In this article by Abid H. Mujtaba, author of Lego Mindstorms EV3 Essentials, we'll have look at a powerful framework designed to grant an extraordinary degree of control over EV3, namely LeJOS:

(For more resources related to this topic, see here.)

Classic programming on EV3

LeJOS is what happens when robot and software enthusiasts set out to hack a robotics kit. Although lego initially intended the Mindstorms series to be primarily targeted towards children, it was taken up with gleeful enthusiasm by adults. The visual programming language, which was meant to be used both on the brick and on computers, was also designed with children in mind.

The visual programming language, although very powerful, has a number of limitations and shortcomings. Enthusiasts have continually been on the lookout for ways to program Mindstorms using traditional programming languages. As a result, a number of development kits have been created by enthusiasts to allow the programming of EV3 in a traditional fashion, by writing and compiling code in traditional languages.

A development kit for EV3 consists of the following:

A traditional programming language (C, C++, Java, and so on)
Firmware for the brick (basically, a new OS)
An API in the chosen programming language, giving access to the robot's inputs and outputs
A compiler that compiles code on a traditional computer to produce executable code for the brick
Optionally, an Integrated Development Environment (IDE) to consolidate and simplify the process of developing the brick

The release of each robot in the Mindstorms series has been associated with a consolidated effort by the open source community to hack the brick and make available a number of frameworks for programming robots using traditional programming languages. Some of the common frameworks available for Mindstorms are GNAT GPL (Ada), ROBOTC, Next Byte Code (NBC), an assembly language, Not Quite C (NQC), LeJOS, and many others.

This variety of frameworks is particularly useful for Linux users, not only because they love having the ability to program in their language of choice, but also because the visual programming suite for EV3 does not run on Linux at all. In its absence, these frameworks are essential for anyone who is looking to create programs of significant complexity for EV3.

LeJOS – introduction

LeJOS is a development kit for Mindstorms robots based on the Java programming language. There is no official pronunciation, with people using lay-joss, le-J-OS (claiming it is French for "the Java Operating System", including myself), or lay-hoss if you prefer the Latin-American touch.

After considerable success with NXT, LeJOS was the first (and in my opinion, the most complete) framework released for EV3. This is a testament both to the prowess of the developers working on LeJOS and the fact that lego built EV3 to be extremely hackable by running Linux under its hood and making its source publicly available. Within weeks, LeJOS had been ported to EV3, and you could program robots using Java.

LeJOS works by installing its own OS (operating system) on the EV3's SD card as an alternate firmware. Before EV3, this would involve a slightly difficult and dangerous tinkering with the brick itself, but one of the first things that EV3 does on booting up is to check for a bootable partition on the SD card. If it is found, the OS/firmware is loaded from the SD card instead of being loaded internally.

Thus, in order to run LeJOS, you only need a suitably prepared SD card inserted into EV3 and it will take over the brick. When you want to return to the default firmware, simply remove the SD card before starting EV3. It's that simple! Lego wasn't kidding about the hackability of EV3.

The firmware for LeJOS basically runs a Java Virtual Machine (JVM) inside EV3, which allows it to execute compiled Java code. Along with the JVM, LeJOS installs an API library, defining methods that can be programmatically used to access the inputs and outputs attached to the brick. These API methods are used to control the various components of the robot.

The LeJOS project also releases tools that can be installed on all modern computers. These tools are used to compile programs that are then transferred to EV3 and executed. These tools can be imported into any IDE that supports Java (Eclipse, NetBeans, IntelliJ, Android Studio, and so on) or used with a plain text editor combined with Ant or Gradle. Thus, leJOS qualifies as a complete development kit for EV3.

The advantages of LeJOS

Some of the obvious advantages of using LeJOS are:

This was the first framework to have support for EV3
Its API is stable and complete
This is an active developer and user base (the last stable version came out in March 2014, and the new beta was released in April)
The code base is maintained in a public Git repository
Ease of installation
Ease of use

The other advantages of using LeJOS are linked to the fact that its API as well as the programs you write yourself are all written in Java. The development kits allow a number of languages to be used for programming EV3, with the most popular ones being C and Java.

C is a low-level language that gives you greater control over the hardware, but it comes at a price. Your instructions need to be more explicit, and the chances of making a subtle mistake are much higher. For every line of Java code, you might have to write dozens of lines of C code to get the same functionality.

Java is a high-level language that is compiled into the byte code that runs on the JVM. This results in a lesser degree of control over the hardware, but in return, you get a powerful and stable API (that LeJOS provides) to access the inputs and outputs of EV3. The LeJOS team is committed to ensure that this API works well and continues to grow.

The use of a high-level language such as Java lowers the entry threshold to robotic programming, especially for people who already know at least one programming language. Even people who don't know programming yet can learn Java easily, much more so than C.

Finally, two features of Java that are extremely useful when programming robots are its object-oriented nature (the heavy use of classes, interfaces, and inheritance) and its excellent support for multithreading. You can create and reuse custom classes to encapsulate common functionality and can integrate sensors and motors using different threads that communicate with each other. The latter allows the construction of subsumption architectures, an important development in robotics that allows for extremely responsive robots.

I hope that I have made a compelling case for why you should choose to use LeJOS as your framework in order to take EV3 programming to the next level. However, the proof is in the pudding.

Summary

In this article, we learned how EV3's extremely hackable nature has led to the proliferation of alternate frameworks that allow EV3 to be programmed using traditional programming languages. One of these alternatives is LeJOS, a powerful framework based on the Java programming language.

We studied the fundamentals of LeJOS and learned its advantages over other frameworks.