WEBVTT 00:00.880 --> 00:01.440 Hi everyone! 00:01.440 --> 00:02.720 Welcome back to this tutorial. 00:02.720 --> 00:08.720 In this tutorial I will explain you about the stateful and stateless function that are part of the streams 00:08.720 --> 00:09.240 API. 00:10.200 --> 00:13.600 The first question is does streams have an internal state? 00:13.960 --> 00:15.160 The answer is yes. 00:16.040 --> 00:19.400 Does all the streams functions maintain an internal state? 00:19.760 --> 00:21.400 The answer is no. 00:21.440 --> 00:25.280 We have covered a lot of different stream functions that are part of the streams API. 00:25.640 --> 00:32.520 As part of the previous tutorials, let's take a look at the example and find out where exactly the 00:32.520 --> 00:37.720 stream state context resides as part of the stream pipeline. 00:37.760 --> 00:42.880 So this example is going to convert a list of student to list of string. 00:42.880 --> 00:47.080 We have coded the same example in one of the prior tutorials. 00:47.080 --> 00:52.720 If you take a look at it, we are starting the stream here and then we are performing a map call. 00:53.120 --> 01:00.440 This is going to convert a list of student to a list of names, basically stream of students to a stream 01:00.440 --> 01:01.240 of string. 01:01.440 --> 01:06.070 And then we are performing an uppercase operation on that stream of string. 01:06.350 --> 01:06.950 And there we are. 01:07.030 --> 01:09.870 And then we are performing the select operation. 01:10.150 --> 01:12.790 This whole thing is called a stream pipeline. 01:13.230 --> 01:16.310 And these two are intermediate operations. 01:16.550 --> 01:19.590 And then collect is the terminal operation. 01:20.190 --> 01:25.950 So the stream state resides in the context of the intermediate operations. 01:25.950 --> 01:31.990 Intermediate operations is where the streams state is going to reside. 01:33.350 --> 01:39.230 I would like to highlight that not all the functions maintain the stream internal state. 01:39.270 --> 01:40.350 The explain here. 01:40.870 --> 01:44.190 The example here doesn't maintain the internal state. 01:44.190 --> 01:48.390 I will explain why in a moment. 01:48.470 --> 01:51.950 The intermediate operations are categorized into two. 01:51.990 --> 01:56.270 One is a stateful functions and another one is a stateless functions. 01:56.270 --> 02:04.020 Whatever that are mentioned here are the stateful functions distinct sorted skip and limit. 02:04.020 --> 02:06.580 These are stateful functions. 02:06.860 --> 02:07.420 Whatever. 02:07.700 --> 02:12.940 The other functions that are not listed here are called stateless functions. 02:13.220 --> 02:17.420 The examples are map, filter, and etc.. 02:18.220 --> 02:23.940 I'm going to show you an example of stateful functions first. 02:23.980 --> 02:28.060 We have coded this example as part of the distinct and sorted tutorial. 02:28.700 --> 02:33.660 This example is going to convert a list of student to list of string. 02:34.620 --> 02:39.820 We all know that stream function call here is going to pass the elements one by one. 02:40.260 --> 02:45.340 Before I get into the details of it, I would like to explain this example again and then we'll discuss 02:45.340 --> 02:47.540 more about the stateful functions. 02:47.660 --> 02:50.540 Here is where we are calling the stream method. 02:50.700 --> 02:55.860 And then the stream method is going to pass the students one by one. 02:56.060 --> 03:00.660 So the map here is performing student activities. 03:00.660 --> 03:04.260 So activities is of type list if you remember from the example. 03:04.360 --> 03:08.880 And then this is going to pass a list of students to the flatMap. 03:09.080 --> 03:10.880 And then we are flattening the stream. 03:10.960 --> 03:15.880 This is going to pass a stream of string to the distinct operation. 03:16.080 --> 03:21.960 Here is where we are performing the distinct operation which is going to remove the duplicates. 03:21.960 --> 03:28.000 And then we are performing the sorted operation to sort the list of activities in the alphabetical order. 03:28.160 --> 03:31.200 And then we are performing the collect operation. 03:31.560 --> 03:39.320 So basically this is going to give you a list of unique student activities as a result. 03:39.360 --> 03:39.720 Right. 03:39.880 --> 03:49.120 So in this pipeline until the distinct function, the whatever functions that we are calling are stateless, 03:49.440 --> 03:56.640 distinct function is where the stateful context comes into picture, because that's when the streams 03:56.680 --> 04:03.600 API need to preserve the state of the previous items, whatever the previous stream elements that it 04:03.600 --> 04:04.160 processed. 