Summary of Java Foundation (2) -- object oriented

Chris 3 2020-11-10 10:45:04
summary java foundation object oriented


1. object-oriented

(1) Concept
The so-called object-oriented is a programming idea , Through this kind of thinking, we can simplify the complicated things in our life , From the original executor to the commander , Object oriented is based on process oriented .
We often talk about object-oriented programming (OOP,Object Oriented Programming)

Object oriented emphasizes results , My understanding is that object-oriented is to encapsulate all kinds of things in our life into an object , It includes his various attributes and methods , Provide these methods and properties to the outside world , So that we can create (new) This object calls these methods and properties , So even if we don't know how the various properties and methods are encapsulated, we can use them .
(2) Three characteristics :
1、 encapsulation : Encapsulation is to combine the attributes and operations of an object into an independent whole , And try to hide its internal operation details .
2、 Inherit : Inheritance is , The child class inherits the characteristics and behavior of the parent class , Some ways to make a subclass parent . But in the parent class private The method of decoration , Cannot be subclasses with , And in the parent class static The method of decoration , Can't be overridden by subclass , Even if the subclass has a static method with the same name as the parent class , There's no connection between the two .
3、 polymorphic : Polymorphism is the ability of one behavior to have many different forms or forms . Polymorphism is the same interface , Use different instances to perform different operations , Polymorphism is the embodiment of various forms of object . When a method is invoked in a polymorphic manner , First check that the method is present in the parent class , without , Compile error ; If there is , Call the subclass's method with the same name .
The benefits of polymorphism : Can make the program has a good extension , And the object of all classes can be treated in general .

2. Construction method

(1) Concept : The construction method is a special method , It is a method with the same name as the class and the return value type is of the same class type . The creation of objects is accomplished by constructing methods , Its function is to complete the creation of objects or the initialization of objects . When a class instantiates an object, it automatically calls the constructor . Constructors, like other methods, can be overloaded .
(2) form :
It can be with or without

`` Modifier Class name ([ Parameters ]){

 Code ……

}``

3. Keywords in object-oriented

(1)private: Is a permission modifier , Used to modify member variables and member functions , Privatized members can only be accessed in this class , Want to modify or get the member , It can only be provided to the public get and set Method .
(2)this and super:this A reference object that represents the object of this class .
In the constructor ,this() Must be on the first line .
(3)extends: Is an inherited keyword , Indicates that the child class inherits the parent class , The child class inherits the characteristics and behavior of the parent class , Some ways to make a subclass parent . But in the parent class private The method of decoration , Cannot be subclasses with , And in the parent class static The method of decoration , Can't be overridden by subclass , Even if the subclass has a static method with the same name as the parent class , There's no connection between the two . This is also one of the three characteristics of object-oriented .
(4)static: Static keywords , You can modify member variables 、 Member methods and static code blocks ( Load as class loads , And it's loaded only once , It is generally used for project initialization ), The feature is that the modified variables and methods will be loaded with the loading of the class , Better than object loading , Load only once and it will always exist , No new space will be opened up , Global unique global share , It can be called directly by the class name , In addition, static can only call static , Non static can be called at will ,static Can't drink this perhaps super The common use , Because there is static There may be no object yet .
(5)final:final Modifiable class 、 Methods and member variables , Specific is to be final Modified class , uninheritable , By final Methods that modify cannot be overridden , By final The modified variable is a constant , Value cannot be changed .
(6)abstract: Abstract keywords , Used to modify a class or method ,Java Methods without method bodies can be defined in , This method is implemented by its subclasses . The method without method body is called abstract method , Classes that contain abstract methods are called abstract classes . Abstract class can be understood as a special class with only method declaration and no method body . The feature is that there can be no abstract methods in the abstract class ( There are subclasses to implement ), If there are abstract methods in the class , Then the class must be defined as an abstract class , After subclasses inherit abstract classes , Or it's still an abstract class , Or rewrite all the abstract methods , Abstract classes cannot be instantiated .

4. Interface (interface)

(1) Concept
Java Because multiple inheritance is not allowed , So if you want to implement the functions of multiple classes , It can be implemented by implementing multiple interfaces .

Java Interface and Java abstract class It stands for abstract type , It is the concrete representation of the abstraction layer that we need to put forward .OOP object-oriented Programming for , If you want to improve the reuse rate of the program , Increase the maintainability of the program , Extensibility , It has to be interface oriented programming , Abstract oriented programming , Use the interface correctly 、 Abstract class these too useful abstract types are used as java At the top of the hierarchy .
(2) characteristic
1、 Interfaces are all abstract methods
2、 adopt interface Keyword creation interface
3、 adopt implements Let the subclass implement
4、 Can be interpreted as , Interface is a special abstract class
5、 The interface broke through java The limitations of the single inheritance
6、 Interface and class can be implemented more , Interface and interface can inherit more
7、 Interface is the rule of exposure , It's a set of development specifications
8、 The interface improves the function extension of the program , Reduced coupling
9、 There is no constructor in the interface . Default when creating objects that implement classes super(), Is the default of the call Object The nonparametric structure of .
10、 There is no member variable in the interface , They're all constants . therefore , When you define a variable without a modifier , It will be added by default :public static final
11、 Methods in the interface , Default is abstract , If you don't say it's abstract Of , That will automatically make up
12、Java The limitation of Zhongshan inheritance can be solved through the interface . Interfaces can be inherited or implemented in many ways , You can even inherit and implement more at the same time .

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本文为[Chris 3]所创,转载请带上原文链接,感谢

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