need to have Java 6 or Java 7 (also known as JDK6/JDK7) installed on your system. Get it here:
Note that only the version
labeled JDK will work. Not the one labeled JRE,
nor the one that comes with NetBeans, nor the EE version.
Debian/Ubuntu users should install the JDK via the package management system rather than using the links above. See the installation instructions.
Details about system
requirements are here.
On MacOS X 10.5+, a
recent Java SDK version is installed by default. You
do not normally need to install Java on this system. See here for more information.
Previous BlueJ versions.