Christoph Bartneck, Ph.D.

This tutorial will show you how to install and configure all the software necessary for you to develop Java programs for the LEGO Mindstorms NXT. Java is much more powerful and flexible than the original NXT-G software that LEGO provides. We will be using the Eclipse editor to write the actual Java code and to download this into the NXT. This combination enables you to conveniently write software and to quickly test it on the NXT. Of course there are many other Java editors, but we consider this combination to be particularly convenient. This tutorial focuses on the Windows XP platform, A Mac OS X tutorial is also available. We have two additional steps for Windows Vista. You will work through the following five steps:

1. Installing Java on your computer
2. Installing LEGO NXT USB driver on your computer
3. Install Lejos on your Computer and on your Mindstorms NXT
4. Install and configure Eclipse on your Computer
5. Write, download and run a “HelloWorld” program
6. Extra steps for Windows Vista

The order of these steps is very important. Please go through them one by on. In addition to this tutorial you may also want to consider reading these books that are of particular use:

Maximum Lego NXT by Brian Bagnall view at Amazon.com	Building Robots with LEGO Mindstorms NXT by Mario Ferrari view at Amazon.com
Robot Programming by Joe Jones & Daniel Roth view at Amazon.com	Extreme NXT by Michael Gasperi, Isabelle L. Hurbain, and Philippe E. Hurbain view at Amazon.com

1. Install Java on your Computer

Download and install Java SE (Standard Edition) JRE (Java Runtime Environment). You do NOT need to install the JDK (Java Developer Kit). We will install Eclipse later, which does already include all the tools you need to write and compile your programs. After the installation you also do NOT need to set any Path or Classpath variables for Java itself. Java will be installed in “C:\Program Files\Java”. However, if you cannot even get Java to work on your machine, enter the path to your java bin directory into the Path variable.

If you already have Java installed you need to verify that you have at least version 5, preferable of the JRE, but the JDK will work as well. We will install Eclipse, which does require at least this version.

2. Install Lego USB driver on your computer

The NXT can be connected to the computer via USB or Bluetooth. The communication via USB is more reliable and faster than via Bluetooth. The later also depends on each specific computer model. USB is in comparison more standardized. You must first install the driver and only then connect the NXT with the USB cable. You do NOT need to install the any software that comes on the Mindstorms CD because we will NOT program in NXT-G (Lego’s own software that is based on Labview). We only need to install the USB driver which is available on the Mindstorms’ website. If you already installed the original Mindstorms software then you do not need to uninstall it, just check if the Mindstorms’ website has an update for the USB driver.

Download the Mindstorms NXT Driver.

Unzip the file and start setup.exe

Click through the installation program

Windows might ask you to restart the computer after the installation. Next, connect the NXT to the computer with the USB cable. Check the correct installation of the driver by checking the device manager. Right-click on “My Computer” and select “Properties”. Switch to the hardware tab. Click on “Device manager“. It should list “Lego Devices => Lego Mindstorms NXT”.

3. Install Lejos on your Computer and on your Mindstorms NXT

Download Lejos. At the time this tutorial was created, version Beta 0.4 was the latest release. By now, newer versions are available and we cannot guarantee that this tutorial will work for the newer versions.

Create a directory called “ProgramFiles” on your hard disk. The already existing directory “Program Files” does contain a space character between “Program” and “Files” that sometimes causes problems in the usage of Java technology. You will later also install Eclipse into the “ProgramFiles” directory.

Unzip the Lejos ZIP file. This will reveal a directory called “lejos_nxj” . Place it in your “ProgramFiles” directory.

We now need to inform Java of the existence of the Lejos library. Right-click on “My Computer” and select “Properties”. Click on the “Advanced” tab. Click on “Environment Variables” at the bottom. Create a new “System variable” by clicking on “New” at the bottom. Do NOT create a “User variable” at the top.

Name the new system variable:

LEJOS_HOME

and give it the value:

C:\ProgramFiles\lejos_nxj

In short, Lejos needs to know where it has been installed. Notice that we now already use the new directory “ProgramFiles” that does not contain a space character. When you are done, click “OK”.

Verify that the new system variable shows up in the list. Next, we need to add the LEJOS_HOME variable to the system variable “Path”. Select “Path” from the list and click on “Edit”. Add

;%LEJOS_HOME%\bin

to the end of the value. The different values are separated by a colon and no colon is necessary at the end of the line.

Now we will test if Lejos has been installed correctly by using the DOS command prompt. Click on the Window’s “Start” button and then on “All Programs => Accessories => Command Prompt”. Type in:

lejosdl

If the result look like the one in the picture below then everything is fine.

We will now replace the original firmware that came with your NXT with Lejos. You only need to do this once. Lejos will then function as the operating system of your NXT. It implements a Java Virtual Machine on which you can run your Java programs. Make sure that you NXT is connected to your computer with an USB cable and that it has been recognized by the operating system. In case of doubt check the device manager again. You first need to bring the NXT to the firmware upload mode. Use a paper clip to press and hold the hidden button. The NXT will then play a soft pulsing sound that signals its readiness.

Type:

lejosfirmdl

into the command prompt and press enter.

