WEBVTT 00:00.850 --> 00:01.390 Hi everyone. 00:01.390 --> 00:02.650 Welcome back to this tutorial. 00:02.650 --> 00:08.830 In this tutorial, we'll talk about the reduce function which is part of the streams API. 00:08.950 --> 00:11.500 Reduce is a terminal operation. 00:11.710 --> 00:16.780 We have seen one another terminal operation in the previous tutorials, which is the collect. 00:16.780 --> 00:22.240 This is the second terminal operation which we are going to discuss as part of this tutorial. 00:22.540 --> 00:29.530 So this is basically used to reduce the contents of the stream to a single value. 00:29.620 --> 00:31.060 That's what it is going to do. 00:31.060 --> 00:36.520 Let's say you want to find a sum of all the elements in the stream or perform multiplication of all 00:36.520 --> 00:42.610 the elements in the stream, or you want to find out who is the highest grade student out of the student 00:42.610 --> 00:43.360 database. 00:43.390 --> 00:48.160 In those kind of scenarios, you can make use of the reduce function. 00:48.160 --> 00:51.790 So the reduce function takes in two parameters as an input. 00:51.790 --> 00:55.960 The first parameter is the default or the initial value. 00:55.960 --> 00:58.120 And the second parameter is the binary. 00:58.120 --> 00:59.110 Operator. 00:59.140 --> 01:05.960 A quick summary of the binary operator is that binary operator is like a function which takes two inputs 01:05.960 --> 01:10.340 of the same type and returns an output of the same type. 01:10.550 --> 01:12.020 Let's go ahead and code this one. 01:13.840 --> 01:19.420 So have the previous example here want to move away from the presentation mode? 01:20.850 --> 01:24.190 Was this what I'm going to do for the purpose of this tutorial? 01:24.210 --> 01:29.010 I'm going to create a class called Stream Reduce. 01:31.190 --> 01:33.980 Dream, reduce example. 01:34.520 --> 01:38.990 And then I'm going to make this class executable by adding public static void main. 01:39.440 --> 01:40.040 There we go. 01:40.100 --> 01:44.750 Now what I'm going to do, I'm going to create a list of numbers. 01:45.350 --> 01:46.280 List of. 01:47.120 --> 01:48.200 Integers. 01:51.730 --> 01:55.570 And then this integer list is going to have. 01:57.360 --> 01:58.920 Restart as list. 01:58.920 --> 02:01.260 I'm going to add like four numbers in it. 02:01.470 --> 02:04.020 135, seven. 02:05.100 --> 02:07.020 Let me put this in presentation mode. 02:09.950 --> 02:17.250 So what I'm looking for is that I want the multiplication of all the numbers as a result. 02:17.270 --> 02:18.530 That's what I'm looking for. 02:18.560 --> 02:21.800 So how can we achieve that using the reduce method? 02:21.930 --> 02:28.460 I'm going to introduce a method called public static int. 02:31.050 --> 02:31.710 Perform. 02:32.860 --> 02:34.000 Multiplication. 02:35.120 --> 02:37.910 And then pass these numbers as an input. 02:38.000 --> 02:40.670 I'm going to pass this list of. 02:41.440 --> 02:42.370 Integer. 02:44.210 --> 02:45.320 An integer list. 02:46.340 --> 02:50.930 You're going to do integer list. 02:51.700 --> 02:53.020 Dot stream. 02:54.470 --> 02:59.650 With the method that we are going to, the function that we are going to look is the reduce. 02:59.680 --> 03:04.300 If you take a look at the reduce, it has many different variations. 03:04.300 --> 03:12.340 The one which we are going to look for is this one which has the identity and the by function, right? 03:12.340 --> 03:18.490 So what I'm going to do, I'm going to use this, the first parameter is a default value or the initial 03:18.490 --> 03:18.820 value. 03:18.820 --> 03:21.040 I want the initial value to be one. 03:21.040 --> 03:24.430 And then it is a by function. 03:24.880 --> 03:27.220 Basically, if you take a look at it, it is an accumulator. 03:27.220 --> 03:33.850 It is going to accumulate all the result and then send the response as a single value. 03:33.850 --> 03:35.290 That's what it is going to do. 03:35.290 --> 03:40.930 So by function, which is going to take two parameters as an input, I'm just giving it a name as a 03:40.930 --> 03:47.200 comma B, you can give any name you want and then what I'm going to do, I'm going to perform the multiplication 03:47.200 --> 03:51.790 of those two values, and then we are going to return the result. 03:52.690 --> 03:59.860 So what this is going to do is this is going to return a new result for every value that is being passed 03:59.860 --> 04:00.700 from the stream. 04:00.700 --> 04:04.120 So it is going to pass the stream values in this order. 