WEBVTT 00:01.080 --> 00:01.800 Hi everyone. 00:01.800 --> 00:02.560 Welcome back. 00:02.600 --> 00:07.840 Let's now explore one of the most powerful built in gatherers, which is fold. 00:07.880 --> 00:14.240 We use it to combine or accumulate elements in a stream while maintaining an internal state. 00:14.240 --> 00:20.040 So when we say stateful reduction, it means the fold operation keeps track of running state. 00:20.160 --> 00:26.040 For example, some count or concatenation as elements pass through the stream. 00:26.160 --> 00:32.720 This is especially useful when you want to compute a single result, like a total duration of all the 00:32.720 --> 00:33.840 movies in the stream. 00:34.000 --> 00:38.360 Our total cost or even build a string from multiple elements. 00:38.520 --> 00:45.360 If you have used the reduce function that's part of the streams before, this concept will feel similar, 00:45.360 --> 00:46.960 but there is a key difference. 00:46.960 --> 00:53.080 Reduce is a terminal operation, meaning once you call it, your stream ends and you get one final result. 00:53.280 --> 00:58.960 But fold is an intermediate operation, meaning it happens inside the stream pipeline. 00:59.080 --> 01:03.440 You can actually continue transforming or filtering forward. 01:03.480 --> 01:05.360 Now let's break down the syntax. 01:05.360 --> 01:09.320 The syntax for fold takes in a supplier and a buy function. 01:09.360 --> 01:15.880 The supplier provides the initial state like zero for summing or an empty string for concatenation and 01:15.880 --> 01:20.320 buy function defines how each element should update the state. 01:20.320 --> 01:21.200 For instance. 01:21.240 --> 01:22.960 Accumulation plus value. 01:23.000 --> 01:29.040 The simple structure gives you complete control over how data is accumulated while the stream flows. 01:29.120 --> 01:32.040 So in this case here we have the code example. 01:32.040 --> 01:36.360 In this example we are calculating the total duration of all the movies. 01:36.360 --> 01:38.640 So in this case we use a gather. 01:38.640 --> 01:41.680 So the movie is going to be the collection. 01:41.880 --> 01:44.960 And we are gathering all the movies in the collection. 01:44.960 --> 01:49.960 So we have the initial state which is zero over here, and then the actual movie. 01:49.960 --> 01:56.120 So what we are doing here is that when the first element is processed, we process the zero plus the 01:56.120 --> 01:57.080 movie duration. 01:57.080 --> 02:02.670 But after that we are going to be continuously getting all the movies and adding the duration to it, 02:02.710 --> 02:06.030 and then we call the find first to get the final result. 02:06.230 --> 02:11.070 If there are no movies in the movies collection, then we will get the value as zero. 02:11.230 --> 02:14.430 So this is how we are gathering the duration. 02:14.430 --> 02:19.950 Basically, in this case, we are performing a stateful operation of maintaining the state of what the 02:19.950 --> 02:20.990 duration was. 02:21.190 --> 02:26.510 When each and every movie is being gathered by this gather operator. 02:26.550 --> 02:27.750 I hope the explanation is clear. 02:27.750 --> 02:31.070 Let's go ahead and explore this in action by getting back to IntelliJ. 02:31.110 --> 02:32.630 So I'm back in IntelliJ now. 02:32.790 --> 02:36.150 So in this case we are going to be exploring the fold operation. 02:36.150 --> 02:41.710 So let's go ahead and comment out this window sliding so that our console looks cleaner. 02:41.710 --> 02:43.510 So let's go to the demonstrate fold. 02:43.510 --> 02:45.230 So in here what I'm going to do. 02:45.230 --> 02:47.790 So I'm going to be performing the same operation. 02:47.790 --> 02:50.350 So in this case movies dot stream. 02:50.510 --> 02:53.870 And we are going to be calling the gather operator here. 02:53.870 --> 02:56.830 So gather dot fold and the fold. 02:56.830 --> 02:58.550 You can see it takes in a supplier. 02:58.550 --> 03:02.430 So the very first thing is we basically provide our initial state. 03:02.430 --> 03:04.430 The initial duration is going to be zero. 03:04.590 --> 03:07.550 And after that we can provide the bi function. 