Day06 class variables instance variables local variables constant variables naming conventions

Variable

Variable is the most basic storage unit in a program ,

Its elements include ：

1. Variable name
2. Variable type
3. Scope

Notes on declaring variables ：

1. Variable types must be declared , It can be a basic type , It can also be a reference type
2. There must be a variable name , The identifier of variable name must be legal ,$ _ Start with upper and lower case letters . Cannot contain keywords .
3. Declared variables must have values .
4. Variable declarations are complete statements , So every statement needs ; ending
5. Within the same function , Variable names are unique .

public class demo{
    
    public static void main(String[] args){
    
        byte a1 =127;  // perhaps -128
        short a2 =32767; // perhaps -32768
        char c1 ='a'  // According to the character , Single quotation marks 
        int a3 = 2147483647;  // perhaps -2147483648
        long a4 = 9223372036854775807L;// perhaps -9223372036854775808. Pay attention to add L Follow int distinguish 
        float a5 = 3.14F; // Pay attention to add F Follow double distinguish 
        double a6 = 3.14;//
    }
}

public class demo_day3 {
    
    public static void main(String[] args) {
    
        //int a,b,c; // Although it can be written like this , But not recommended 
        int a=1;
        String name  =" Kaiwei ";
        System.out.println(name);
    }
}

Three variable forms ： Class variables , Instance variables , local variable

1. Class variables . On the method , It can be used directly in the method
2. Instance variables . On the method , Class must be introduced , Can be used
3. local variable . In the method , Direct declaration can be quoted

//  Three variables  ,  Class variables , Instance variables , local variable 
public class demo_day3 {
    
    static double salary = 20;// Class variables , You can reference directly in the method 
    String name = "kaiwei" ;// Instance variables , You must reference variables before you can use them. Understand 
    public static void main(String[] args) {
    
        int a1 = 20;// Reference local variables 
        System.out.println(a1);
        demo_day3 Baby =new demo_day3();// Reference instance variable , I'm not familiar with it for the time being. Just know how to write it 
        System.out.println(Baby.name);
        System.out.println(salary);// Reference class variables 
    }

The default value of uninitialized variables

The number will return 0

The string returns null

Boolean values will return false

Constant

Understood as a special variable , After setting the value , It is not allowed to be changed while the program is running

final double HIGHT = 300

public class demo_day3 {
    
    static final double HIGHT = 400;  // Declare class variables   One high is 400 The constant 
   // final static double HIGHT =400; // The meaning is the same as above ,static It's a modifier , It doesn't matter before or after 
   public static void main(String[] args) {
    
       System.out.println(HIGHT);

   }
}

Variable naming conventions

All constants , Variable , Class name Must see the name to know the meaning . Replace with English words

1. Class member variable ： First character lowercase and hump principle .monthSalary For example, monthly salary
2. local variable ： First character lowercase and hump principle .
3. Constant ： Capital letters and underscores .MAX_VALUE
4. Class name ： Capital letters and hump principle .Hello
5. Method name ： Lowercase and hump principle .run（）,runRun（）

Streamlining

Constants are all uppercase .

Class names begin with uppercase

Members of the class Follow Method name local variable equally The first letter Hump principle