1

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So you know understand inheritance.

2

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But the next question is designed to look.

3

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And the question would be how do you know which of the classes should be a superclass on which one should

4

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be at subclass for example can staff be a superclass and user be a subclass.

5

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Similarly if there is already a class which is Dola but your team member is available from an external

6

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library then would it be OK to make one of your classes a subclass of this other class.

7

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These are some important questions we shouldn't blindly create inheritance relationships which could

8

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be dangerous.

9

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There are a few tests that you need to apply and we will look at only one of those tests which is called

10

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us the easy test the easy test is the fast on the most fundamental tests that you need to apply to determine

11

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if classes can be related via inheritance.

12

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Let's look at it using older user classes just test Spaf is a user is used to check if stuff can be

13

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a subclass of user.

14

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That is what the staff is it hypoth user or in other words with a staff is it specialized Russian officer

15

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.

16

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Then we read it all.

17

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It completely makes sense that staff is a user on so they can be connected via inheritance.

18

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So use is more abstract and generalized stuff is more specialized.

19

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Next is whether they did a resistance which also sounds perfectly right.

20

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Next is it.

21

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It is a user which is also Karak.

22

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In fact this just is automatically valid due to the first two Tests.

23

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That is if staff is a user and editor is a staff then editor is also the user.

24

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In other words when you claim an inheritance tree from editor you would encounter use or at some point

25

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.

26

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And that means there is an easy relationship between editor and user.

27

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So obviously chief editor is also a user.

28

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The final two examples are two random examples which make perfect sense.

29

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Sojourned is a doctor.

30

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Triangle is a shape.

31

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Not that easy.

32

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Books in only one direction for example.

33

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Is a user but the user is not as tough as we know that a regular user is simply a user of the website

34

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and does not how some of the capabilities of a staff member.

35

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So it test is a fundamental test which you need to apply to check for inheritance relationship.

36

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In addition there are also one or two other tests that you should apply and these are described in item

37

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16 of effective Java.

38

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But let's not worry about them for now.

39

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As it is a bit early to get into those concepts and we can look at them later.

40

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If is it test is failing then it means that you definitely must not use inheritance relationship.

41

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The easy test in the hostile examples shown here.

42

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That is a bookmark under you are two classes then bookmark is not a review.

43

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Similarly a review is not a bookmark.

44

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However a bookmark can have every view that is a bookmark and have an instance variable which is off

45

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pipe review and this case to say that there is has a relationship between them and object oriented programming

46

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.

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Such a relationship is also referred to as composition and in some cases composition is preferred over

48

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inheritance.

49

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It is actually the main subject of item 16 and we can look at it later.

50

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The final example shown here is about the relation between two classes bathroom on top.

51

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Now if you see double extends about the row it kind of sounds reasonable but as soon as you play easy

52

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test you will see that the relationship does not work in either direction.

53

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Bathroom has up is the right relationship and that's about it.

54

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Do make sure to always play the easier test.

55

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Thank you