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

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.

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

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

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)