[day01] preface, introductory program, constant variables

13. Program notes and HelloWorld explain



14. The concept and characteristics of keywords

15. The concept and rules of identifier

16. The concept and classification of constants

17. Printout of constants

For character constants ： There must be only one character between two single quotation marks , No, no, No ; Not even two
For empty constants ： Null constants cannot be used directly for printouts

18. Basic data type
There are two types of data ：
1. Basic data type
2. Reference data type ： character string 、 Array 、 class 、 Interface 、Lambda

A string is a reference type , Not a basic data type .

19. The concept and definition format of variables

20. Basic use of variables

You cannot create variables with the same name repeatedly

Variables without assignment cannot be used

21. Variable considerations

When using variables , There are a few caveats ：
1. If you create multiple variables , Then the names between variables cannot be repeated
2. about float and long In terms of type , Letter suffixes F and L Don't lose it.
3. If you use byte perhaps short Variable of type , Then the data value on the right cannot exceed the range of the type on the left
4. Variables that are not assigned , Can't be used directly ; Be sure to assign the value after , Can be used
5. Variables cannot be used beyond the scope of the scope
【 Scope 】： Start with the line that defines the variable , Until the end of the brace to which it belongs
Variables must be defined before using
If the variable is out of the range of braces , Out of scope , Variables can no longer be used
6. You can create multiple variables in one statement , But in general, it is not recommended to write