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Hello.

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The students that they were going to start, important and very interesting topic, I'm talking about

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functional programming and this lesson, we are going to understand what functional programming is.

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Well, understand how it is different from object oriented approach after this lesson.

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And you know what the difference between method and function is?

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And only after we will understand the basics of functional programming, we will talk about functional

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programming in Java.

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We are going to review Jaberi till function package and understand its structure will not have enough

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time to learn all types from that package today.

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But you will understand the structure of the package and what it exists for.

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The things that we are going to learn in this class and will help us to boost our learning of functional

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programming in Java.

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Let's start and let's understand first what is a functional programming.

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Functional programming is a programming paradigm where programs are constructed by applying and composing

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functions in functional programming.

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Functions are treated like the main core element of each program, and they can be assigned to variables

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and can be returned from the Massett, the same as other data type.

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To understand functional programming better.

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Let's discuss concepts that are specific to functional programming.

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The first concept is a concept of first class function.

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Usually this term is applied to programming language that can treat functions as first class citizens.

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That means language supports and functions as an argument to other functions, return and functions

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from other functions as value and assign functions to variables.

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The next concept that is related to functional programming is pure functions.

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These are functions that have following properties.

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The first one, its evaluation has no side effects.

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That means the function doesn't modify some state variable values outside the function.

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And the second property is that original value of pure function is the same for the same arguments.

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That means, given the same input, will always return the same output.

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This can be used for code.

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Optimization, for example, is a result of pure function is not used.

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It can be removed without affecting other expressions.

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That also means that pure functions are completely independent from outside state and can be used in

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parallel or asynchronously.

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It is easy to move them around in your code, making your programs more flexible and adaptable to future

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changes.

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The third concept of functional programming is a recursion and functional programming.

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There is no for and while loops.

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But what is used instead calling the same function from another function is called recursion and can

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be used for iteration.

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The fourth concept is that functional programs do not have assignment statements.

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That means the value of a variable is immutable and doesn't change state once it was defined.

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This is called referential transparency.

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The next concept in our list is lazy evaluation.

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What does that mean?

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Each programming language has strategies to evaluate code blocks and when we call a function, programming

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language should define the strategy to produce arguments.

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There are two possible main evaluation methods.

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They are strict, sometimes called EGA evaluation and non strict so-called lazy evaluation.

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What is the difference in strict evaluation?

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Arguments are processed immediately when we invoke a function and give a list of arguments.

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Each argument is processed and calculated before colon's a function, whereas in lazy evaluation expression,

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evaluation is deferred.

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Evaluation is delayed as long as possible until it is necessary to evaluate the expression to produce

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a result.

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That's only five key concepts in different literature.

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You can find the different list of concepts or just different names for the same concepts that I have

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just described.

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I would say that the list of the current concepts is enough to understand the functional programming.

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What advantages and disadvantages functional programming has been through professionals.

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We have to understand advantages and disadvantages of functional programming approach the disadvantages

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advantages it is worth to mention that because of using pure functions that don't change any state and

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are entirely dependent on the input, they are simple to understand the simplicity of functional programming

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is one of the advantages that is mentioned.

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Most often.

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The testing is easier than the bargain is.

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Easier is it is the first advantage.

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The second advantage is that because of the pure functions, implementation of concurrency is much easier.

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We shouldn't worry about state being changed from the different threats of execution in parallel.

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To be honest, these are the biggest advantages you can Google advantages of functional programming

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if you wish, but all of them can.

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Be grouped in these two advantages that are listed, somebody would also mention that programs are more

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concise and more bulletproof.

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But I would rather said that this is associated with a simplicity that we have just discussed and the

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logical benefits of PUA functions.

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Somebody would also notice that absence of classes in functional programming makes the program code

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easy to read.

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But in my humble opinion, these advantages depend on the type of software you develop.

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While functional programming works great for simple programs and for beginner programmers, I would

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like to see how people who write articles that functional programming is the best.

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I would love to see how they would implement some complex enterprise solutions that requires the mind

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driven design and object oriented approach.

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And remember, team, we are professionals.

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That's why we don't say that functional programming or object oriented approach is better than something

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else.

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We just say that each approach works the best for specific cases and we need to know when to apply a

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different approach if needed.

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Talking about these advantages of functional programming, I'd like to mention a few items here.

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The first thing that you probably might think of is that in case all values are immutable and functions

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doesn't change state that would cause significant performance issues during the processing of a big

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amount of data because a lot of values will be created that are not used.

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That would also require a specific mechanism for memory management.

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The second disadvantage is partially we talked through imagine that huge online store with customer

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management, personalized promotions, specific rules for discount management, affiliate program,

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warehouse management, etc..