04:04.120 --> 04:06.310 One three. 04:08.970 --> 04:09.640 And seven. 04:10.140 --> 04:18.710 Now, let's talk in detail about how these values A and B are populated with what values in that stream. 04:18.720 --> 04:24.540 So let's talk about the first iteration, meaning it is going to pass the first element in the stream. 04:24.540 --> 04:30.540 What will happen is that a equal to one, which is the default value, The identity value will be assigned 04:30.540 --> 04:33.630 here and be equal to one from stream. 04:36.100 --> 04:37.240 And what is happening here? 04:37.240 --> 04:39.760 It is performing the multiplication operation. 04:39.760 --> 04:40.000 Right. 04:40.000 --> 04:41.410 And the result will be returned. 04:43.370 --> 04:44.990 Result, one is returned. 04:49.550 --> 04:50.720 And what is the next step? 04:50.840 --> 04:52.340 The next step is. 04:54.170 --> 04:58.070 Yay equal to result from previous step. 04:58.100 --> 05:03.500 The result from the previous step is one and be equal to three. 05:05.170 --> 05:12.240 Is again from Dream and the result is going to be it is going to perform the multiplication here and 05:12.240 --> 05:15.040 the result three is returned. 05:19.570 --> 05:20.770 And what is the next step? 05:20.770 --> 05:22.000 The next step is. 05:23.860 --> 05:31.570 The value of E is going to be three from the result and the value of B is going to be five. 05:34.430 --> 05:37.100 So the result is going to be this value. 05:37.100 --> 05:42.950 The B is from stream and the result is going to be 15. 05:43.310 --> 05:43.970 Right. 05:45.250 --> 05:49.180 Result, 15 is returned. 05:49.780 --> 05:50.530 That's good. 05:53.110 --> 05:54.400 So what is the next thing? 05:54.430 --> 06:02.320 The next thing is is going to be 15 because that's a result it produced as part of the multiplication 06:02.320 --> 06:06.730 operation of three into five and B is going to be seven. 06:07.600 --> 06:09.430 The value is coming from stream. 06:09.430 --> 06:17.260 So the B value is always from the stream and the value is a result of the reduce operation for each 06:17.260 --> 06:19.020 and every element in the stream. 06:19.030 --> 06:21.700 The result is going to be one No. 06:21.710 --> 06:22.090 Five. 06:24.560 --> 06:33.200 So this is the background operations that stream performs for us in order to perform the multiplication 06:33.200 --> 06:36.050 of all the values in the string. 06:39.560 --> 06:41.380 So let's go ahead and check it. 06:41.390 --> 06:43.490 Let's run this and then find out the result. 06:44.360 --> 06:52.580 Now what I'm going to do, I'm going to perform a sort of perform multiplication and then I'm going 06:52.580 --> 06:53.990 to pass the integers. 06:54.650 --> 06:58.280 So if I run this, it is going to give me the. 06:59.370 --> 07:02.220 Result, which is the multiplication of all the values. 07:02.220 --> 07:03.680 That's what it did here. 07:03.690 --> 07:07.830 Now here we have given the initial value as one. 07:07.830 --> 07:13.920 Basically, when the first element is received from the stream, it is going to use this as an initial 07:13.920 --> 07:14.340 value. 07:14.340 --> 07:17.010 It is going to perform one into one. 07:17.010 --> 07:18.780 If I pass zero, what will happen? 07:18.780 --> 07:19.380 Right? 07:19.830 --> 07:20.610 We pass zero. 07:20.610 --> 07:27.690 The whole result is going to be zero because if you perform zero into one is going to be zero and all 07:27.690 --> 07:30.750 the elements is going to be multiplied with the zero value. 07:30.780 --> 07:35.280 That's the reason why the final result, whatever we have, we got it as zero. 07:35.520 --> 07:41.940 Now what we will do is I'm going to change it to change it back to its original value. 07:42.180 --> 07:42.870 There you go. 07:42.870 --> 07:48.930 Now, if we don't pass this right, there is another variation which we saw for the reduce method. 07:48.930 --> 07:51.060 If we don't pass it, what will happen? 07:51.940 --> 07:57.760 To perform multiplication without identity. 07:58.840 --> 08:02.020 Is that set a variable name that we have. 08:02.050 --> 08:05.620 So without identity, I'm going to remove this. 08:05.740 --> 08:07.280 So here we got some issue. 08:07.300 --> 08:09.700 Let's take a look at the issue that we got. 08:10.500 --> 08:12.140 So required. 08:12.150 --> 08:17.190 And so the type which it is going to return is INT But what do we have we found optional? 08:17.310 --> 08:21.750 So optional is a object which got released as part of Java eight. 