> 00:11:04,920 for the constructor, to the code from samePlace. 140 00:11:04,920 --> 00:11:08,065 And by convention, we always put the pointer by the constructor first. 141 00:11:08,065 --> 00:11:13,480 'Kay? Once we're done with the object if we 142 00:11:13,480 --> 00:11:17,069 don't need it any more in our code, and lose all references to it. 143 00:11:17,069 --> 00:11:21,870 Our garbage collection routines will find it, find this place in memory, and say 144 00:11:21,870 --> 00:11:25,840 doesn't, nothing seems to be referencing it, we can reclaim this space, so we can 145 00:11:25,840 --> 00:11:30,020 use it for other things. And it will take back the space that had 146 00:11:30,020 --> 00:11:34,670 been allocated for the object. However, the space allocated for the v 147 00:11:34,670 --> 00:11:38,930 table will continue to reside in memory, because we might create a new point 148 00:11:38,930 --> 00:11:42,430 sometime later. This space remember, is not reclaimed for 149 00:11:42,430 --> 00:11:44,740 the object. That's a space that is allocated once, 150 00:11:44,740 --> 00:11:52,785 for every object of this type. They will all be pointing to the same 151 00:11:52,785 --> 00:11:58,060 vtable. Alright, next step let's say we created 152 00:11:58,060 --> 00:12:04,000 this new point. We've we have a place in memory for its x 153 00:12:04,000 --> 00:12:07,540 and y variables and we have a vtable pointer. 154 00:12:07,540 --> 00:12:12,620 And now what we're going to do is call this object constructor, that piece of, 155 00:12:12,620 --> 00:12:18,126 that first method, that piece of code that is going to run on to initialize 156 00:12:18,126 --> 00:12:21,580 this object. To do that, what we're going to do is 157 00:12:21,580 --> 00:12:27,835 call the function constructor with a single argument, which is going to be the 158 00:12:27,835 --> 00:12:31,805 pointer to the object we've just allocated in memory. 159 00:12:31,805 --> 00:12:34,250 Okay? which contain those, that contains those 160 00:12:34,250 --> 00:12:42,740 two fields. The Constructor code is going to run with 161 00:12:42,740 --> 00:12:47,430 a pointer to this location. So it will know that x is the very next 162 00:12:47,430 --> 00:12:50,580 element, and that y is the one after that. 163 00:12:50,580 --> 00:12:56,355 And the compiler will set things up so that those offsets are appropriately 164 00:12:56,355 --> 00:13:00,959 calculated, and we can put the 0 values in those two locations. 165 00:13:02,160 --> 00:13:04,930 Now, where do the, where does the code for the other methods go? 166 00:13:04,930 --> 00:13:10,100 how do we find that? Well, methods in Java are just functions, 167 00:13:10,100 --> 00:13:15,330 as in C, but as you've noticed from the previous slide, there's an argument 168 00:13:15,330 --> 00:13:18,120 that's implicit. Although we had defined constructor 169 00:13:18,120 --> 00:13:22,350 without any arguments, the constructor without any arguments, we did give it an 170 00:13:22,350 --> 00:13:25,420 argument which was the newly allocated space in memory. 171 00:13:27,420 --> 00:13:33,110 This pointer is always refered to by the name "this" in Java, that's a built in 172 00:13:33,110 --> 00:13:38,810 keyword that says we're talking about the reference to the current object. 173 00:13:40,070 --> 00:13:42,580 Alright? So, all of our methods really have this 174 00:13:42,580 --> 00:13:48,410 as the first argument. So the, the methods samePlace, which was 175 00:13:48,410 --> 00:13:54,890 defined with a single argument p. Really has two arguments, this and P. 176 00:13:54,890 --> 00:14:01,354 so, this two pointers, point to those two objects in in memory. 