Object oriented thinking

# Summary of knowledge points Chapter II
 
1. object : Representing something " complex " Data is called objects , That is, some basic data types / The new composite data type formed by putting the composite data type into a class becomes an object
 
2. Object oriented languages have many advantages , Such as code reuse 、 extensible 、 Easy to maintain, etc
 
3. Three characteristics of object-oriented language ： encapsulation 、 inheritance 、 polymorphism
   encapsulation : Encapsulation immediately realizes the hiding of data , Some data is hidden from other parts of the program , Prevent others from modifying important data
   inheritance : To support code reusability 、 Reduce the total amount of code in the program
   polymorphism :Java There are two kinds of polymorphism: Method rewriting and method overloading
4.static The modified variable is a class variable
 
5. stay Java You can use private To modify a class ( It is usually used in one class Inside , Ensure data security )
 
6.System Classes of cannot be instantiated , That is, you cannot create System The object of , Usually we can go directly through System. visit System Method or property of
 
7. Every class must have a constructor , If you don't write it yourself , The system will generate a constructor by default , So a class must have a constructor , And the constructor has no return value
 
8. class 、 attribute 、 Methods can be modified by more than one modifier such as private static……
 
9. A static method of a class ( Yes static modification ) Can't access non static member variables , But an instance method ( nothing static modification ) You can access any member variable
 
10. Static variables are shared by all instance objects of the same class
 
11. Instance variables can only be accessed through objects , But class variables ( Static variables ) It can be accessed through classes or objects
 
12. The instance method of class represents   The behavior of class objects