08:22.080 --> 08:27.240 Don't worry about it for now because I'll be covering optional the optional as a separate section in 08:27.240 --> 08:28.170 the future tutorials. 08:28.170 --> 08:36.930 For now we just have to know instead of this we are going to return optional of type integer. 08:39.550 --> 08:40.120 Okay. 08:41.550 --> 08:48.690 So what this is going to do, main reason why this optional is introduced as part of Java eight is to 08:48.690 --> 08:52.830 handle the null values that it is going to return gracefully. 08:53.010 --> 08:53.970 Basically. 08:56.060 --> 08:57.170 It is value. 08:57.290 --> 09:01.790 What I'm going to do instead of this, I'm going to do perform. 09:02.920 --> 09:05.500 Multiplication without the identity. 09:06.010 --> 09:10.060 Then I'm going to pass this integer list as an input to that. 09:10.330 --> 09:16.660 So this is going to return optional of integer. 09:17.230 --> 09:21.160 And then let's say we give it a name called result. 09:21.990 --> 09:26.200 So in here, if you perform, I'm going to bring this one down. 09:26.260 --> 09:32.700 So if you want to check what are all the methods it supports the optional object supports. 09:32.700 --> 09:38.040 For now we're just going to explore a couple of methods and then we'll explore all the different methods 09:38.040 --> 09:44.610 in our future tutorials, which I'm going to cover Optional as a separate section. 09:45.750 --> 09:55.470 Dot get will give you the actual value inside that one is present will give you the actual is that value 09:55.470 --> 09:56.300 present or not? 09:56.310 --> 10:01.320 This is going to give you a boolean and this is going to give you the actual value. 10:02.220 --> 10:04.680 So these are the two methods which you have to know. 10:04.680 --> 10:05.550 For now. 10:05.580 --> 10:08.940 We will talk more about option in the future tutorials. 10:08.970 --> 10:10.530 Now I'm going to run this. 10:12.990 --> 10:16.220 If I run it, say it gave you the same result. 10:16.230 --> 10:20.730 But if you take a look at it, this value is returned as true. 10:20.760 --> 10:23.220 Let's say I'm going to pass a. 10:24.950 --> 10:27.200 Empty list and then we'll see what happens. 10:30.010 --> 10:30.910 But one. 10:32.180 --> 10:32.910 Gonna pass. 10:35.010 --> 10:36.390 Create a new list. 10:37.470 --> 10:38.790 This new. 10:40.330 --> 10:40.740 Else. 10:43.460 --> 10:45.230 In this year one. 10:46.200 --> 10:47.060 In its fastest. 10:48.730 --> 10:50.260 So I'm going to do. 10:51.620 --> 10:52.130 Result. 10:52.160 --> 10:53.480 One dot is present. 10:54.110 --> 10:54.860 Run this. 10:56.620 --> 10:58.540 We run it, we take a look at it. 10:58.720 --> 11:00.430 It returns the value as false. 11:00.430 --> 11:02.260 So if you pass. 11:04.090 --> 11:07.360 A list with values, then it is going to perform this. 11:07.360 --> 11:10.120 Otherwise it won't even perform it. 11:10.120 --> 11:12.850 But it will still return that optional object. 11:13.470 --> 11:20.600 You can check whether that optional has any value or not by using the Is present method. 11:20.610 --> 11:24.890 But your question might be this is this might be running in your mind. 11:24.900 --> 11:29.550 If I do get what will happen, this is going to give you exception. 11:31.840 --> 11:32.610 See here. 11:32.620 --> 11:34.150 No such element exception. 11:34.150 --> 11:40.420 So it is always recommended to check whether optional has a value or not. 11:41.740 --> 11:44.860 The option is a value or not and then perform the get method. 11:44.860 --> 11:49.330 So ideally it should be result one dot. 11:51.220 --> 11:51.880 Present. 11:51.970 --> 11:52.780 It is present. 11:52.780 --> 11:53.680 Then perform this. 11:53.680 --> 11:59.800 Otherwise do not even perform so that you can avoid that no value present exception. 12:03.340 --> 12:04.460 That you can avoid this exception. 12:04.480 --> 12:08.650 When I run this, see whether is it still giving me that exception? 12:10.400 --> 12:10.940 There you go. 12:10.970 --> 12:12.510 The exception isn't even there. 12:12.550 --> 12:16.740 So this is one way of using the reduce operation. 12:16.760 --> 12:21.040 So with the initial value and without the initial value. 12:21.050 --> 12:27.290 But this is again, the accumulator in both the scenarios is just a buy function or a binary operator, 12:27.290 --> 12:33.050 which takes two inputs and then results returns the result of the same type. 12:34.930 --> 12:35.230 But this. 12:35.230 --> 12:36.910 We came to the end of this tutorial. 12:36.940 --> 12:38.050 Thank you for watching.