rosjava is the first pure Java implementation of ROS.
From ROS.org: ROS is an open-source, meta-operating system for your robot. It provides the services you would expect from an operating system, including hardware abstraction, low-level device control, implementation of commonly-used functionality, message-passing between processes, and package management.
Developed at Google in cooperation with Willow Garage, rosjava enables integration of Android and ROS compatible robots. This project is under active development and currently alpha quality software. Please report bugs and feature requests on the issues list.
To get started, visit the rosjava_core and android_core documentation.
Still have questions? Check out the ros-users discussion list, post questions to ROS Answers with the tag "rosjava," or join #ROS on irc.oftc.net.
rosjava was announced publicly during the Cloud Robotics tech talk at Google I/O 2011.
Looking for a robot platform to experiment with ROS, Android, and cloud robotics? The Willow Garage TurtleBot is a great mobile perception platform for getting started with robotics development.
Visit the rosjava_core wiki for instructions.