1) Single Responsibility Principle

Reduce the complexity of the class. A class is only responsible for one responsibility and improve the readability of the class. Maintainability reduces risks caused by class changes. Under normal circumstances, we should abide by the single responsibility principle. Only the class logic is simple enough to violate the single responsibility principle. If the number of methods in a class is relatively small, the single responsibility principle can be maintained at the method level.

2) Interface isolation principle

The client should not rely on interfaces it does not need; the dependence of one class on another class should be based on the smallest interface.

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3) Dependency Inversion Principle(Dependence Inversion Principle, DIP)

1. High-level modules should not depend on low-level modules, both should rely on their abstractions

2. Abstractions should not depend on details

3. Details should depend on abstractions

4 , Every logic implementation is composed of atomic logic. The indivisible atomic logic is the low-level module (usually an interface, abstract class), and the assembly of atomic logic is the high-level module. In the Java language, abstraction refers to interfaces and or abstract classes, neither of which can be directly instantiated. Details are implementation classes. The classes generated by implementing interfaces or inheriting abstract classes are details and can be instantiated directly.

4) Liskov Substitution Principle

If for every object o1 of type S, there is an object o2 of type T, so that all objects defined with T When all objects o1 of program P are replaced with o2, the behavior of program P does not change, then type S is a subtype of type T.

All references to a base class must be able to transparently use objects of its subclasses.

The second definition is the clearest. In layman’s terms, as long as the parent class can appear, the subclass can appear, and replacing it with a subclass will not cause any errors or exceptions. Users may There is no need to know whether it is a parent class or a subclass. However, the opposite is not true. Where subclasses appear, the parent class may not be able to adapt.

When using inheritance, try not to override parent class methods.

5) Open-Closed Principle (ocp)

A software entity such as a class, module and function should be open for extension and closed for modification.

Software implementation should be open to extensions and closed to modifications. This means that a software entity should achieve changes through extensions, rather than by modifying existing code.

6) Demeter's Law

is also called the Least Knowledge Principle (LKP). It can also be expressed as an object should have the least knowledge about other objects. Understanding, that is, a class should know the least about the class it needs to couple or call.

1. Only talk to your immediate friends (Only talk to your immediate friends) Among classes, what classes count as friends? Classes that appear in member variables and input and output parameters of methods are called member friend classes. The classes that appear inside the method body do not belong to friend classes.

2. There is also a distance between friends. Don’t expose it too much, otherwise the scope of influence will increase during the second modification. This also requires that public methods between classes cannot be exposed unscrupulously

3. If a method is in the relationship between classes, placing it in its own class will not increase the relationship between classes, nor will it have any impact on this class. If there is a negative impact, place it in its own class.

4. Perform serialization operations carefully, for RMI (Remote Method Invocation)

Best practice: The core of Dimit's law lies in the decoupling between classes. Only weak coupling between classes The reuse rate will increase. The required result is to generate a large number of transfer or jump classes.

7) Principle of synthesis and reuse

The principle of synthesis/aggregation reuse is to use some existing objects in a new object to make it a new object part; new objects achieve the purpose of reusing existing functions by delegating to these objects.

The short description is: try to use composition/aggregation and try not to use inheritance.

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