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Hello.

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Yes, students in this class and we are going to learn what lambda expressions and message references

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are today.

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We are going to have really a lot of practice after this lesson.

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You will be able to write lambda functions like a pro.

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I will explain to you what functional interfaces are and we will learn the difference between regular

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and functional interfaces by the end of the class and you will know how to create your own functional

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interfaces.

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We are going to learn what the function is and what Massett reference is.

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I prepared special examples that will help you to grasp the knowledge faster.

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I'm going to show you how you can write lambda functions and how you can use math references.

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Let's start.

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And today, the best way to learn the topic is start from examples.

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That's why I suggest switching to the eclipse and start our lesson.

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To understand how to use Lambda functions and Macit references, we have to understand first what functional

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interface is.

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As we briefly discussed in previous lesson, functional interface is an interface that has a single

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object method declared in it.

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Take a look at this example, this interface is called Distance Calculator.

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It is declared to have Iraqi of types that can calculate distance of delivery of customer's order in

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miles.

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I can calculate the shortest path, the most optimal road route for interstate delivery, distance from

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the closest store or from the specific store and lots more.

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All this might be implemented in calculate distance method in other types that will implement the Easton's

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calculator.

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This method also takes two objects of city type.

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The store inside coordinates.

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We have city time declared here.

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This is a type created just for our example.

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The state of the city can be described with the help of the coordinates, longitude and latitude, and

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we have getters and setters here.

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Now you have the context of real life business case.

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I believe that understanding of real life example will help you to grasp new knowledge faster.

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That is great.

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You can see new annotations here, functional interface.

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What is it for?

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First of all, this is a great sign to notify clients of your code that originally regionalisms interface

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is created to be a target type for lambda functions and Macit references.

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And the second reason to use the sanitation is that in case somebody would try to add additional abasic

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Macit, there will be a compilation error.

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Let me comment this line of code that contains declaration of additional Massett.

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And here you can see we have a compilation error now.

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That's why functional interface annotation helps us to prevent cases when someone wants to add additional

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behavior to the type that we already used in other parts of the program as a tag for lambda function.

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Hope it is clear what function interface is.

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Let me comment this MassArt again to proceed.

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Pay attention to the fact that functional interface can contain as much default methods as possible

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and as much static methods as possible.

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This does not violate the rule for functional interface.

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That said, regarding functional interface, that was easy and great.

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Let me congratulate you.

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Now you know how to create your own custom functional interface.

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And now when you know what functional interface is, let's learn what lambda expressions are.

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Let me open the next examples that I have prepared before this lesson.

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I have a few more classes grouped here in one file, just for the sake of example, to not switch tabs

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between different files.

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Imagine that we have class with a name or the management, and this class requires an instance of distance

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calculator type to perform all operations related to the management we can pass in terms of distance

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calculator type, either with the help of constructor or with the help of the setup here.

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That means you need to create an object of type distance calculator in your program, but you know that

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you can't create objects on the basis of interface, right?

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That's why you declare concrete class, let's say the full distance calculator and implement distance

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calculator interface here.

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Now when we create object of type order management, we great object of type distance calculator.

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But what to do in case we need to change this since calculator to take into account specifics of inter-state

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delivery.

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In this case, we have to create another concrete class and we need this just override, calculate distance

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method.

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Only once is it worth it.

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We've seen that this doesn't work of our efforts and we decide to declare anonymous class and create

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an instance of anonymous class.

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This time we take into account logic for interstate distance calculation.

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OK, but can you see how much lines of code it takes?

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It is super weird that we have to declare a concrete class just to give implementation of one single

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method.

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We can create thousands of concrete classes to implement dozens of functions.

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But do we need to overwhelm our application with additional classes if we just need one function in

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one single place?

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No, that's where lambda function is going to help us.

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But what is lambda function?

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It is also called anonymous function.

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It doesn't have any name.

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Usually anonymous functions are passed to higher order functions.

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Also, in case the function is only used once a limited number of times, it will be easier to Islam

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the expression rather than the clear and concrete class in need to create an object of time distance

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calculator.

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We have to just implement one method, take a look here and declare a new variable of distance calculator

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type to describe new logic for calculate distance method.

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And here you can see the land expression.

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Here it is lambda function, anonymous function.

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We can call this in different ways by using different names.

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Let me explain in the syntax here.

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You can see.

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Parentheses that we usually use for MassArt invocation, Autobus arguments, you can see that I Passey

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Q1 and Q2 arguments where I this I didn't create any objects.

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This is just local MASAD variables similar to the ones we use in massive declaration.

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Using these variables I can get access to objects at my function should work with inside the body.

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I will show you that in a second.

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Why I don't write types of these local variables because compiler knows that I want to give implementation

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of the method from distance calculator type and the first argument.

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And this method has typesetter the same as the second argument.

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That's why Compiler knows that these local variables should be of typesetter.

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But in case I will add types here, there is no harm in this, just like this.

