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This article mainly introduces the UML class diagram example analysis, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let the editor take you to understand it.

Distinguish between operation operation and method method

The operation is the thing proposed to the object (process description), while the method is the process body. For example, the getPrice () of a superclass is an operation, while the getPrice () of all its subclasses is a method.

The operation can be divided into constant operation query and modified operation modifier. The difference is that they can change the observable state. One of the advantages of the constant state operation is that it changes the execution order of the constant state operation without changing the behavior of the system, so it is beneficial to highlight the constant state operation, and it is common practice to make the modified operation without a return value, so that the one with the return value is the constant state operation, although this can sometimes feel inconvenient. (Meyer's "modified operation-constant state operation separation principle").

Second, dependence

Class diagrams in UML essence typically have dependencies in the following two weeks: one class sends a message to another class; one class uses another class as the data part.

Overly complex dependencies can lead to a 'ripple effect'. The result of this effect is that if there is any change in the future, it will affect the whole body.

"words commonly used to express dependence"

The call source invokes the operation in the target

Create source creates an instance of the destination

Derive source is exported by destination

The instantiate source is an instance of the target (if the source is a class, the class itself is an instance of the class, that is, the target class is a unary class)

The permit destination allows the source to access the private characteristics of the destination

An realize source is an implementation of a specification or interface defined by a target

The refine source can be a design class with the target of the corresponding analysis class

The substitute source can replace the destination

Trace is used to track changes such as requirements to classes or changes in a model that are connected to changes elsewhere

The use source requires the target to be implemented.

Principles on which the design depends:

one. Minimize dependencies, especially when they span large areas of the system

two. Beware of circular dependencies, as they can cause changes to the loop

three. Trying to show that all dependencies in a class diagram are futile, with too many dependencies and too many changes, when the dependency and the topic are drawn from a direct connection.

four. It is also a good habit to strictly separate appearances from areas.

III. Aggregation and combination

The aggregate aggregation of the class graph in the essence of UML is the relationship between the whole and the part. Many developers think aggregation is important, even if they are based on different reasons. Therefore, UML contains aggregations. However, aggregation has no semantics in UML! Unlike a composite composition, it means that a class can be a component of multiple other classes, but any instance must be a component of only one owner, that is, it belongs to only one owner. For example, an instance of a point can be part of a multilateral row or a circle, but not both. This rule is also called a "non-shared rule". Second, if this multilateral row is deleted, you should automatically ensure that all points it has are deleted as well.

Omit aggregations in UML and use only combinations. For aggregations that appear in other people's class diagrams, different authors or development teams may have different purposes for using aggregations because they should be carefully analyzed.

IV. Classification and generalization, multiple classification and dynamic classification

Beware of the idea that all subclasses are is-a relationships. Sometimes the use of inheritance can cause inappropriate or confusion of responsibilities. Look at the following phrase:

1) Shep is a sheepdog

2) the sheepdog is a dog

3) Dogs are animals

4) the sheepdog is a genus of Breed

5) Dog is a breed of Species

It can be inferred from 1-4 that Shep is a genus, while 2-5 can be deduced that sheepdog is a species, which does not seem appropriate. This is because not all relationships here are generalized (the sheepdog type is a subcategory of the dog type), and some are classified (the object Shep is an instance of the sheepdog type). Generalization is transitive, but classification is not.

Why would you associate Shep objects with other classes? Look at the definition of classification, which refers to the relationship between objects and their types. Mainstream programming languages assume that an object belongs to only one class. However, in multiple classifications, an object can be expressed as several types, and they are not necessarily connected by inheritance.

When to use the class diagram

one. Try to use concise expressions (classes, associations, attributes, generalizations, constraints) and less advanced diagrams

two. Don't draw a class diagram for everything, but focus on the key aspects

The danger of using class diagrams is that you may be preoccupied with the structure and ignore the behavior. So, while drawing class diagrams in the essence of UML, * together with some form of behavioral technology.

Design according to the contract

Assertions are designed according to the contract and use three specific assertions: pre-condition pre-condition, post-condition post-consition and invariant invariant. Pre-conditions and post-conditions are used for operations. A postcondition is a statement that you should have done it beforehand after the operation is executed (such as the formula of calculation, you enter the parameters, and I will give you the result according to this formula), which is used to express what I don't do rather than how we do it. In other words, it is a useful way to separate the interface from the implementation. A precondition is a statement of how we expect things to happen (such as requirements for input values) before the operation is executed. The precondition makes it clear who is responsible for verifying the correctness of the input condition (such as the value range of the input parameter setting). This is important, and if there is no such clear statement of responsibility, it may be 'too little verification' (no inspection) or 'too much verification' (check both engines).

Through the pre-condition and post-condition, we can get a deep understanding of the exception Exception: when the exception occurs when the operation is enabled, the pre-condition is satisfied, but the operation can not send back the result that the post-condition is satisfied. Invariants are added to the pre-and post-conditions related to all common operations of a given class. In method execution, an invariant can be false, but it should be true when any other object can do anything to the recipient.

Assertions play a unique role in the construction of subclasses. One of the dangers of inheritance is that you may define a new subclass, but it is not compatible with the operation of the superclass. Assertions reduce the chances of this happening. Class invariants and postconditions must be used for all subclasses. Subclasses can choose to reinforce these assertions, but not weaken them. On the other hand, the precondition can not be strengthened, but can be weakened.

Thank you for reading this article carefully. I hope the article "sample Analysis of UML Class Diagram" shared by the editor will be helpful to everyone. At the same time, I also hope that you will support and pay attention to the industry information channel. More related knowledge is waiting for you to learn!

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