UML Model content

· Model content

Semantics and representationThe model includes two main aspects: semantic information (semantics) and Visualized Expression (Representation ).

In terms of semantics, a set of logical components are used to express the meaning of an application system, such as classes, associations, States, use cases, and messages. Semantic model elements carry the meaning of the model, that is, they express the semantics. Semantic Modeling elements are used for code generation, validity verification, and complexity measurement. Their visual appearance is irrelevant to most tools that process models. Semantic information is usually called a model. A semantic model has a lexical structure, a set of highly formal rules, and a dynamic execution structure. These aspects are often described separately (that is, the document defining UML), but they are closely related and are part of the same model.

Visualized expressions display semantic information in the form of observation, browsing, and editing. The representation element carries the visual expression of the model, that is, the semantics is expressed in a way that can be directly understood by people. They do not add new semantics, but organize expressions in a useful way to emphasize the model examples. Therefore, they play a guiding role in understanding the model. The semantics of expression elements comes from semantic model elements. However, because a model graph is drawn by a person, the expression element is not entirely from the logical element of the model. The arrangement of expression elements may express another meaning of the semantic relationship. These semantic relations are not obvious or ambiguous, so that they cannot be formally expressed in the model, but can be enlightening.

Context)The model itself is a computer system product and is applied in a large context that gives the meaning of the model. This includes the internal organization of the model, comments to each model throughout the development process, a set of default values, assumptions for creating and manipulating the model, and the relationship between the model and its environment.

The model requires an internal organization that allows multiple working groups to use a model at the same time without too much mutual involvement. This decomposition of the model is not required in terms of semantics-compared with a model that is decomposed into multiple coherent packets, the information expressed by a model with a large single block structure may be equally accurate, because the determination of the boundary of the organizational unit may complicate the work of accurately defining semantics, therefore, the information displayed in a single block model may be more accurate than the packet structure model. However, it is impossible for multiple working groups on a large single block model to work effectively without mutual interference. Second, a single block model does not have reusable units suitable for other situations. Finally, some modifications to the big model often lead to unexpected consequences. If the model is properly decomposed into small subsystems with good interfaces, the consequences of modifications to a small and independent unit can be tracked and determined. In any case, decomposing a large system into a hierarchical organizational structure composed of carefully selected units is the most reliable method for designing a large system that has been invented for thousands of years.

The model captures the semantic information of an application system, but it also needs to record various information in the model development process, for example, the designers of a class, the debugging status of the process, and the usage permissions of various personnel. This information is at most the peripheral information of the system semantics, but it is very important for the development process. Therefore, the establishment of a system model must take two aspects into account. The easiest way to do this is to add project management information to the semantic model as a comment, which allows you to describe model elements in any way using non-modeling languages. In UML, text strings are used to represent comments.

The commands used by the text editor or browser are not part of the programming language. Similarly, the commands used to create and modify models are not part of the semantics of the modeling language. The attributes of model elements have no default values. In a specific model, they all have values. However, for the actual development process, it is required that you do not need to detail all relevant details when creating and modifying the model. The default value exists at the boundary of the modeling language and the modeling tool that supports this language. In the model creation commands used by modeling tools, they are the true default values, although they may surpass a single modeling tool and become the common language used by the modeling tool as the user expects.

Models are not built and used in isolation. They are part of the big environment where models are located, including modeling tools, modeling languages and language compilers, operating systems, computer network environments, and restrictions on specific system implementations. System information should include information about all aspects of the environment. Part of the system information should be stored in the model, even if the information is not semantic information, such as the Project Management comment (discussed above), code generation prompt, model packaging, editing tool default command settings. Other information should be stored separately, such as program source code and operating system configuration commands. Even the information in the model can be interpreted in different places, including the modeling language, modeling tool, code generator, compiler or command language. This book uses UML to explain the model itself. However, when implementing the physical implementation of the system, other resources used for interpretation are required. These resources are invisible to UML.