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Основы Java. Работа в Dropbox. Интенсив по Java

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Channel: Skillbox Программирование

курсы java

программирование на java

обучение

программирование java

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java для начинающих

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основы программирования

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уроки программирования

00:00:01

Hello friends, today we continue

00:00:04

to delve into the topic of

00:00:06

object-oriented

00:00:07

programming and consider such

00:00:11

concepts as interfaces and abstract

00:00:13

classes in this place. For those who have not watched

00:00:18

previous videos dedicated to the class

00:00:21

method of classes and

00:00:22

object-oriented

00:00:23

programming, I

00:00:24

advise you to review these videos,

00:00:28

and for those who have not yet I

00:00:31

haven’t subscribed to the channel, I advise you to subscribe

00:00:33

because not a single large complex topic

00:00:36

can be discussed in one video

00:00:38

and will inevitably be broken down into

00:00:41

some sequence. Well,

00:00:45

now let’s move on to the topic, let’s start with abstract

00:00:47

classes since they are more similar to

00:00:50

the classes that we are already accustomed to

00:00:52

using abstract class this is a class

00:00:57

from which you cannot create an instance, you

00:00:59

simply cannot write

00:01:02

new the name of an abstract class, the only

00:01:07

way to use an abstract class

00:01:08

is to create a descendant class,

00:01:11

well, it would seem, why such complexity,

00:01:14

why create some abstract

00:01:17

classes if you can create just the most

00:01:19

ordinary class,

00:01:21

but abstract classes are used

00:01:24

when related classes 1 part of

00:01:27

the methods are the same and others differ;

00:01:30

those methods that coincide

00:01:33

can be implemented in the base abstract

00:01:35

class and those that differ can be

00:01:37

implemented in descendant classes. The most

00:01:41

common example is, for example,

00:01:45

figures, that is, a figure in itself is an

00:01:49

abstract entity, but a circle or

00:01:52

square is quite concrete Essentially,

00:01:56

well, for example, if we

00:01:59

want to calculate the area of a figure, then the

00:02:02

area of the figure for a square and for a

00:02:04

circle will be calculated by different

00:02:07

formulas,

00:02:08

but on the other hand, if it is assumed

00:02:12

that this figure should be drawn

00:02:14

on the screen, then for example, the color of the figure

00:02:18

can be implemented by the same

00:02:22

function and using the same method, and then the

00:02:26

method for obtaining the color of a figure can be

00:02:29

transferred to the base abstract class and the

00:02:31

method for calculating the area can be transferred to the

00:02:37

descendant class. Abstract classes can

00:02:42

contain abstract methods.

00:02:44

These are methods for which there is only

00:02:47

a definition but no content, that is, there is

00:02:50

nothing in curly brackets.

00:02:52

This is a way to force yourself or other programmers to implement

00:02:56

these methods in descendant classes.

00:02:59

If you

00:03:01

work in a team, this is necessary so

00:03:05

that somewhere in some methods that

00:03:10

work not with concrete classes but with

00:03:13

abstract ones, you can

00:03:15

assume that such methods

00:03:17

will definitely exist, for example for the same

00:03:21

figure, you can write an

00:03:23

abstract class that will contain an

00:03:26

abstract function to get the area and

00:03:29

thereby force everyone who will

00:03:32

inherit from this class

00:03:35

to implement this function; this method

00:03:38

will be different for all types of

00:03:40

figures; consider the interface and the interface and is

00:03:46

similar to abstract classes but is

00:03:49

defined by a keyword interface

00:03:51

instead of

00:03:52

abstract class, now let's look at their

00:03:57

difference and this question is almost always

00:04:01

asked in interviews: how does an interface differ

00:04:03

from abstract classes?

00:04:07

One of the main differences between an interface and an

00:04:09

abstract class is that in an

00:04:10

interface, methods are only defined, but there

00:04:13

can be no implementation at all;