177 00:14:01,354 --> 00:14:04,392 And if we had equivalent C code, it would look like this. 178 00:14:04,392 --> 00:14:09,520 we would follow this pointer to the element x, the p pointer to the element 179 00:14:09,520 --> 00:14:16,140 x, compare those two. Do the same thing for the y's, and then 180 00:14:16,140 --> 00:14:19,973 do the logical and. Okay, so this is how we find the 181 00:14:19,973 --> 00:14:23,680 variables. And, the way we go about finding the code 182 00:14:23,680 --> 00:14:31,224 is through the vtable Pointer. How do we subclass an object? 183 00:14:31,224 --> 00:14:37,520 Well, here you'll notice I've created a new class called PtSubClass, that extends 184 00:14:37,520 --> 00:14:40,280 Point. It has a couple of different things from 185 00:14:40,280 --> 00:14:44,390 Point. One, it has a new element called 186 00:14:44,390 --> 00:14:49,250 aNewField. it also overrides the samePlace method 187 00:14:49,250 --> 00:14:53,020 that we had previously defined for points. 188 00:14:53,020 --> 00:14:57,850 And, it has a totally new method called sayHi. 189 00:14:57,850 --> 00:15:03,890 So where does this new new element go, this new integer, aNewField? 190 00:15:03,890 --> 00:15:08,170 And where does the pointer to the two other methods the code for the two 191 00:15:08,170 --> 00:15:10,312 methods go. How do we find those? 192 00:15:10,312 --> 00:15:14,370 Alright? So, what we're going to is just add these 193 00:15:14,370 --> 00:15:20,625 fields and pointers on to the ends of the object in memory, and to the end of the 194 00:15:20,625 --> 00:15:24,480 vtable. So let's take a look at what the, the map 195 00:15:24,480 --> 00:15:28,720 in memory is going to look like. Our new object is going to look just like 196 00:15:28,720 --> 00:15:33,090 Point, except that it has an extra nth at the end. 197 00:15:33,090 --> 00:15:37,890 So when we allocate space for this object, we need a little bit more space 198 00:15:37,890 --> 00:15:42,620 to also be able to store aNewField. The reason we pack it on at the end, is 199 00:15:42,620 --> 00:15:48,590 that if we ever want to cast this as a Point, we don't have to worry about the 200 00:15:48,590 --> 00:15:51,660 offsets being different than they were before. 201 00:15:51,660 --> 00:15:55,042 so that x and y are still in the same relative positions. 202 00:15:55,042 --> 00:16:01,320 The vtable for this object is going to be different than the v.table for the 203 00:16:01,320 --> 00:16:07,390 previous object. this vtable has three elements, not two, 204 00:16:07,390 --> 00:16:13,480 because we have a Constructor, we have samePlace, but we also have sayHi. 205 00:16:13,480 --> 00:16:16,860 Now, where do we point to? Well, Constructor is just going to 206 00:16:16,860 --> 00:16:20,640 inherit the previous Constructor. We didn't change that at all. 207 00:16:20,640 --> 00:16:26,390 So, this is going to be a pointer to the same region of code, the same routine 208 00:16:26,390 --> 00:16:30,700 that we had for Point. samePlace, though, is going to point to 209 00:16:30,700 --> 00:16:36,275 this new code that we're using to override the old version of samePlace. 210 00:16:36,275 --> 00:16:42,004 And sayHi will of course point to a totally different area. 211 00:16:42,004 --> 00:16:47,250 So, we've tacked on a new field at the end of the object definition, extra space 212 00:16:47,250 --> 00:16:53,320 and memory for that. we have a new vtable for Point subclass. 213 00:16:53,320 --> 00:16:57,470 It is not the same vtable as Point, because this has different methods 214 00:16:57,470 --> 00:17:02,830 associated with it. however, we are re-using, the code for 215 00:17:02,830 --> 00:17:07,628 the Constructor by pointing to the same, procedure in memory. 216 00:17:07,628 --> 00:17:14,590 samePlace points to a new piece of code, of, for this new samePlace and, of 217 00:17:14,590 --> 00:17:20,800 course, we have a pointer for the new code for sayHi.