UML Easy to read

This is a tutorial on the basics of UML; now that we're doing a project too tight, we haven't really done a real class-level detail design, let alone using UML to implement canonical modeling; This article is mainly about the relationship between several classes that I have been confused about before, Let's make it clearer when we really use UML for example class diagram design, and here are some of the relationships:

Inherited

Refers to the ability of a class (called a subclass, sub-interface) to inherit the functionality of another class (called the parent class, the parent interface) and to augment its own new functionality, which is the most common relationship between a class and a class or interface and interface, in Java where such relationships are explicitly identified by the keyword extends, In the design of the general is not controversial;

Realize

Refers to a class class implementation of the interface interface (can be multiple) function, the implementation is the most common relationship between the class and interface, in Java such relationships through the keyword implements clearly identified, at design time is generally not controversial;

Depend on

It can be simply understood that a class A uses another Class B, which is contingent, temporary, and very weak, but changes in class B affect A; For example, if someone wants to cross the river and borrow a boat, then the relationship between the man and the ship is dependent; At the code level, For Class B as a parameter is used by Class A in a methods method;

Association

He embodies a strong dependency between the two classes, or the semantic level of the class and interface, such as me and my friends; this relationship is more than relying on the contingency, the relationship is not temporary, generally long-term, and the relationship between the two sides is generally equal, the association can be one-way, two-way At the code level, the associated class B appears as a class attribute in the association Class A, or the association Class A refers to a global variable of the type being associated with Class B;

Polymerization

Aggregation is a special case of association relationship, he embodies the whole and part, the relationship of ownership, that is, the relationship between the whole and the part is has-a, they can have their own life cycle, some can belong to multiple whole objects, can also be shared for multiple whole objects, such as computer and CPU, The relationship between the company and its employees, and the expression at the code level, and the association is consistent, can only be differentiated from the semantic level;

Combination

The combination is also a special case of the relationship, he embodies a contains-a relationship, which is stronger than aggregation, also known as strong aggregation, he also embodies the relationship between the whole and part, but at this time the whole and part is not divided, the whole life cycle end also means that part of the life cycle end , such as you and your brain; At the level of the code, the relationship is consistent and can only be differentiated from the semantic level;

There is little doubt about the inheritance and realization of these two relationships, they embody a kind of class and class, or the vertical relationship between the class and the interface; the other four relationships embody the class and class, or the reference between the class and the interface, the transverse relation, is more difficult to distinguish between, there are many things between the relationship to prepare for positioning is very difficult, These kinds of relationships are semantic level, so from the code level does not completely distinguish between the various relationships, but in general, the following relationships show the degree of strength: Combination > Aggregation > association > dependency;

In UML class diagrams, the following relationships are common: generalization (generalization), implementation (realization), Association (association), Aggregation (Aggregation), composition (composition), dependency ( Dependency)

1. Generalization (generalization)

Generalization relationship: An inheritance relationship that represents a general and special relationship that specifies how subclasses can characterize all the characteristics and behaviors of a parent class. For example, the tiger is a kind of animal, that is, the characteristics of tigers have animals in common.

Arrow pointing to: Solid line with triangular arrows, arrow pointing to Parent class

2. Implementation (Realization)

Implement relationship: is a class-to-interface relationship that indicates that a class is an implementation of all the characteristics and behaviors of an interface.

Arrow pointing: Dashed line with triangular arrows, arrow pointing to Interface

3. Association (Association)

Association: An owning relationship that enables a class to know the properties and methods of another class, such as: teachers and students, the relationship between husband and wife can be two-way or one-way. Two-way associations can have two arrows or no arrows, and one-way associations have an arrow.

"Code embodiment": member variable

Arrows and points: solid lines with normal arrows, pointing to the owner

, teachers and students are two-way association, the teacher has many students, students may also have more than one teacher. But the relationship between a student and a course is a one-way link, a student may have to take many courses, the course is an abstract thing he does not own students.

To associate itself with:

4. Aggregation (Aggregation)

"Aggregation relationship": is the relationship between the whole and the part, and some can leave the whole and exist separately. If the car and the tires are the whole and part of the relationship, the tires leaving the car can still exist.

The aggregation relation is one kind of association relation, is the strong association relation, the Association and the aggregation cannot distinguish syntactically, must examine the concrete logical relation.

"Code embodiment": member variable

"Arrows and points": solid lines with hollow diamonds, diamonds pointing to the whole

5. Combination (composition)

"Combinatorial relationship": the relationship between the whole and the part, but part cannot leave the whole and exist alone. If the company and the Department is a whole and part of the relationship, no company there is no department.

A combinatorial relationship is a relationship that is stronger than the aggregation relationship, which requires that an object representing the whole in an ordinary aggregation relationship is responsible for the life cycle of the object that represents the part.

"Code embodiment": member variable

Arrows and points: solid lines with solid diamonds, diamonds pointing to the whole

6. Dependency (Dependency)

Dependency: A relationship that uses a class that requires the assistance of another class, so try not to use two-way interdependence.

"Code representation": A local variable, a parameter to a method, or a call to a static method

Arrows and points: dashed lines with arrows pointing to the user

The order of strength of the various relationships:

Generalization = implementation > Composition > Aggregation > Association > dependencies

The following UML diagram shows the various class diagram relationships in a more vivid way:

UML Easy to read