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Imagine that this kind of program is written with only functions.

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Would that really make code readable?

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Would that make the code easier to maintain and extend?

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Or onboarding of new engineer to the project will require a month or two for learning all functions

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that exist in the project.

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That's why among the disadvantages of functional programming sometimes mentioned that in particular

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cases, programs that are even with the help of only functions are hard to read.

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The third important disadvantage that is worth to mention is that while writing of pure functions might

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look easy, it is hard task to combine pure functions and make them work together to achieve some specific

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goal and integrate them with the rest of application.

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Now, you know, advantages and disadvantages of functional programming.

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Functional programming is a form of declarative programming.

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After you understood what a functional programming is, it will be easy for you to understand the definition

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of declarative programming.

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Declarative programming is a programming style of building the structure and elements of computer programs

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that express the logic of a computation without describing the order in which individual statements,

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instructions or function calls of program are executed or evaluated.

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Does it make sense now on the opposite of declarative programming, we have imperative programming in

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computer science.

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Imperative programming is a programming paradigm that uses statements that change a programs state.

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To make you understand imperative programming, I would like to use object oriented development approach

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as an example.

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We have objects that have state and masses to modify the state.

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Usually Java is considered to be an imperative programming language, and in Java we usually use term

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method instead of function to describe some specific block of code that can be executed.

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What is the difference between the function and method or this is the same?

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A function is a piece of code that is called by name and method is a piece of code that is called by

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name that is associated with an object.

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And I believe you can see that these two definitions are close, but not the same.

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There are two main points to consider here.

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The first one is that MassArt is called on the object.

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That's why Massager is considered to be closer to object oriented programming approach.

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Function can exist without any object and can be called independently.

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And the second point, the method is able to operate on data that describes the state of the object.

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Function works with arguments that are pastorate.

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Does it make sense?

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Help now?

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The difference between function and MassArt is clearer.

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Let's talk now about functional programming and Java.

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Functional programming was introduced in Java eight and I'd like to add more specifics here.

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In my opinion, it will sound correctly.

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It will say that functional like syntax was introduced in Java version eight.

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Java still remains imperative language.

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Java still uses object oriented approach, and even functional programming in Java is implemented in

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object oriented manner.

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That is really impressive.

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Java combines is.

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So declarant functions and it's pure object oriented style that deprives the Java feeling during the

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code in Java introduced syntax improvements, namely now we can use lambda expressions and methods references

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during the development.

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We're going to learn what is this?

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And a separate lesson.

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And we'll practice in writing our own lambda expressions and create references.

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Java also introduced new package with the language function that's called an overview of this package

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to understand what was introduced there.

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And here you can see official documentation of your function package.

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Why do we need it in this package?

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Functional interfaces are grouped.

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What are functional interfaces?

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In short, every interface that declares only one abstract method is a functional interface in our next

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lesson.

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So we are going to learn in detail what functional interfaces are.

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But for this lesson, it is enough to imagine interfaces.

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It always has only one abstract method.

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And when I will say functional interface, you will understand what I am talking about.

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The functional interfaces provide target types for lambda expressions and methods references.

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Let me explain this in simple words.

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This package group, all types that can be used during the declaring of lambda expressions and method

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references, all types in this package follow specific naming convention and this is specified here,

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will not learn this right now because in my opinion it is better to learn during the practice.

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You can see that this package was introduced in Java version eight.

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And here you can see a list of types that we are going to learn.

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Clear explanations written right here next to type name.

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I can leave the link to this documentation in attachments to the lesson and you would read it after

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the lesson.

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Don't try to understand everything because we are going to have lessons that will teach you how to work

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with types from this package.

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But you can try to understand why we need some types here.

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For example, here you can even see a function type.

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This type is developed to represent the function that accepts one argument and produces a result.

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While learning stream API will use some of these types to declare a lambda expressions.

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And now when you can see the whole list of types that allows us to create an object of these types,

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you can understand why.

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A few minutes ago I said that functional programming Java implemented with a feeling of object oriented

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paradigm.

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You can see that function is also a separate type and I can create an object of function type.

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I believe this information is more than enough to understand what functional programming is and what

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we are going to learn.

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Let's recap what we have learned today, that iminent what functional programming is.

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We discussed main concepts of functional programming.

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After that, we discussed advantages and disadvantages of functional programming.

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Also today, we learned what is imperative and declarative programming.

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We learned what the difference between the function and the method is.

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And at the end of the lesson, I told you what functional programming in Java is and hold an overview

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of Jabbawockeez function package.

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I believe now we have enough theoretical knowledge to practice our learning of functional programming

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in Java.

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That's all what I wanted to discuss with you during this lesson.

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Thanks a lot for your attention.

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See you in the next lesson.