2021-11-05 understand main method syntax, code block and final keyword

= = = = = = = One 、 understand main Method syntax = = = = = = = =

One 、 explain main The form of the method :public static void main(String[] args)
1. main Method is called by the virtual machine
2. java The virtual machine needs to call the main() Method , So the access rights of this method must be public
3. java Virtual machine is executing main) Method, you don't have to create an object , So the method must be static
4. This method receives String Array parameters of type , The array saves the execution java The parameters passed to the running class when the command is executed
5. java Program parameters executed 1 Parameters 2 Parameters 3
Two 、 hot tip
1) stay main() In the method , We can call main The static method or static property of the class in which the method is located .
2) however , Non static members in this class cannot be accessed directly , You must create an instance object of the class , This object can be used to access the non static members of the class

= = = = = = = Two 、 Understand code blocks = = = = = = = =

One 、 Basic introduction ：
Coded blocks are also called initialization blocks , Belongs to a member of a class [ That is, part of the class , It's similar to the method , Encapsulate logical statements in the method body , adopt {} Surrounded .
But different from the method , No method name , No return , No parameters , Only the method body , And you don't have to call... Explicitly through objects or classes , But when loading classes , Or implicitly call... When creating an object .
Two 、 Basic grammar ：
[ Modifier ]{
Code
};
Note description :
1) Modifiers are available , What to write , I can only write static
2) Code blocks fall into two categories , Use static The modification is called static code block , No, static Embellished , It's called a normal code block / Non static code block .
3) A logical statement can be any logical statement ( Input 、 Output 、 Method call 、 loop 、 Judgment, etc )
4); The number can be written , You can omit it .

3、 ... and 、 Basic understanding ：
1) Equivalent to another form of constructor ( Complementary mechanisms to constructors ), You can do initialization
2) scene : If there are repeated statements in multiple constructors , It can be extracted into the initialization block , Improve code reuse .
Four 、 Notes and details ：
1) static Code blocks are also called static blocks , The function is to initialize the class , And it executes as the class loads , And only once . If it's a normal code block , Every time an object is created , Is executed .
2) When the class is loaded [ Important back !]
① When creating an object instance (new)
② Create subclass object instances , The parent class is also loaded
③ When using static members of a class ( Static attribute , Static methods )
3) Common code block , When creating an object instance , Will be implicitly called .
Created once , It will be called once .
If you only use static members of a class , Ordinary blocks of code do not execute .
Summary :1.static The code block is executed when the class is loaded , Only execute — Time
2. A normal block of code is called when an object is created , To create a , Call once
3. Class loaded 3 In this case , Need to remember .

= = = = = = = 3、 ... and 、final keyword = = = = = = = =

One 、 Basic introduction ：
final Modifiable class 、 attribute 、 Methods and local variables .
In some cases , Programmers may have the following requirements , Will be used to final:
1) When you don't want the class to be inherited , It can be used final modification .
2) When you don't want a method of a subclass to be covered by a subclass / rewrite (override) when , It can be used final Keyword modification .
3 When you don't want the value of an attribute of a class to be modified , It can be used final modification .
4) When you don't want a local variable to be modified , have access to final modification

Two 、 Precautions and use details ：
1）final Modified attributes are also called constants , It's usually used XX_XX_ XX Named after the
2) final Modified attributes are defined , The initial value must be assigned , And it can't be changed in the future , The assignment can be in one of the following positions 【 Select a position and assign an initial value 】:
① Definition time : Such as public final double TAX_RATE=0.08;
② In the constructor
③ In the code block .
3) If final Decorated attributes are static , Then the initialization position can only be ：
① Definition time ② In static code blocks
Cannot assign value in constructor .
4) final Class cannot inherit , But you can instantiate objects .
5) If the class is not final class , But it contains final Method , This method cannot be overridden , But it can be inherited .