00:04:16

no second main difference is

00:04:19

that when inheriting a parent

00:04:22

class there can be only one abstract

00:04:27

class, this is or usually but it can

00:04:28

only be one and there are as many parent interfaces

00:04:31

as you like, the

00:04:34

meaning of creating interfaces is to create a

00:04:37

hierarchy or a set of classes that are

00:04:39

united by a common meaning, that is, like

00:04:42

shapes and a

00:04:44

square and a rectangle and a circle is all

00:04:47

shapes and a set of methods with different

00:04:51

implementations, well, the example was already given above, that

00:04:56

is, for example, calculating the area or the

00:04:58

drawing method, and so on, but now let’s

00:05:03

move on to the practical part,

00:05:05

I’ll illustrate the examples already mentioned above,

00:05:10

when demonstrating the examples, we will go in a

00:05:13

different sequence, that is

00:05:16

first we will demonstrate interfaces and

00:05:19

then abstract classes, and in fact,

00:05:22

these rules must be followed almost

00:05:26

always because the abstract class is a

00:05:30

narrower entity and therefore we need to

00:05:37

start tying our hands and making an

00:05:41

abstract class only

00:05:42

if there is really a need for it, well, let

00:05:46

's start with interfaces, let’s assume

00:05:50

we have a vital need to make

00:05:54

some kind of method that will take a

00:05:56

list of shapes as input and print

00:06:01

some of their parameters in the console: the name of the

00:06:03

shape, the area, the color, and so on, what we

00:06:07

must do as

00:06:11

object-oriented programmers, we

00:06:14

must design this structure that

00:06:17

is called from top to bottom from the general to the

00:06:19

specific, respectively, that’s where we

00:06:23

start, we make an interface, we create an

00:06:30

interface, we call it sew, that is,

00:06:33

a figure, and here we begin to discuss

00:06:39

what methods, in principle, we will need

00:06:42

from a figure, well, for example, let this be a

00:06:46

method getting a name, a good method,

00:06:54

then let this there will be a meter for obtaining

00:06:58

an area

00:07:03

and let this be a method for obtaining color,

00:07:14

well, this is how we figured out approximately what it

00:07:17

should be and for now, without bothering,

00:07:21

we write a method,

00:07:33

let this method take as input a

00:07:36

list of figures in the list we went through in the

00:07:45

previous lesson, so

00:07:55

now we import using the for

00:08:04

loop

00:08:09

we output the data to the console,

00:08:14

each figure,

00:08:31

first we output the name like this

00:08:41

in the second line we output to the area

00:08:54

in the third line we output the color in

00:09:07

this way well, and this is

00:09:11

actually one of the miracles of

00:09:14

object-oriented

00:09:15

programming when we still don’t

00:09:18

know anything about any specific figures but already

00:09:21

We were able to write a

00:09:22

method that somehow operates with these shapes, here’s

00:09:26

what we

00:09:31

can do next, in order to

00:09:35

call this method we need to

00:09:36

somehow create a list, a list of

00:09:41

shapes, create it,

00:10:02

missed the words new, what’s next, how to

00:10:06

fill out this list if you write it

00:10:12

like this, if you try make an instance of the

00:10:23

shape interface, the compiler will tell us

00:10:27

that we can’t do this cannot be

00:10:29

instantly ted what to do next, we

00:10:35

need to define not abstract but

00:10:38

concrete shapes, for example, let’s create a

00:10:43

circle class and inherit this class from the

00:10:53

shape interface, here I’ll point out another difference, the fact is

00:10:59

that when we inherit from a

00:11:01

class, we write the word

00:11:03

extends which so and but and then some

00:11:08

class, and thereby we emphasize that we are

00:11:11

extending the base class,

00:11:14

if we inherit from an interface we

00:11:18

write implements, that is, it implements in

00:11:28

this way, now we see that food and

00:11:33

underlines in red underlines in

00:11:36

red for the reason that the

00:11:39

circle class, although it inherits from the

00:11:43

interface, does not implement any of

00:11:46

the methods that are defined in the interface,

00:11:50

we need to implement all these abstract

00:11:53

methods,

00:11:54

so before we give it some initial

00:11:58

content for them, let’s do this

00:12:06

name let returns everything

00:12:14

in order for us to have an area, we

00:12:17

must have a radius

00:12:24

[music]

00:12:33

so the radius must be filled somewhere, let it be

00:12:35

filled somewhere in the constructor

00:12:57

for now, what to do is just

00:13:05

fill in the radius and when calculating

00:13:12

the area we return it using the formula

00:13:17

314 1592

00:13:22

6 multiplied by radius once again multiplied by

00:13:28

radius wrote fro instead of retour and the

00:13:38

color color well, we also need to store the color somewhere so

00:13:53

we also need to pass the color in the

00:13:55

constructor so that we can

00:14:15

return the color,

00:14:22

well, there are

00:14:24

no errors, the class is defined,

00:14:27

we define the 2nd class, let it be there will be

00:14:33

a rectangle

00:14:41

like it also implements a figure with us,

00:14:53

let it return the name of the

00:14:59

corresponding rectangle y

00:15:01

now as for the area, in order to

00:15:05

know the area we need to know

00:15:08

the height and width of the rectangle, we set up

00:15:13

these two variables

00:15:28

and of course we should also have a color

00:15:43

so we have the area is calculated as the height

00:15:49

multiplied by the width, that is, the formula is

00:15:53

completely different than for a circle, and the

00:15:58

color is the color like a sphere,

00:16:07

now

00:16:10

these parameters also need to be determined in the

00:16:12

constructor,

00:16:23

we will have everything there, we will have

00:16:27

the width,

00:16:29

we will have the color, well, this

00:16:45

way now look here in this place

00:16:52

we can write

00:16:55

New York we define the radius and

00:17:02

some color is red for example as

00:17:07

you can see here polymorphisms

00:17:11

inheritance works in one person

00:17:13

since circle is a figure

00:17:16

since it inherited from the

00:17:18

shape interface it is also a figure and this is the

00:17:22

design perhaps we will add the newly

00:17:28

created figure to the list of figures,

00:17:35

but we can also use a slightly simpler figure,

00:17:39

ask the structure to be built like this, for example,

00:17:42

and define a rectangle with

00:17:54

sides 11 and let the color be blue.

