Six relationships of classes -- the difference between dependency and Association

For a lot of new contacts UML Children's shoes , You may not understand the associations and dependencies between classes , Let's talk about the dishes today .

 

rely on

Expressed as parameters in the function (use a), Is the connection between classes , Indicates that one class depends on the definition of another class , A change in one class will affect the other . For example, if A Depend on B, be B Embodied as local variables , Method parameters 、 Or static method calls . Like TV (TV) Channel dependent (channel) Common dependencies are as follows ：
（1） class B Pass in the class as a parameter A Methods . I personally call it “ Parameter dependence ”.
（2） class B Exists in a class as a local variable A The methods of . I personally call it “ Local dependence ”.
（3） class A Calling class B A static property or method of . I personally call it “ Static dependency ”.
UML The implementation in the figure uses a line with The dotted line of the arrow points to the dependent class , as follows ：

relation （ delegate ）

Expressed as variables (has a), Join between classes , It lets one class know the properties and methods of another class . For example, if A Relating to B, be B Embodied in A Global variable of , Such as person Classes and company class .
There are two-way Association and one-way Association ：
1、 Two way connection ： Both classes know the common properties and operations of the other class .
2、 One way Association ： Only one class knows the public properties and operations of another class .
Most associations should be one-way , One way relationships are easier to establish and maintain , It is helpful to find classes that can be taken .
UML The implementation in the figure uses a solid line ( Some places use A solid line with an arrow ) Connect the same or different classes , as follows ：

       This is indeed a bit messy , But Xiaocai suddenly found a better entry point , Take it out and share it .

       Readers who have been exposed to design patterns , You will often see such scenes ： In instantiation A Class time , need B Class as a parameter of the constructor , This explanation A Class needs to hold a B Class reference . For example, the agency model 、 decorate Patterns, etc , They all do that . for example Java Medium IO The flow adopts the decoration mode , So we often see such statements ：new BufferInputStream(new FileInputStream("c:\\1.db"));

person And company The relationship between is also called delegation relationship （ What I call myself ）.

       This holds references , Is a simple relationship . In the code, it's shown as ： stay A Class has a member variable , The type of variable is B class ,A Class holds B Class reference , Just explain A Classes and B Class is associated .

       use UML The figure is as follows ：

       A little explanation , Because it is A Class holds B Class reference , Therefore, the correlation is from A Class ( from A Class induced ), So the arrow should start from A Class point B class .

       Usually , This simple one-way association is enough , But the relationship is mainly used in database design .

       In database design , Whether it's one-on-one 、 One to many 、 Many to many , Are not one-way .

       From the perspective of tables , Both of them can determine the other end from either end . Take one to many , With one The primary key of the end , According to many Check the foreign key of the end table many End data ; With many End foreign key , According to one Check the primary key of the end table one End data .

       From the perspective of entity classes , You can also determine the other end from either end . Take one to many ,one The end entity class will hold a many End reference collection , for example private Set<B> bs;, Query to one End , You can read directly from this collection many End ;many The end entity class will hold a one End references , for example private A a;, Query to many End , You can identify each directly from this reference many Terminal one End .

       thus , It becomes a two-way Correlation , use UML When drawing relationships , Add arrows on both sides , This is too ugly , Simply do not add .

       For example, the relationship between department entity class and employee entity class , You can say that ：

 

       Because it is the association relationship between database entity classes , Therefore, we need to add the quantitative relationship ,1 representative one End ,0..n representative many End , Explain that a department can have multiple employees , But an employee can only belong to one department , adopt UML The figure describes one to many .

       This is the typical application of Association .

       I have to mention that , Associations can also be broken down into aggregation and composition ( The specific concepts of the two are searched by the readers themselves ).

       Small dish discovery polymerization 、 Combination can be understood from another angle .

polymerization

       Let's start with aggregation , It is a weak correlation , The general meaning is that the whole and the part can exist independently . If we change the angle , It can be regarded as a cascading operation of the database .

       Take the group and its members for example , When deleting a group , Delete the members of this group , This is obviously unscientific , Because the group and its members are weakly related , The group can have any one member , A team member can also go to any group , This group no longer exists , You can go to another group , They are not necessarily related , It can be called aggregation .

       therefore , When we were designing the database , Cascading deletion is often not set , in other words , Deleting a group does not delete group members .

       UML The figure is as follows ：

       Hollow diamond for aggregation , Point to one End .

Combine

       Let's talk about combination , Composition is a strong association , The general meaning is that the whole and the part are inseparable , Can't exist independently . Also understand from cascade operation .

       Take students and student ID cards for example , If a student drops out , No longer belong to this school , Then consider deleting the student information , When deleting , The student ID information corresponding to the student will also be deleted , You can add cascading deletion . Because the student ID card belongs to a student's proprietary information , The student no longer exists , The student ID card cannot be used by others , So it's a strong correlation , It can be called combination .

       UML The figure is as follows ：

       The last thing to talk about is dependency .

       If A Class , Used B class , Then say A Classes depend on B class , They are dependencies .

       A Class uses B class , Maybe the parameter of the method is B class , It is also possible to get a B Class instance . But in either case ,B Class in A Classes are all based on local variable In the form of .

       therefore ,A There are B Type of local variable , Just say A Classes depend on B class .

       UML The figure is as follows ：

       Dashed arrows indicate dependencies , The arrow points to the dependent class .

       Sum up , There is a simple rule of judgment ： A class appears in another class as a member variable , The two are related ; One class appears in another class as a local variable , The two are dependencies .

        Be careful ： This article is for the convenience of explanation , Always take class as an example , This is not a good design thinking . In development , In order to achieve better " open - Closed principle ", Generally, the interface is defined , Interface dependent , Depend on abstraction , Not according to specific programming , I hope readers will not be misled by the side dishes ！！