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In case if Jason's calculator wouldn't declare any Masset parameters and calculate distance measured

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in that case, we would skip Masad arguments here and would leave just blank parentheses like this.

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Here is LAMDA and here goes returned value.

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Here is some damage calculation.

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For example, I perform subtraction of longitude of 31 and 32.

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That doesn't give us a true distance between cities, but makes you understand that you can perform

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any calculations here.

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And the result of this calculation is returns from the MassArt worse given return keyword here, because

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Compellent knows that at one point of implementation of the absolute masset of distance calculate the

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time and compiler knows that this method should return value.

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Technically creation of object of anonymous glass.

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And this is the same because calculate distance implementation is the same as here.

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The next point you have to know about our Syntex in case our anonymous function consists of more than

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one line, then we need to use Brace's.

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And here you can see an example.

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Also, in this case, we have to right return keywords.

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And if you see that in this case, visually, this LAMDA function is closer to the method we have here

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in anonymous class where missing only a return type value method, name type of local variables and

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access modifier.

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So in case I write here, public double calculate distance and in case I would add types to local variables,

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does it make more sense like this?

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But for lambda expression, we don't need to specify access modifier, it is already accessed here,

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let me remove it also.

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We don't have to specify return that because our Java compiler can understand that we are going to implement

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Macit from a distance, calculate the functional interface and the method inside this interface has

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double return type.

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Can you understand now why it is important that functional interface has only one abstract method?

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Because in case functional interface has more than one method, then it is not clear what message I

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try to implement with the help of anonymous function.

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Is that clear?

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So I can remove, recontact and also I can remove massive name because it is clear what method I want

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to implement and I don't need any other identifier to access this function.

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And I believe you already understood why I can remove all types of my local variables, because compiler

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knows that these variables will be of type according to the interface here you can see that I can call

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calculate distance method on my reference here and I believe you already understood that these lines

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of code will be executed.

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Let me prove this.

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I run the program and here you can see console output from the lambda function.

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And when I need implementation of distance calculator interface and I need it only one time only it

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is easier for me to create lambda function rather than declare a concrete class and create an instance

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of a concrete class.

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For example, here in the set method, I can pass any function I need and that will be enough for order

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management because all the management is interested not in the fact of get an object of type discount

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calculator.

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Order management is interested in getting behavior to calculate distance and that gives a specific definition

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of behavior with the help of lambda function.

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Usually it is not instantly clear for my students where we will get arguments for the lambda function.

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I want you to understand that lambda function.

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It is not a method invocation with arguments.

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It is just the definition of the method with parameters similar to this thing we do when we implement

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any interface in concrete class.

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So the city objects will be passed here to this function, wherever it will be needed.

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According to the logic described in the order management.

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Sometimes we only know that some type like order management just needs logic to calculate distance between

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two cities.

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And sometimes we really don't care where order management will get those cities.

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Probably they will be passed to other methods of order management or will be present in all the details.

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But we let all the management object decide when to call our function and we let all the management

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bother about what just to pass there.

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Does it make sense?

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Hope you understood what lambda function is.

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Now, let me explain to you what a reference is.

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Imagine that you already have mass.

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That's exactly the same what you want.

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Probably it is from some external library or probably it is just from another class.

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Then why do we need to declare a concrete class to override MassArt or create lambda function?

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Why don't we just tell the order management object to use the mass that already exists?

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In all cases, when all the management needs to calculate distance, we can do so.

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And for this we have to use MassArt references.

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Let me show you the next example.

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Here is the example of some third party class.

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Imagine that it is called Google Dist. calculator and it has Masset for distance calculation.

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It is called get distance between cities.

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You can see that we have static and non static MassArt.

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That is, for the sake of example, to cover a few cases.

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Please pay attention to the fact that type of return value and type of method parameters is the same

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as we have in distance calculator interface.

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This is important.

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And now when I need to set new distance calculator into my or the management object, I just pass the

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reference to this method.

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Let me explain this.

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This index here here is the name of our class that contains the message that we need, followed by double

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column.

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This is a syntax of reference I'm referencing to the distance between a static method in Google distance

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calculator class.

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Does it make sense?

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And again, this is not in location.

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That's why I don't pass any arguments here, or the management just needs something that can take two

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cities and will return double value.

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That's all what order management object expects from distance calculator type and this method from Google

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Distance Calculator do the exact things that order management wants.

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That's why there is no compilation error here, pay attention that I use class name to make a reference

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to static method in case I want to make a reference to non-cyclical method.

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I have to create object.

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And here you can see that I created object.

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Now I can use variable of my object to make a reference to the non static method.

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Does it make sense?

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Hope now it is clear how Massata references work.

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They learned a lot today.

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Let's recap what we have learned today, the day we learned what functional interface is and we even

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created a custom functional interface.

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Now you know what the function is and how to write your own lambda expression.

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After that, we learned what math reference is.

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On the real example, we learned how to apply your knowledge.

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That's it for this lesson.

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Thank you for your attention.

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See you in the next lesson.