BlueJ is an editor and generator of Java programs. This program is capable of editing any existing java application, or generating a new one, by generating new classes or selecting them from other packages. In order to use this editor you must have any version of Java Development Kit on your system. You can download the JDK for free from www.sun.com. Optionally, you can choose to download and install the Sun Java Wireless Toolkit (WTK). This way, you will install not only the Java Developer Kit, but the external standard libraries and device emulators needed for developing Java ME applications (also called MIDlets). These applications will run on mobile devices, like cellphones.
From this version onwards, BlueJ supports developing this kind of applications. The program will let you build new projects or edit saved ones, adding, modifying or deleting classes. You will also be able to open non-BlueJ projects, in order to edit them. This tool contains links to the developer's page (the University of Kent), where you will be able to find lots of documentation, manuals and resources to master this piece of software.