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leJOS
A Java virtual machine small enough to replace the RCX firmware. It grew out of TinyVM, and it lets you write robot code in Java, with threads and objects, and send the classes over the tower.
One of three replacement runtimes people wrote for this brick, beside legOS · brickOS in C and pbForth in Forth. It outlived the RCX: leJOS became the main Java platform for the NXT and the EV3 as leJOS NXJ. The RCX generation came first.