03:07.550 --> 03:10.270 So the bi function takes in two input. 03:10.270 --> 03:14.230 And then it's going to be giving you an output which is in this case is integer. 03:14.230 --> 03:19.630 So in this case now we are going to be calling the find first this is going to be an optional integer. 03:19.830 --> 03:25.430 If it's not present then we are going to be calling or else the value is going to be zero okay there 03:25.430 --> 03:25.870 we go. 03:26.030 --> 03:26.430 There we go. 03:26.430 --> 03:29.070 So in this case we are doing the stream of movie. 03:29.350 --> 03:32.310 And here this is going to be maintaining the state. 03:32.310 --> 03:37.590 So this particular operation is the one which is going to stream through all the movies and then compute 03:37.590 --> 03:41.070 the duration of all the movies by running this logic. 03:41.390 --> 03:47.110 Once that is done, it's going to be calling the find first, and then it's going to return the total 03:47.110 --> 03:47.630 duration. 03:47.630 --> 03:50.350 So I'm going to be assigning this to a variable. 03:50.350 --> 03:52.910 So I'm going to name this one as total duration. 03:52.950 --> 03:53.590 There you go. 03:53.590 --> 03:56.590 So this is going to give you the duration over there. 03:56.710 --> 03:59.310 Now I'm going to be printing the value. 03:59.350 --> 04:03.390 So in this case, total duration of all the movies is this minutes. 04:03.550 --> 04:04.270 So this is good. 04:04.270 --> 04:06.150 Let's go ahead and execute the program. 04:06.150 --> 04:08.950 Let's go to the top and let's execute this program. 04:08.950 --> 04:14.270 So here you can see that the total duration of all the minutes is 173 two minutes. 04:14.430 --> 04:19.510 So you can actually do a total of all these things that will give you this result. 04:19.550 --> 04:19.910 Okay. 04:19.950 --> 04:23.790 Now let's explore some other example in the same fold operation. 04:23.790 --> 04:24.910 Let's go to this one. 04:24.910 --> 04:29.510 So what I'm going to do is I'm going to be calculating the average rating okay. 04:29.550 --> 04:31.990 So I'm going to pretty much copy the similar code. 04:32.030 --> 04:34.190 But I'm going to change things a little bit. 04:34.190 --> 04:38.870 So in this case what we are going to calculate is a average rating. 04:38.870 --> 04:44.510 And in here instead of duration what we are going to do is we are going to be calculating the rating 04:44.510 --> 04:45.750 over here okay. 04:45.790 --> 04:47.830 And this is going to be of type double. 04:47.830 --> 04:49.270 That's why I see this error. 04:49.270 --> 04:51.110 So because rating is of type double. 04:51.110 --> 04:55.230 So in this case we can change the supplier to represent a double. 04:55.230 --> 04:55.870 There we go. 04:56.260 --> 04:58.340 And this is also going to be a double. 04:58.340 --> 05:01.540 So what this is going to do this is going to give you an average rating. 05:01.540 --> 05:01.900 Right. 05:01.900 --> 05:04.580 This is going to give you an addition of all the ratings. 05:04.580 --> 05:05.660 So what do we need to do. 05:05.660 --> 05:09.420 So I'm going to do a division of movie dot size. 05:09.420 --> 05:14.740 So this is going to give you the average rating of all the movies that we have in our collection. 05:14.740 --> 05:16.620 So let's go ahead and print this value also. 05:16.620 --> 05:20.180 So average rating of all the movies is string dot format. 05:20.180 --> 05:22.740 And this is the average rating okay. 05:22.820 --> 05:27.260 So now the next use case we can code multiple use cases based on the fold operation. 05:27.260 --> 05:31.020 So now I want to just concatenate all the titles okay. 05:31.060 --> 05:33.660 So what I'm going to do I'm going to do something similar. 05:33.860 --> 05:37.140 But in this case it's not going to be the rating here. 05:37.140 --> 05:39.660 It's going to be the movie title okay. 05:39.700 --> 05:41.300 So in this case movie title. 05:41.500 --> 05:45.300 And this is going to be an empty string as a initial state okay. 05:45.460 --> 05:49.540 So I'm going to change this one to a variable with the name accumulator. 05:49.540 --> 05:50.580 So accumulator. 05:50.580 --> 05:52.900 So there is a little logic I need to write actually. 