* Step One: Install Java Standard Edition Development Kit (JDK)
JDK is a prerequisite software for the BlueJ. It includes tools useful for developing and testing programs written in Java and running on the Java platform.
Oracle's website provides free download for the latest stable version for JDK, please click here.
After download the JDK for your operating system, run the distribution files to complete the installation.
Upon completion of JDK installation, let's move the Step Two！
* JRE and JDK
You probably will find both JRE and JDK avaialble in Oracle's download webpage. To run the BlueJ, JRE is not sufficient. You have to make sure that JDK is installed.
* Beta/non-beta version
You probably will find many versions of JDK in Oracle's download webpage. The non-beta (stable) version of JDK is recommended for installation rather than the beta one.