What is the convention for word separator in Java package names?

Question

How should one separate words in package names? Which of the following are correct?

1. com.stackoverflow.my_package (Snake Case using underscore)
2. com.stackoverflow.my-package (Kebab Case using hyphens)
3. com.stackoverflow.myPackage (Camel Case)
4. com.stackoverflow.MyPackage (Pascal Case)

What is the general standard?

Answer 1

All three are not the conventions.

Use com.stackoverflow.mypackage.

The package names do not follow camel casing or underscores or hyphens package naming convention.

Also, Google Java Style Guide specifies exactly the same (i.e. com.stackoverflow.mypackage) convention:

5.2.1 Package names

Package names are all lowercase, with consecutive words simply concatenated together (no underscores). For example, com.example.deepspace, not com.example.deepSpace or com.example.deep_space.

— Google Java Style Guide: 5.2 Rules by identifier type: 5.2.1 Package names.

Answer 2

Here's what the official naming conventions document prescribes:

Packages

The prefix of a unique package name is always written in all-lowercase ASCII letters and should be one of the top-level domain names, currently com, edu, gov, mil, net, org, or one of the English two-letter codes identifying countries as specified in ISO Standard 3166, 1981.

Subsequent components of the package name vary according to an organization's own internal naming conventions. Such conventions might specify that certain directory name components be division, department, project, machine, or login names.

Examples

• com.sun.eng
• com.apple.quicktime.v2
• edu.cmu.cs.bovik.cheese

References

• java.sun.com - Code Conventions/Naming

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Note that in particular, anything following the top-level domain prefix isn't specified by the above document. The JLS also agrees with this by giving the following examples:

• com.sun.sunsoft.DOE
• gov.whitehouse.socks.mousefinder
• com.JavaSoft.jag.Oak
• org.npr.pledge.driver
• uk.ac.city.rugby.game

The following excerpt is also relevant:

In some cases, the internet domain name may not be a valid package name. Here are some suggested conventions for dealing with these situations:

• If the domain name contains a hyphen, or any other special character not allowed in an identifier, convert it into an underscore.
• If any of the resulting package name components are keywords then append underscore to them.
• If any of the resulting package name components start with a digit, or any other character that is not allowed as an initial character of an identifier, have an underscore prefixed to the component.

References

• JLS 6.1 Package Names

Answer 3

Anyone can use underscore _ (its Okay)

No one should use hypen - (its Bad practice)

No one should use capital letters inside package names (Bad practice)

NOTE: Here "Bad Practice" is meant for technically you are allowed to use that, but conventionally its not in good manners to write.

Source: Naming a Package(docs.oracle)

Answer 4

The official naming conventions aren't that strict, they don't even 'forbid' camel case notation except for prefix (com in your example).

But I personally would avoid upper case letters and hyphenations, even numbers. I'd choose com.stackoverflow.mypackage like Bragboy suggested too.

(hyphenations '-' are not legal in package names)

EDIT

Interesting - the language specification has something to say about naming conventions too.

In Chapter 7.7 Unique Package Names we see examples with package names that consist of upper case letters (so CamelCase notation would be OK) and they suggest to replace hyphonation by an underscore ("mary-lou" -> "mary_lou") and prefix java keywords with an underscore ("com.example.enum" -> "com.example._enum")

Some more examples for upper case letters in package names can be found in chapter 6.8.1 Package Names.

Answer 5

Underscores look ugly in package names. For what is worth, in case of names compound of three or more words I use initials (for example: com.company.app.ingresoegresofijo (ingreso/egreso fijo) -> com.company.app.iefijo) and then document the package purpose in package-info.java.

Answer 6

Concatenation of words in the package name is something most developers don't do.

You can use something like.

com.stackoverflow.mypackage

Refer JLS Name Declaration