Java conventions are that enum names are all uppercase, which immediately causes problems if you want to serialize the enum in another form. But when you serialize those enum values, you want to use lowercase values of "left", "top", "right", and "bottom". No problem—you can override Side.

Should enum be uppercase or lowercase?

Because they are constants, the names of an enum type's fields are in uppercase letters. You should use enum types any time you need to represent a fixed set of constants.

Should enum always be uppercase?

Enums are a type and the enum name should start with a capital. Enum members are constants and their text should be all-uppercase. If they are their own class start with upper case, if they are members lower case.