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There are 13 official diagrams in UML, but there are not many commonly used ones. When software development uses UML, it pays attention to software design rather than UML, so the design of a software does not need to use all the diagrams in UML. UML is just a tool to assist us in software design.

Everyone is different. I have six kinds of pictures commonly used, but only three are the most commonly used.

Friends who read this post should have some understanding of object-oriented, so we should know the following equation:

Class = attribute (member variable) + behavior (member method)

For example, human beings have attributes: name, sex, age, etc.; behavior: eat, drink, pull, scatter, sleep, etc.

In fact, there are only three commonly used ones: use case diagram, class diagram, sequence diagram and activity diagram.

Use case diagram: analyze the requirements and obtain the class diagram

Class diagram: derive the class diagram from the use case diagram and analyze the member variables and member methods of each class. Like humans.

Sequence diagram: realize the message passing between objects to complete the corresponding function. For example, Li Si and Zhang San said, "you need a key to open the door. Zhang San gave the key to Li Si, and Li Si opened the door." Three objects: door, Zhang San and Li Si send messages to complete the function of opening the door.

Activity diagram: a logical implementation flowchart of a member method in a class, usually represented by an activity diagram. For example, human eating behavior, how to eat?

Secondly, there are three kinds of diagrams that are also commonly used: state diagrams, deployment diagrams, and writing diagrams

State diagram: describes the state experienced by an object in its lifetime from initialization to recycling, including early childhood, youth, and old age.

Deployment diagram: to achieve the physical operation and communication of the program, that is, how to deploy the program after development is completed, so that users can use it.

Collaboration diagrams: basically useless, because the sequence diagrams that collaboration diagrams can describe can be described. However, because the function is equivalent to the sequence diagram, and the sequence diagram is more important, this diagram is also listed.

In the development of a software, we must first know what to develop for and what problems to solve. That is, the demand analysis part. The most commonly used requirement analysis is the use case diagram: User Case Digram

After that, through the requirement analysis, we can get the class diagram.

After getting the class diagram, the object is generated by the class to complete every function in the program, which is the cooperation between objects. Therefore, the object-oriented method is also called message-object method. Objects cooperate with each other, deliver messages, and complete the function.

The behavior in each object, or the behavior in each class (member method), the flowchart in which a function is implemented in the method is generally represented by an activity diagram.

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