Detailed introduction to inheritance and polymorphism in java

Inheritance (extends):

　1: object is the parent (base) class of all classes.

　2: The subclass inherits all the contents of the parent class except (private modifications and construction methods).

　3: When a subclass manually creates a constructor, it must call the parent class constructor.

4: In Java, a class can only have one direct parent class (only single inheritance), and can have multiple indirect parent classes.

5: Subclass instantiation order: instantiate the parent class first, then instantiate the subclass; static first, then the members.

Polymorphism (referring to a phenomenon):

1: There must be an inheritance relationship; there must be a method Rewriting; the parent class reference points to the child class object (the condition for polymorphism to occur).

2: Method rewriting:

Return value type The basic types are the same.

Reference type parent class>=subclass.

　3: Permission modifier of subclass >= parent class.

　4: The parent class reference can only call the overwritten (rewritten) content within the subclass.

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　ett in any mistakes, please correct me, I am a young man with little talent and little knowledge）

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