What are the defining rules of Java enums?

The rules for defining Java enum are as follows:

1. Define an enumeration type using the keyword enum.
2. Constants must be listed on the first line of the enumeration type, separated by commas, and end with a semicolon.
3. Constants are typically written in uppercase letters, and if they consist of multiple words, they are usually separated by underscores.
4. Enumeration types can have constructors, methods, and fields.
5. The enum constants inside an enum type are singletons.
6. Enum types can also implement interfaces, and each enum constant needs to implement the abstract methods in the interface.