00:18:02

Now if we call the method that

00:18:10

prints the figures with and pass it a list of

00:18:14

these figures, we will get the following

00:18:17

we collect and launch as you can see, we

00:18:25

saw a description in the console of the figures

00:18:33

that we entered,

00:18:39

well, this is how

00:18:40

designing from above is carried out

00:18:43

designing from above, it’s good because

00:18:46

we have defined the print ships method

00:18:51

which takes figures as input and

00:18:53

now this method will not change

00:18:56

depending on what kind of figure we will still

00:18:58

define, we can add another figure of a

00:19:01

triangle, oval, square, whatever we

00:19:06

want, but this will no longer change

00:19:09

and thus we got rid of

00:19:13

a lot of work, as if all this

00:19:16

changed depending on the number of

00:19:18

figures, but let’s move on, it’s already quite

00:19:23

obvious that besides what is different in

00:19:30

these classes, there is a part that is the

00:19:34

same for these classes, but if this

00:19:39

part were not there, then I would advise,

00:19:41

of course, to leave the shake interface and not

00:19:45

worry about it, but since this part is

00:19:48

the same for us, we end up with duplication

00:19:50

in the code,

00:19:51

then everything is better - so put this into some

00:19:55

common entity and this common entity

00:19:59

will be our shape, we remake the

00:20:03

spike interface into an abstract class like this,

00:20:11

then

00:20:15

if we have a function in an abstract class,

00:20:18

or rather a method in an abstract class is not

00:20:20

defined, then it should be marked

00:20:26

as abstract like this That’s how I

00:20:36

didn’t put the public keyword in the interface

00:20:38

because in the

00:20:40

default interface all methods are public,

00:20:46

just the interface doesn’t make any

00:20:48

sense without this,

00:20:52

so on and so forth, while we will have the get color method in

00:20:59

common and we define it, we

00:21:10

define it in the same way as and in the

00:21:14

classes of us in the classes of the heir, then

00:21:18

and in the same way we fill out this field in the

00:21:23

constructor

00:21:41

and the get color method will return

00:21:44

light. Now now this get color method

00:21:54

can be removed

00:22:03

from the heir classes and everything

00:22:08

connected with this color, respectively, and we can

00:22:14

correct the constructors from

00:22:23

everywhere by color

00:22:29

now it would seem like a penny

00:22:31

saving, but in reality when there are a

00:22:33

lot of class hierarchies, there is a lot of duplicate code,

00:22:37

then this leads to

00:22:39

significant savings and simplification, so

00:22:43

now since the shape has ceased to be

00:22:46

an interface, it has become an abstract class,

00:22:49

we need to replace

00:22:50

implement with extends

00:23:00

and here the most interesting thing happens,

00:23:05

here I defined the constructor for the

00:23:09

abstract class shape and here you can

00:23:13

ask me how I defined

00:23:15

the constructor if you cannot create

00:23:19

instances of this class, how

00:23:22

can this constructor be called and

00:23:24

this constructor can be called as follows

00:23:27

in the constructor of the inheritor class the

00:23:33

following super construction is written

00:23:36

and the color is passed to this super

00:23:42

in a super way, this is a

00:23:51

link to the methods of the base class, let

00:24:15

's define the same thing for the rectangle,

00:24:20

now we remove from the constructors those

00:24:22

parameters that have disappeared, collect

00:24:31

and run, well, the result is about the same,

00:24:36

only there is less code,

00:24:44

so we showed what

00:24:49

abstract classes are like them apply as

00:24:52

abstract classes how abstract

00:24:55

classes

00:24:56

differ from interfaces how you can

00:24:59

design from top to bottom and write

00:25:02

methods that take

00:25:05

abstract entities as input which may

00:25:09

not even be in the program yet which can

00:25:11

be defined later,

00:25:13

if you liked the lesson, like it,

00:25:17

if you didn’t like it, write comments and

00:25:20

ask questions for everyone bye bye

00:25:24

good luck

Description:

Программирование на Java для начинающих. Продолжение темы объектно-ориентированного программирования. Базовые сведения об интерфейсах и абстрактных классах. Пример использования, указание на различия.

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