Java basics: identifiers

Identifier:
The character sequence that Java uses when naming various variables, methods, classes and other elements is an identifier
technique: wherever you can name it, it is called an identifier.
Define the rules for identifiers: the
upper and lower case of 26 English letters, 0-9, _ or $
can not start with numbers.
Keywords and reserved words cannot be used, but keywords and reserved words can be included.
Java is strictly case-sensitive, and the length is unlimited.
The identifier cannot contain spaces.
Exercise: miles,Test,a++,__a,#R,#44,apps,class,public,int,x,y,radius