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Prefer composition to inheritance

2022-04-23 19:16:00 【Eighteen movements of Tathagata divine palm】

Java It's a object-oriented Language . encapsulation 、 Inherit 、 Polymorphism is the three characteristics of object-oriented . Whether studying or working, it may need to be reused , The first word I think of is ： Inherit . But in design patterns , Find that composition is a good way to reuse , It applies to most situations where we need to reuse , So use composition first rather than inheritance .

Inherit

Inherit （Inheritance） It's a hierarchical model that connects classes and classes . It refers to a class （ be called Subclass 、 A subinterface ） Inherit another class （ Called the parent class 、 The parent interface ） The function of , And the ability to add its own new features , Inheritance is the most common relationship between classes or interfaces ; Inheritance is a kind of is-a Relationship .（ Picture from the Internet , Invasion and deletion .）

Combine

Combine (Composition) It's a weak relationship , It embodies the whole and the part 、 The relationship you have , namely has-a The relationship between .
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The difference between composition and inheritance

stay Inheritance structure in , The inner details of the parent class are visible to the child class . So we can usually say that code reuse through inheritance is a kind of white box code reuse . If the implementation of the base class changes , Then the implementation of the derived class will change . This leads to the unpredictability of subclass behavior ;）
Combine By assembling existing objects （ Combine ） Generate new 、 More complex features . Because between objects , The internal details of each are invisible , So we also say that code reuse in this way is black box code reuse .（ Because a type is usually defined in a combination , So I don't know which implementation class's method will be called at compile time ）
Inherit , When you write code, you need to specify which class to inherit , therefore , stay Compile time Then the relationship is established .（ The implementation inherited from the base class cannot be dynamically changed at run time , Therefore, the flexibility of application is reduced .）
Combine , Interface oriented programming can be used when writing code . therefore , The combinatorial relationship of classes is generally in The run-time determine .

Comparison of advantages and disadvantages

Group close Turn off system	Following bearing Turn off system
advantage ： Do not break the package , Loose coupling between whole class and local class , Relatively independent of each other	shortcoming ： Break the package , Subclasses are closely coupled with their parents , Subclasses depend on the implementation of their parents , Subclasses lack independence
advantage ： It has good scalability	shortcoming ： Support extended , But often at the cost of increasing the complexity of the system structure
advantage ： Support dynamic composition . At run time , The whole object can select different types of local objects	shortcoming ： Dynamic inheritance is not supported . At run time , A child cannot choose a different parent class
advantage ： The whole class can pack the local class , Encapsulate the interface of the local class , Provide new interfaces	shortcoming ： A subclass cannot change the interface of a parent class
shortcoming ： The whole class cannot automatically obtain the same interface as the local class	advantage ： Subclass can automatically inherit the interface of the parent class
shortcoming ： When creating objects of the whole class , You need to create objects of all departments	advantage ： When you create an object of a subclass , There is no need to create an object of the parent class

summary

Why composition takes precedence over inheritance , Or use more combinations and less inheritance , Because composition is really more flexible than inheritance , It's also more helpful for code maintenance . Then whether inheritance is good for nothing , Not necessarily .