PHP study notes -- core concepts of object-oriented

Inheritance and polymorphism both reuse classes. One of them is class-level reuse, and the other is method-level reuse. Combination and inheritance of classes. Low coupling, decoupling, tight coupling. Problems with inheritance:
1. Inheritance destroys encapsulation.
2. Inheritance is tightly coupled.
3. Inheritance and extension are complicated.
4. Improper use of inheritance may violate real-world logic. When creating a combined object, the combination needs to create local objects one by one, which adds some code to a certain extent, but inheritance does not require this step, because the subclass automatically has the methods of the parent class. How to use inheritance:
1. Carefully design it specifically for the inherited class. The abstraction layer of the inheritance tree should be relatively stable, generally no more than three layers.
2. For classes that are not specifically designed to be inherited, prohibit them from being inherited, that is, use the final modifier. Using the final modifier can not only prevent important methods from being illegally overwritten, but also give the editor opportunities to find optimizations.
3. Prioritize using combination relationships to improve code reusability.
4. The subclass is a special type, not just a role of the parent class.
5. Subclass extensions, rather than overriding or invalidating the functions of the parent class.
6. The bottom code mostly uses combination, and the top-level/business layer code mostly uses inheritance. Using combination at the bottom level can improve efficiency and avoid bloated objects. Using inheritance for top-level code can improve flexibility and make business use more convenient. Multiple inheritance mechanism: Traits. Polymorphism:
Meaning: When objects of the same class receive the same message, they will get different results. And the news was unpredictable.
Overloading is not something in object-oriented, and it is not the same concept as polymorphism. It is a form of polymorphism.
Polymorphism is a programming way of describing the same object through multiple states or phases. Its true meaning is: in actual development, you only need to care about the programming of an interface or base class, and do not have to care about the specific class to which an object belongs. Implementation of polymorphism:
1. Polymorphism refers to the reification of the same type of objects at runtime.
2. The PHP language is weakly typed, making it easier and more flexible to implement polymorphism.
3. Type conversion is not polymorphism.
4. In PHP, the parent class and the child class are regarded as the "stepfather" and "stepson" relationships. They have an inheritance relationship, but there is no blood relationship. Therefore, the subclass cannot be transformed upward into the parent class, thereby losing the most typical feature of polymorphism.
5. The essence of polymorphism is if...else, but the level of implementation is different. Interface defines a set of specifications to describe the functions of a "thing", requiring that if the "thing" in reality wants to be usable, it must implement these basic functions. Abstract classes and interfaces. In a sense, Traits and interfaces are both a disguised implementation of "multiple inheritance". Several concepts of interfaces:
1. Interfaces exist as a specification and contract. As a specification, the interface should ensure availability; as a contract, the interface should ensure controllability.
2. An interface is just a declaration, once you use the interface keyword, you should implement it. It can be implemented by the programmer (external interface) or by the system (internal interface). The interface itself does nothing, but it can tell us what it can do.
3. There are two shortcomings in the interfaces in PHP. One is that there are no contract restrictions, and the other is that there are not enough internal interfaces. Reflection:
In object-oriented programming, objects are given the ability to introspect, and this introspection process is reflection.
Reflection, intuitive understanding is to find the departure point and source based on the arrival place.
Reflection refers to extending the analysis of PHP programs in the running state of PHP, exporting or extracting detailed information about classes, methods, properties, parameters, etc., including comments. This function of dynamically obtaining information and dynamically calling object methods is called reflection API.

The role of reflection: can be used for document generation. (hook?)

Its uses include: automatically loading plug-ins, automatically generating documents, and can even be used to expand the PHP language

PHP has a Token function, which can achieve some reflection functions through this mechanism. Why use reflection? What is the necessity for reflection?

The purpose of reflection is to expand unknown applications

Exception and error handling:
The concepts of exception and error are different.

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