DrJava requires Java 2, version 1.4 or later. Note that you will need to have the JDK (not the JRE) installed if you wish to use the compiler or debugger within DrJava.
We recommend downloading and using Sun's JDK 1.5.0, currently in beta releases, (from http://java.sun.com) for Solaris, Linux, and Windows. Other users should use the Java virtual machine that comes with their operating system (including Mac OS X).
DrJava uses two Java Virtual Machines (one for the main program, and one for the Interactions Pane) that use Java's Remote Method Invocation (RMI) to communicate with each other. RMI uses TCP/IP as the default transport mechanism, so you must have those drivers installed. Without TCP/IP, DrJava will not start properly.