Access modifiers and the use of optional modifier static

In Java, there are many modifiers, here is a brief talk about the choice of several access modifiers to use, and optional modifier static use.

I. Access modifiers: This is the access to the methods and variables in Java that control access to classes and classes, to expose only the interface, to hide the details of the internal implementation, the access control is divided into 4 levels (only applicable to class and class members decorated with private, default, protected , public), and then briefly introduce their selection in the form of a table:

	This class	Same package		Different packages
		Sub-class	Non-sub-class	Sub-class	Non-sub-class
Private	√	X	X	X	X
default	√	√	√	x	x
protected	√	√	√	√	x
Public	√	√	√	√	√

Description: Inside the table, √ indicates the scope that can be accessed with this modifier;

X means that the range that is not accessible can be decorated with this modifier;

Two. Use of optional modifier static:

　1.static modifier Properties (variables):

	Use (example: public static String name;)	Do not use (example: public String name;)
The grammatical	It can be accessed with a dot of the object, but it is more directly accessed by the class name.	This property must be accessed with a dot of the object.
On-Memory	This property is not placed on the object, but instead exists in the static area of the data segment and is shared by the whole class.	This attribute is placed on each object (each object has its own name property)
The grammatical	This property is related to the class and is independent of the object.	This property is part of an object
Variable generation time	is when the class is loaded.	is created at the time of the new object.
Constant property	Do not consider the direct design: public static final ...;	-----------------------------------------
Property type	Static properties, class properties	Normal property, non-static property

　　2.static Modification Method:

	Use	Do not use
The grammatical	Although it can be called with an object, it is more directly called with the class name	Must be called using an object
Operation on	Static methods can only manipulate static static properties and other static methods they invoke, and cannot use This/super	Both static and non-static properties can be manipulated, either static or non-static methods can be manipulated
Load on	Static method is loaded first	Non-static method is loaded after
Use the	A method has nothing to do with the object, it is only designed to use static,	If a method is related to an object, it can only be designed as a non-static
Design on	If the method is a tool class, the tools method should be designed as static,	If the implementation part of the method needs to manipulate properties or other methods related to the object, the method should be designed as a non-static

　3. Mechanism:static method is loaded first, so non-static content cannot be manipulated.

　Common features of 4.static:

① all static are related to the object, are class-level.

② usually static in the loading period will have special treatment

③ constructor method cannot use static

④static modified in the static area of the data segment at load time

Access modifiers and the use of optional modifier static