WEBVTT

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Hi everyone.

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Welcome back to this tutorial.

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In this tutorial we will code and learn about the flatMap function which is part of the streams API.

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Basically the flatmap is similar to the map function.

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It is going to transform from one type to the another type, or it can transform from one form of the

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same type to another form of the same type, but it is used in the context of stream, where each and

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every element in the stream represents multiple elements.

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So examples are if you have a stream of list or stream of arrays, basically each and every element

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in the stream represents a list.

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Each and every element in the stream represent arrays.

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Because we all know that lists are arrays can have multiple elements.

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I just provided arrays and list as an example.

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It can be any type of collection that represents multiple elements.

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Let's go ahead and code this one.

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I have the IntelliJ open.

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What I'm going to do is in the streams package, I'm going to create a new class called Streams flatMap

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example.

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Streams.

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Map.

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Example, I'm going to make this class executable by adding public static void main.

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There you go.

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Let me put this in presentation mode.

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So now what I'm going to do, let's go ahead and visit the student database class.

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Student database.

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In here.

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If you take a look at it, the student database has name, grade level GPA, gender and the activities.

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These are the activities which represent a list.

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What I want is I want to print all the activities in a list.

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That is my goal.

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So I'm going to use the Streams API in order to perform that activity.

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So I'm going to create a method called.

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Public static.

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So activities are of type string, list of string and then print.

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Student activities.

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I'm gonna.

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For this.

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There you go.

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Good.

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Now what I'm going to do, I'm going to use a student database class, and then I'm going to call the

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get all students.

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This is going to give you the list of all the students that are available as part of the student database.

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I'm going to call the stream method, which is going to create a stream.

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So the next step, what is that I'm going to do?

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I'm going to do map of.

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You don't.

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So what is that we want?

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We want the activity.

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Student get activities.

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Right.

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Okay, so.

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Let's write the code comments here.

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Basically, this is going to give you a stream of student.

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Right, That's good.

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And then we are mapping the student to this list of string meaning, which has all the activities.

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This is going to give you a stream of list of.

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Think so.

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Each and every element in the stream is going to be a list of string.

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Now, if I try to do a collect, right, so let's say I'm going to do a collect.

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Of tallest.

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So this is what we have done before.

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If we just want to collect the names, we just do this.

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What?

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Static method?

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Okay.

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And then let's do the.

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List of string.

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Going to be student.

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Activities.

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Now, if we.

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As soon as I did that.

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Let's return this one to return.

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Student activities.

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So here it is, complaining.

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Right.

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Let's go ahead and take a look at that error.

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No instances of type variable exist.

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So that list of string conforms to string.

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Basically, it is looking for a stream of string, not the stream of list of string.

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That's when the flatmap comes into picture.

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So here each and every element in the stream is going to reference a list of string.

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In those kind of scenarios you need to use flatMap.

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So I'm going to use flatMap.

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Basically, we have to flatten that stream.

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That's the actual purpose that it is going to do behind the scenes.

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And then we are going to do list.

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Colon.

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Colon.

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Spring.

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So we all know.

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So here also I'm using a list we're calling a stream method.

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It creates a stream of student here.

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This is a list of string.

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When you flatten it by calling the flatMap, it is going to create a stream of string here.

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Now.

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It will be able to collect each and every element in that activities.

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List.

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Now what we will do, we will print.

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Okay.

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So I'm going to call.

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Gonna have the sea salt.

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You typing this out?

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It's going to give me the shortcut.

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I'm going to give the print student activities.

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And I'm going to call this method.

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Now, if I run this, basically it is going to give me all the.

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List of activities that students are participating.

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And let's do a quick recap of what we did.

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So flatMap.

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We have used Map and flatMap here, actually.

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So flatMap is used in scenarios where each and every element in the stream, like in this example,

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it represents a list.

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Let's say if we have a set, you can still use set colon.

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Colon string.

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So in those kind of scenarios, you will use flatMap instead of a regular map.

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That's the one thing which we have to register in our mind.

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With this.

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We came to the end of the tutorial.

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Thank you for watching.