General

Code must be compatible with GPL version 3 or later and must include a proper GPL header with proper copyright attribution.

Write unit tests for anything non-trivial.

Git

Configure your repository with your name and email address, also configure core.whitespace with trailing-space,space-before-tab,tab-in-indent,tabwidth=4.
Fix all whitespace errors with git diff --check before committing.
Write well formatted commits http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
Feel free to clean up your commits (e.g. amend/squash/rebase) if that makes code reviewing easier.
Please create a new branch for your change. Any unrelated change belongs to a separate branch and a separate pull request.
Run mvn test to verify it builds OK and no tests fail before sending a pull request.
When sending a pull request, include a concise, informative description of the change being made.

Try to mimic the current code style which strives for something like http://geosoft.no/development/javastyle.html, with the following exceptions:

#8. Fields must be prefixed with _ and referenced without an explicit this.
#61. Indentation must be 4 and must consist of only whitespace.
#65. else clause must be on same line as previous closing bracket.
#72. Single line statement if-else, for or while statements must always use brackets.
#73. case 100: // NOT case 100 :
#74. Method names can not be followed by a whitespace when it is followed by another name.

Misc:

No tabs, no carriage returns, no trailing whitespace.
Lines should be at most 100 characters long.
Default to immutable fields and strive for possibly immutable objects.
Try to limit unnecessary usage of null. Assert fields that aren't allowed to be null aren't assigned null. Any (internal) method that return null may have its name suffixed with orNull. Any public API method that may return null should use an Optional instead.
Include javadoc for any unchecked exceptions that may be thrown for a method.