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(4)Composition: Composition relation

Combinatorial relations are also a type of associative relations, which are stronger than aggregative relations. As we mentioned earlier, aggregation relations represent the relationship between the whole and the parts, and composition relations represent the relationship between the indivisible whole and the parts on the basis of aggregation relations. That is, the object representing the whole is responsible for representing the lifecycle of the parts of the object.

"The object representing the whole is responsible for keeping alive, and in some cases annihilating, the object representing the part. An object representing a whole can at some point pass an object representing a part to another object, which is responsible for the lifecycle of the object representing the part. In other words, objects representing parts can only form a combinatorial relationship with one object at a time. And the latter is exclusively responsible for its life cycle. Java and patterns

Let us take the relationship between man and arm as an example. The class diagram of the combination relationship is as follows:

(5)Dependency: Dependency

A dependency indicates that one class depends on the definition of another class. Dependency is unidirectional. People eat apples, so people depend on apples. The class diagram is as follows:

In general, dependent objects often exist in the form of local variables and method parameters in the object, unlike associations, which do not exist in the form of member variables in the object. This is worth noting. In addition, each dependency has a name. The name of this dependency is eats.

That's the relationship between class diagrams and common class diagrams. A recent written test actually required drawing typical class diagrams of all the relationships between classes. I broke down at that time and posted these two blogs to review and review.

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