Consists of letters, numbers, underscore (_) or dollar sign ($). Cannot start with a number. Case-sensitive. Unlimited length. Cannot be a reserved keyword in Java. To facilitate later over-limit issuance or maintenance, general identifier naming is best to be meaningful. In the Java programming process, although the compiler supports Chinese naming, it is best not to use Chinese to avoid unnecessary trouble. Common naming habits are: ① Package names are generally composed of lowercase letters and a small number of numbers, such as: org, shan, dao, etc., preferably an organization name, company name or function module name; ② Class names and interface names generally consist of one or Made up of several words, following the "hump rule". ③ Except for the first letter of the method name, the first letter of the other words is capitalized, which is similar to the class name, that is, the small camel case rule: such as toSend ④ If the attribute name is a variable of a basic data type, it is generally lowercase and refers to the data type The variables are generally named similarly to the class name, such as "int name"; or "String PersonModel", etc. Only local variables can be abbreviated, such as "int i;" or "int j", etc. Camel case naming is when a variable name or function name is a unique identifier composed of one or more words connected together. The first word starts with a lowercase letter; the first letter of the second word is capitalized or each The first letters of each word are all capital letters, for example: myFirstName, myLastName. Such variable names look like camel humps rising one after another, hence the name. The naming rule of camel case can be regarded as a convention, not absolute or mandatory, in order to increase recognition and readability.