04:04.400 --> 04:05.030 Because Gus. 04:05.430 --> 04:08.110 This drink is going to remove the duplicates, right? 04:08.190 --> 04:13.390 So every time an element is passed to this distinct function, it needs to know whether that element, 04:13.430 --> 04:19.830 whatever that is being passed as a current element, is already available in the previously processed 04:19.830 --> 04:25.150 elements, so that it can consider that one as a duplicate and remove it as a result. 04:25.350 --> 04:28.470 And then here we are performing the sorted operation. 04:28.470 --> 04:33.670 Started operation by default is going to sort the result in alphabetical order. 04:33.950 --> 04:40.550 So again it needs to know the previous elements that it processed because so that it can insert or it 04:40.550 --> 04:45.270 can modify the order based on the current element that is being passed. 04:46.430 --> 04:53.590 So distinct and sorted is where it needs to know the state of the previously processed elements. 04:53.710 --> 04:58.990 This is where the stream API state context comes into picture. 05:02.870 --> 05:05.710 Now let's discuss about stateless functions. 05:06.190 --> 05:12.620 The stream functions that does not maintain any state are called stateless stream functions. 05:12.820 --> 05:15.660 Let's dig in further and understand what does it mean? 05:15.900 --> 05:21.820 So in this example again we are going to convert a list of students to a list of string. 05:21.860 --> 05:25.300 So here if you take a look at it first college names dot stream. 05:25.300 --> 05:26.380 We are creating a stream. 05:26.380 --> 05:30.220 And then the stream elements are going to be parsed one by one. 05:30.380 --> 05:37.140 So here we have the map call which is just going to transfer a stream of student to a stream of string. 05:37.820 --> 05:39.140 The stateless operation. 05:39.260 --> 05:43.100 It doesn't need to know what are the elements that it processed prior. 05:43.140 --> 05:45.140 The next thing is again we are converting. 05:45.340 --> 05:52.900 We are calling the map method, and then we are converting a list of stream of strings and then applying 05:53.180 --> 05:55.260 an uppercase operation on top of it. 05:55.300 --> 05:57.460 Basically the type is going to be the same. 05:57.580 --> 06:00.460 It is just that we are performing an uppercase operation to. 06:00.460 --> 06:02.260 It is again stateless. 06:02.660 --> 06:04.820 So here it doesn't even apply. 06:05.260 --> 06:11.520 So we don't even have to consider this Because the state context comes into picture only for the intermediate 06:11.520 --> 06:12.320 operation. 06:12.440 --> 06:16.960 In this example, the intermediate operations are just map. 06:17.520 --> 06:19.520 I would like to explain more about it. 06:19.520 --> 06:24.520 So we will go to the example that we coded previously and then discuss more on it. 06:26.640 --> 06:30.760 So I'm going to open a streams flatMap example here. 06:32.120 --> 06:33.840 Let me put this in presentation mode. 06:38.920 --> 06:39.640 So in here. 06:40.080 --> 06:42.720 So this is the example which we discussed as part of the presentation. 06:42.720 --> 06:47.400 But I would like to give you some more code comments and then make it clear. 06:47.760 --> 06:53.000 So here we are performing didn't get activities. 06:53.000 --> 06:59.880 This is of type list is going to convert a stream of student to stream of list of string. 07:00.160 --> 07:02.800 So this is a stateless operation. 07:03.880 --> 07:06.640 And then here we are flattening the stream. 07:06.640 --> 07:11.190 Basically we are converting a stream of list of string to stream of string. 07:11.190 --> 07:13.110 It is again stateless. 07:14.710 --> 07:18.550 So here is where we are going to perform the distinct operation. 07:18.550 --> 07:23.230 Basically distinct operation is going to remove the duplicates right. 07:23.630 --> 07:25.790 So this is a stateful operation. 07:25.790 --> 07:28.950 It needs to know about the previously processed elements. 07:29.910 --> 07:35.070 So that if it is a duplicate it can remove that element from the result. 07:35.070 --> 07:42.590 And next thing is a sorted sorted here is going to sort the result in the ascending order. 07:42.790 --> 07:48.550 So it needs to know the state of the previously processed elements so that it can sort accordingly as 07:48.550 --> 07:50.910 per the current element. 07:50.910 --> 07:55.470 And the next one is a terminal operation which is going to perform the collect. 07:56.030 --> 08:00.510 And then it is going to give you the list of unique activities as a result. 08:01.110 --> 08:05.830 So this is all about the stateful and stateless functions. 08:06.630 --> 08:08.590 With this we came to the end of this tutorial. 08:08.710 --> 08:09.670 Thank you for watching.