Common naming rules: See name EENOW

Identifier: is to give classes, interfaces, methods, variables, and so from the name.
    
    Composition rules:
        A: capital or small letters
        B: numeric characters
        C: $ and _
        
    Note:
        A: can not start with a number
        B: Java is not a keyword in
        C: Java language strictly case-sensitive
        
    common naming convention: see known name meaning
        , for example: I would like to define a student class
            class student {}
            class S {}
            
        package: in fact, the folder for the same class name to distinguish
            all lowercase
        
            single-stage: liuyi
            multistage: cn.itcast
                       CN
                              itcast
                    
        class or interface:
            one word: the first letter of the word must be capitalized
                example: Student, Dog
            multiple words: first letter of each word must be capitalized
                example: HelloWorld, StudentName
            
        A method or a variable:
            a word: first word lowercase
                example: main, age
            plurality of words: starting from the second word, the first letter of each word capitalized
                example: studentAge, showAllNames ()
                
        constants:
            a word: all caps
                Examples : the PI
            plurality of words: each capital letters, separated by _
                example: STUDENT_MAX_AGE