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Hello, adjustments in this lesson, we're going to learn interfaces from a dual function package,

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Ziya function and by function in this lesson, we are going to have a lot of practice to help you understand.

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These types better will use map implementation for all examples that will give your understanding how

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you can apply your knowledge in real life cases.

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And while learning different types will create a lot of lambda functions and MassArt references, we're

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going to learn default methods from functional interface.

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You will learn what is the difference between and then and compose masses will discuss it identity message

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from the function interface and I will explain how it works and what benefit we can get when we use

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it at the end of the lesson and will show you how you can create.

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Compare that with the help of function that start.

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And to learn this topic, it is better to learn it from the source code and then real examples.

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We'll start our lesson from the function time function interface is a function interface and declare

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one abasic method that takes some value as an argument and can return something.

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There are tensions.

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That function interface is parametrized by two types.

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In case you don't remember what generics are, please look at generic lesson to refresh your knowledge.

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Two types and indeed here in parameterization to put a stress on the fact that type of the argument

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may be different and type of return to value.

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The function is usually applied where you need to produce some result of different type on the basis

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of the argument.

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Let me show you a function example.

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I created separate file for all examples that we are going to review today.

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I will leave links to this file in less than attachments.

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And as I said, an agenda of the lesson will use classes from Java Collections Framework to show you

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how you can apply the knowledge of functional interfaces.

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I created an object of hash map type.

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Our hash map is parametrized by integer type four keys and string type four values.

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I fill out the map with the values for the sake of our demo.

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In this line of code, I want to print the value that is associated with the key for I believe you can

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understand that no will be printed to console because I didn't put any value for key for now on my map.

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I want to go computer absent MassArt.

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Let's look at the source code of this matter.

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To understand what it is for this method should compute the value for the key in case the key is not

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already associated with the value is mapped to now.

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And this method will compute the value using the function that we pass to this method.

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You can see and Masset arguments that this method takes key as a first argument and also it requires

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a function type argument.

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And here you can see apply Massata Scholte on our function.

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The function that will be given to this method is parametrized by any type that is compatible with the

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type of our key and any type that is compatible with the type of the value in this map.

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And knowing how a function works, we already can understand that key will be passed to our function

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and will use it should be associated with that key will be mapped to it.

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Now, let's get back to our example.

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I created all examples here before the lesson to save the time during the video recording.

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Let me explain to you what is written here.

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I call Computer Fab's and Massett and pass the key.

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And the second argument is a function from the syntax standpoint.

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You can see that in case we have only one argument we can skip parentheses here is a local variable

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key and I have to return string tie because the return value should be compatible with the value type

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of our map of the sake of the Dharma.

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Let me return string representation of four.

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Does it make sense?

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So now the execution, this lambda function will be called inside compute if absent MASP.

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Let me show your source code one more time.

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Here apply method of our function will be called and key will be passed.

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It will return string value that is not now and after that it will be inserted into the map.

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Is it clear?

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Let me run the program and print to console the value that is associated with the key for.

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And here you can see that before we call compute the absolute mass.

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There was no value associated with this key.

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And after we call the method, we can see the values that is associated with our key hope.

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This example brings you understanding how we can create lambda functions for function interface.

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You so housemate's computer.

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Perhaps MassArt is written.

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It uses function interface as an abstraction and we can pass an implementation of the interface that

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we want.

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Now I want you to understand that Lambda functions and MassArt references are interchangeable by saying

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this.

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And that instead of LAMDA function here, we can use MassArt reference.

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Let me show you how here in this demo class, I declare a static method that takes some integer as an

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argument and return string.

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It looks like this method meets the requirements of function, interface and work with Tibs that are

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compatible with the current map key and value types.

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If compute absent Masset need some functions that can take integer and return string, Azana will tell

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it to use convert into string method.

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And this line you can see that I used Masset reference to the static MASP.

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Does it make sense?

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Hope you understood how MassArt references work.

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Let's move on.

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The next thing I want to talk with you about is by function.

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Let me open the source code of my function interface.

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You can see that this is also functional interface.

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That is parametrized by three types.

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It has one abstract method that takes two arguments and returns result.

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Now you can understand the naming convention individual function package depends on Ariki with a different

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prefix to interface name.

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Just to refresh your knowledge, I want to remind you what everybody is pretty is a term that is used

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in computer science to specify a number of arguments taken by function.

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Regular function takes one argument.

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My function takes two arguments.

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This prefix is widely used in Java dual function package.

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Now you will understand what it means.

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You can see the two arguments here and return type.

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All of them might be of a different type.

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Let me show you the bi functional practice.

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Let's return to our demo file now.

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I want to compute new value on the basis of existing key and existing value.

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I call compute of present Masab Unmap object.

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I suggest investigating the source code and documentation for this matter, as we can see here in documentation

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in case value for the specified key is present.

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And now this method will compute a new map.

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And given the key and its current mapped value from the massive declaration, we can understand that

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we have to pass some key to this method and by functions at this parametrized by time that is compatible

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with the key and types that are compatible with the value based on the BI function interface.

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We can understand that the first and the second time in Diament operator are types of the arguments

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and the third type from Diamond Operator is a type of return value.

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You can also understand the same after investigation of the source code of compute of present MassArt.

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Now, let's write our own lambda expression in our demo class, take into account my function requires

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two Masset arguments.

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I declare two local variables here and we have to return string that will be used as a map and value

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two key three just for the sake of the.

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Let's create custom string values that consist of the key itself and previous value associated with

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the key separated by column.

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And you can see in concert that the value associated with the key three is updated.

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I believe you already understood how to use massive reference in this situation.

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But just in case, I also wrote an example here to reach complete understanding.

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Here is a method that is compatible with the expected return type and type of method parameters.

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And here you can see that I use this method as method reference instead by function here.