Lejos requires Libusb to communicate with the NXT. If this free software has not yet been installed on your computer, it will start the Libusb installation wizard. Libusb allows programs to access any USB device on Windows in a generic way. Make sure that you do NOT run the test application in the last dialogue. In case that LeJOS is already installed on your NXT you will still have to install LibUSB to be able to download your programs. You can find it in “lejos_nxj\3rdparty\lib\libusb-win32-filter-bin-0.1.12.1.exe”. The name may change when a newer version of libusb becomes available.

Once Libusb is installed the Lejos firmware installer will proceed and hopefully complete the process with success. It should look like the image below.

The NXT will now reboot and shortly display the Lejos Logo before changing to the main menu.

Your NXT is now ready to run Java programs.

4. Install and configure Eclipse on your Computer

Download Eclipse IDE for Java Developers. Eclipse itself is written in Java and does not contain an installer (setup.exe). It comes in a simple ZIP file that you should unzip into the previously created “ProgramFiles” directory. You may want to create a shortcut to “eclipse.exe” on your desktop for your own convenience.

When you start Eclipse you will be asked to choose a workspace. The workspace will contain all the files that you create. Again, you should create a directory in a location so that the path does not contain any space characters ” “. In our example we use the directory “d:\programming\workspace”. You will then be greeted with the welcome screen.

We will now create a Hello World program and upload (others call it download) it into the NXT. First, you need to create a new Java project. Click on “File => New => Java Project”. Give the project a new name, but avoid space characters. We called ours “LeJOS_NXJ”

Next, you need to turn this project into a Lejos project. Right-click onto the project and select “Properties”. Select “Java Build Path” on the left and click on the “Libraries” tab. Afterwards, click on “Add External JARs…” and locate the “lib” directory in your “ProgramFiles\lejos_nxj” directory. Select “classes.jar” and press “Open”. The Lejos library will now be listed.

We stay in the properties and move to the “Java Compiler” section listed in the left panel. Check the box “Enable project specific settings” and choose level 1.3 for the compiler compliance level. This will urge the compiler to optimize for an earlier version of Java. This earlier version is more suitable for the NXT since it was developed for embedded systems. It requires much less resources than the latest version of Java. Click on “Apply” and then”OK” to leave the properties.

The next step is to configure Eclipse for downloading the software into the NXT. Click on “Run => External Tools => Open External Tools Dialog…”.

Select “Program” on the left and then click on the “New” icon. Name the tool “leJOS Download” on top. In the “Main” tab, enter the location of the “lejosdl.bat” file that should be in the “lejos-nxj\bin\” directory ny clicking on “Browse File System”. Enter

${project_loc}\bin

in the “Working directory” and enter

${java_type_name}

in the Arguments section.

Now we will create a shortcut for downloading your program into the NXT. Click on the little green “run” icon (with toolbox) and then select “Organize Favorites…”. Click on “Add…” in the popup window and then check the box in front of “leJOS Download”, which is the external tools we configured in the previous step. Press “OK” and then “Ok” again.

5. Write, download and run a “Hello world” program

First, we need to create the main class of your program. Click on “New => Class”. Name it “HelloWorld” and check the box “public static void(String[] args)”. You will receive an almost empty class. Eclipse is a very smart Java editor that tries to compile your program while you write. Similar to M$ Word, it will underline errors with a red line. Sometimes this automatic compilation does not work and you want to force Eclipse to compile. You can achieve this by simply saving the file.

You now need to to enter a few lines of Java code. In the first line add:

import lejos.nxt.LCD;

This line will enable your program to use the LCD display of your NXT. Next, we need to define what should be displayed and when. Add the following lines to the main method:

LCD.drawString(“Hello World!”, 2, 2);
LCD.refresh();
while(true) {}

Now you can safe the program and then download it into the NXT. The next screenshot walks you through these steps. Before you start the downloading procedure you must make sure that the NXT is connected to the computer and that the NXT is recognized by the operating system. You can verify this in the device manager. You also need to select your main class window in the middle before starting the downloading procedure, so that Eclipse knows which class you want to download. Use the pull down menu (denoted as 4 in the screenshot) to select the downloading favorite your previously created. This will start the downloading procedure.

After the download is complete you can start the program by using the buttons on the NXT. Notice that the program will run forever. To stop it you need to reset the NXT by holding the two middle buttons for a longer time. This will then terminate the USB connection, so you need to wait a little for it to reconnect before you download a new program. Of course there are better ways of ending your program, such as listening to a button press or after a certain delay. Replace the endless while loop with this:

try {
Thread.sleep(5000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
}

This will set your NXT to sleep for 5 seconds before the program ends. You will now see “Hello World!” for five seconds and then the program ends. For more information on programming in Java you should check the leJOS page.

Extra steps for Windows Vista

Before you start make sure the ‘User account control’ is turned off. Go to Control Panel → User Accounts

Uncheck Use User Account Control.

If installing the leJOS firmware on the NXT states that LibUSB is not installed:

1. Download the latest libusb version for Windows (http://sourceforge.net/projects/libusb-win32/files/)
2. Right click on this file. Select Properties -> Compatibility. Click the “Run this program in compatibility mode” box and select “Windows XP (Service Pack2)” from the drop down list.
3. Right click again and select “Run as Administrator”. Follow the installation instructions.

Thanks to Niko Vegt & Maarten Brugmans for this information.

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