Relationships between classes in Java

The relationships between classes in Java include the following:

1. Generalization)

It is very simple, that is, the inheritance we often say. It means that sub-classes can obtain the features of the parent class and expand their own functions.

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Java code: extends and implements

2. Dependency)

When a is responsible for constructing B, A and B form a dependency, that is, a uses B.

 

 

Java code shows local variables, method parameters, and calls to static methods.

3. Association)

Two relatively independent things (a and B) form an association when object a holds object B.

There are two special forms: Aggregation and composition. There is only a conceptual difference between aggregation and combination, and there is no difference in code implementation in Java.

Aggregation: refers to the relationship between the whole and the part,

 

 

Combination: indicates the relationship between the whole and the part of the class, but the partial and the whole of the combination have a unified lifetime, that is, the whole object does not exist, and some objects will not exist,

 

 

In Java code, it is represented as a member variable.

4. Summary

In Java, the combination should be used as much as possible, instead of inheritance, because inheritance will complicate the class relationship too much and undermine encapsulation. Using the combination can also obtain the functions of existing classes, it also makes the new class more stable.

In fact, from the dependency -----> aggregate --------> combination, the relationship between classes is more closely, and the influence between classes is getting greater and greater. In fact, we usually do not distinguish between these relationships, in fact, this definition is not very easy to understand, so it will certainly lead to deviations in understanding. So when we use these things, try to be close to what everyone agrees, this makes it easy for others to understand.