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Now, when you understood what function and function are, I want to draw attention to the methods that

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we didn't discuss.

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Those are default methods from function and by function interfaces.

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The way how they work are similar.

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That's why we will review on the example with function interface.

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Let me open again the source code of function interface.

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Here you can see that we have default methods Campo's and and then methods.

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These methods allow you to compose new function from existing ones to understand this.

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Let me get back to the demo file to show you an example.

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Consider the next case.

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We have product type products, price and idee.

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That is a unique product identifier.

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We also have getters and setters declared here.

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Besides that, I overwrite the string MassArt.

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This will help us during the debugging when we will print product to console for the sake of the demo

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and declared all types in one file do not switch between tops in eclipse.

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That's why you can find this glass at the bottom of our demo file.

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Now imagine that we have a map object that should map product against discount amount so that I could

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get discount by product.

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That's why my map is parametrized with product type four keys and double type four values.

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This example is just to help you to understand the case.

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Imagine that I have a list of products in real life.

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I can fetch this data from the database or pass from some file.

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Each product has its IP and price.

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Now I have to fill out my map.

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How I would do that.

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I create for each loop to iterate over all products and on each iteration I compose two functions.

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The first function just extract the price.

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The second function calculates discount.

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Based on this price, I use constant value for that.

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This means five percent discount and I want to get the amount of money for my discount.

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That's why you see multiplication here.

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After that, I combine these two functions.

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I combine them in the way the first function will be get the price function and the second function

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will be a discount function.

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Probably using that is a lot of good.

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And you could calculate discount in one function here, but focus on the fact that you can combine two

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functions.

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This example is about that you can create new function on the basis of already existing functions.

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You can make them as you wish.

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Next time you don't get price combined with discount function, but you need to get price and for example,

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get that function.

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Can you understand now the benefit of function composition?

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This gives you flexibility in combination of functions and after that I call comparative absence method

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past the key to this method that would be product and functions.

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It calculates the discount.

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Let me print the map that we received to consult and here you can see that each product is mapped to

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the discount.

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Is it clear now when you know how and then method works?

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Let me show you how composed Method works.

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The only difference between these two methods is the order in which functions are combined.

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In case I use function, then call and that method and pass another function.

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This function will be executed after the first one and we'll take the result of the first function as

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an input argument.

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Does it make sense?

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In case I use compose method, then I should specify function that should go first.

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So technically these two functions are the same, but the orders that are used in syntax is different.

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In my second function, I use that discount function, the call compose method and parsing its price

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function to it.

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That means get price function will be executed first and after that the discount function will be executed

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with a result of get price function.

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I hope I didn't get.

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The future now, you know, the difference between Enza and Campo's default mascot's in function interface.

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Let me open my function interface now.

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Here you can see only one default method.

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And then knowing how to create lambda function for my function interface and knowing how.

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And then Masset works in function interface.

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I believe you would be able to apply this method during the development when it will be needed.

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There is also one more default method in function interface that we didn't talk through.

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I'm talking about identity must pay attention to the source code of this message.

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This function always returns its inputs.

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Argument here is simple lambda expression for that.

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But should we use this method instead of similar lambda function?

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Yes, we should, because right in lambda expression will create an instance of function type after

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each occurrence of lambda function, whereas column identity method you would get the always the same

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instance of the function.

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So using function identity instead of this lambda function might save you some memory, but that shouldn't

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drive your decision.

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If you really think that lambda function is more readable than function identity, I would love to show

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you the example of this method and tell you where and when you can apply this method.

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But we need knowledge of Stream API to understand the example.

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Basically, we use this method only when we are forced to pass function, but we don't need to modify

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inputs.

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Argument you will face with such cases while working with stream API.

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Don't worry.

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In the lessons about Stream API, we'll talk about this method.

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And the last but not least examples that I wanted to share with you today is how to create comparator

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with helper function.

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Let me open compared the interface.

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And if you remember, the computer interface has a lot of default methods.

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Some of them use function interface.

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I'm going to show you one example that will give you understand how you can use the rest of the methods

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that they function as an argument.

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Here we have really cool method that most likely you will use.

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It is called comparing it grades compare to on the basis of the function that extracts the comparable

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key that should be used to sort by to help you understand uses.

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Let me show you an example how this works.

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Today we created a list of products and if you remember that, imagine that I want to short list by

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its price.

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That's why I call sort Masset on my list.

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I have to pass Comparator here.

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I don't need to create a concrete class of comparator type.

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I also don't need to implement comparable interface in my product type.

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I can just pass the function that can extract the field that should be used during the sorting.

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And I already have this function.

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Let me pass the message reference together in my product class.

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This guy knows how to retrieve price from each product taken into account.

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I have a list of products here and I don't have it in the file for product.

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I can use a class name to make a reference to Macit Compiler is smart enough to understand that this

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method should be called on.

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Each object from this list to extract K double is comparable type.

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That's why certain will be performed without any issues.

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And here is an analogy written with the help of Lambda function.

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These two lines are similar in console.

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We can see that when I print at least all elements assaulted by price.

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So now you know how to create a comparator by using function.

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That's all examples that I wanted to discuss with you in this lesson.

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Let's recap what we have learned today.

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Today we learned what function and function interfaces are.

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We learned what methods they have, and we know how to create lambda functions and Masset references

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for these interfaces.

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We learn how and then and compose methods work.

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Now, you know the difference between these two methods.

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I believe after this lesson, you also know how identity method works in function interface and also

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today had a lot of practice.

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And in real life example, we learned how we can use function and by function during the work with MAP

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implementations.

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At the end of the lesson, you saw how to create Comparator with the help of a function.

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That's all what I have for you for today.

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Thanks a lot for your attention.

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Have a great day and see you in the next lesson.