• What is leJOS?
• Preliminiaries
• Compatibility
• Installation and Set Up
• Known limitations
• Known bugs
• Documentation
• License
• Questions and Problems

What is leJOS?

• Lego Mindstorms RCX, i.e. Hitachi H8300
• Unix (for emulation of RCX programs only)

• Object oriented language (Java)
• Preemptive threads (tasks)
• Arrays, including multi-dimensional ones
• Recursion
• Synchronization
• Exceptions
• Well-documented API

• A Windows version, which doesn't require CygWin
• A MacOSX version
• Floating point operations (doubles truncated to 32 bits)
• String constants
• Casting of longs to ints and vice versa
• Marks references in the stack (which makes it feasible to implement garbage collection)
• Multi-program downloading
• java.lang.Math class with sin, cos, tan, atan, pow, etc.
• Additional APIs

Preliminaries

• You need a Java Development Kit (JDK) 1.4 or above
• Your PATH must contain the JDK's bin directory
• Make sure you have set JAVA_HOME properly to the root directory of the JDK
• For the vision parts, you need a Java Media Framework (JMF) 2.1 or above (will be brought along by the leJOS distribution)
• In case you want to build the distribution, your PATH must contain the ant binary (ant 1.6 or above)
• Add leJOS's bin directory to your PATH
• Set RCXTTY to your 'tower' device (e.g. usb,/dev/usb/legousbtower0 or COM1)
• Download firmware using firmdl
• Compile using lejosjc instead of javac
• Link and download programs to the RCX using lejos
• Emulate leJOS programs on the host using emulate_lejoslink and emu-lejos
• Exceptions: Use lejos --verbose. Exceptions are shown in the RCX as [XXXX Y], where XXXX = method_signature and Y = exception_class_index MOD 10

Compatibility

Installation and Set Up

1. Set your environment variable LEJOS_HOME to the directory you installed this distribution into
2. Set your environment variable RCXTTY to usb (for USB towers) or COM1 (for serial towers); on Linux, for USB towers RCXTTY might be required to be set to /dev/usb/legousbtower0
3. On Linux, depending on the privilege settings you might need to adjust the execution permissions in the bin folder

Download the firmware to the RCX with firmdl.

Check the installation by switching to the "check" folder of the leJOS distribution and run checkDistribution.bat resp. checkDistributionCheck.sh.
A very simple test program will be downloaded to the RCX. After the download has finished, press the RCX's 'RUN' button. The RCX's LCD should print the strings "SMOKE" and "TEST".

Known limitations

• Garbage collection is not performed yet. Hence, the number of objects in your program should be limited
• Switch statements are not supported
• Arithmetic operations on variables of type long are not supported, although you can cast ints to longs and vice versa
• Maximum array length is 511
• The instanceof operation will always be true for interfaces. In bytecode, CHECKCAST succeeds on all interface classes (which is a bit unsafe)
• The instanceof and checkcast operations are rejected by the linker when they are performed on array classes (e.g. b instanceof byte[])
• There are no objects of type java.lang.Class, which means that the ".class" construct won't work. Class.forName() throws ClassNotFoundException at all times. As a consequence, reflection is not supported by leJOS
• Most JDK APIs are unavailable

Known Bugs

• MONITOREXIT ignores null objects (it assumes MONITORENTER has handled them). Furthermore, it doesn't check monitor validity. Until this bug is fixed (actually, at all times) all monitors should be constants
• Initializers (static or otherwise) are not implicitly synchronized. This could lead to access of statics before they are initialized, when multiple threads use a class

Documentation

License

Questions and Problems

Feel free to take a look at the Lejos website or its project page.