05:52.900 --> 05:55.100 So if the value is empty. 05:55.260 --> 05:59.540 If the accumulator value is empty, which is going to be empty for the very first time, right? 05:59.580 --> 06:02.700 If it's empty, I'm going to be adding the movie title. 06:02.700 --> 06:08.220 Otherwise I'm going to be adding the accumulator plus the movie title. 06:08.260 --> 06:08.460 Okay. 06:08.540 --> 06:13.300 So this is going to give me the movie titles as comma separated values. 06:13.300 --> 06:17.220 So in this case this is also going to be empty string. 06:17.220 --> 06:19.820 We don't want the division over there. 06:19.820 --> 06:22.580 So this is going to be all titles okay. 06:22.620 --> 06:23.900 All titles. 06:23.900 --> 06:29.180 And another thing about our copilot is that let's say this code logic is very complex for you to understand. 06:29.180 --> 06:34.660 So what I'm going to do is I'm going to highlight this code and this particular inline chat will open. 06:34.700 --> 06:35.820 So let's open that one. 06:35.860 --> 06:38.540 And then you can ask explain this code to me. 06:38.580 --> 06:38.780 Right. 06:38.780 --> 06:43.540 When you do that it's going to give you an explanation of what that logic is so that you will be able 06:43.540 --> 06:45.940 to understand what it actually does. 06:45.940 --> 06:46.780 So here. 06:49.860 --> 06:50.300 Okay. 06:50.340 --> 06:55.820 So here if you take a look at it, the fold operations collapses the entire stream of movies into one 06:55.820 --> 06:56.140 result. 06:56.180 --> 06:59.260 By this is the initial value, which is the empty string. 06:59.260 --> 07:02.860 And then here we are actually performing the accumulator. 07:02.860 --> 07:05.180 So bills a comma separated list. 07:05.180 --> 07:10.980 So the terminal operation produces exactly one output regardless of the input size. 07:10.980 --> 07:14.300 It maintains the accumulated straight across this elements. 07:14.300 --> 07:19.380 And in this case if you take a look at it let's say if you have three movies it transforms it to this 07:19.380 --> 07:21.500 particular single string value. 07:21.620 --> 07:25.620 This is different from scan which would emit intermediate results. 07:25.620 --> 07:28.500 So here there is no emission of intermediate result. 07:28.780 --> 07:34.300 It folds the overall list of elements into one single output and then emits that value. 07:34.340 --> 07:34.780 Okay. 07:34.860 --> 07:39.740 So what we have done is we have coded the average rating and all titles. 07:39.740 --> 07:41.780 Let me go ahead and print that one also. 07:41.780 --> 07:44.780 So in this case over there all titles okay. 07:44.820 --> 07:47.700 So now let's go ahead and execute this particular function. 07:47.700 --> 07:52.860 This is going to print all the three functionalities that we have coded using the fold operator. 07:52.890 --> 07:59.970 So in this case, the total duration of all the movies is this, and the average rating is 8.82 stars 07:59.970 --> 08:04.290 and all movie titles, you can see that it's all comma separated nicely. 08:04.290 --> 08:07.490 So if you go to the top, let me explore the code one more time. 08:07.490 --> 08:08.970 Let's go to the fold operation. 08:08.970 --> 08:15.530 So this is a total duration of all the movies where we are simply performing summation of all the durations. 08:15.530 --> 08:22.770 So here we are also performing the same summation concept, but we divided the whole result by the movies 08:22.770 --> 08:25.290 dot size to get the average rating. 08:25.330 --> 08:26.490 And then if you go here. 08:26.650 --> 08:33.530 So here we are performing the string concatenation operation where we are taking each title and then 08:33.570 --> 08:35.810 having them separated by comma. 08:36.010 --> 08:37.130 That's what this one is. 08:37.370 --> 08:40.810 So this is where the folding operation is really handy. 08:41.090 --> 08:47.850 If you want to perform an intermediate operation with state and then produce a final result, then fold 08:47.850 --> 08:49.530 is an excellent option for that. 08:49.570 --> 08:50.970 This marks the end of this lecture. 08:50.970 --> 08:51.970 Thank you for watching.