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LeJOS (pronounced like "Legos") is a programming language used to program LEGO Mindstorms robots. It stands for "Lego Java Operating System" and it is essentially a tiny, lightweight version of Java designed specifically for these kinds of small robots. LeJOS supports multithreading, exception handling, synchronization, and garbage collection. It also includes a variety of libraries for controlling motors and sensors as well as performing tasks such as navigation and data logging. This makes it an ideal choice for anyone looking to develop sophisticated robotics applications using LEGO hardware.

Ask the right questions secure the right LeJOS talent among an increasingly shrinking pool of talent.

First 20 minutes

General LeJOS app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in LeJOS application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

How would you install LeJOS on your system?

To install LeJOS, you would first need to download the appropriate version for your operating system from the official LeJOS website. Then you would need to extract the downloaded file and run the installer. After the installation, you would need to set the environment variables correctly.

What are the key features of LeJOS?

Key features of LeJOS include support for threads, exceptions, synchronization, floats, recursion, and a large subset of Java, including many java.lang classes. It also provides a simple deterministic garbage collector, easy to use I/O, and a very simple graphics model.

Describe the difference between LeJOS and standard Java.

LeJOS is a firmware replacement for Lego Mindstorms programmable bricks, while standard Java is a general-purpose programming language. LeJOS allows you to program the bricks using Java, but it has limitations and does not support the full Java language specification.

How would you program a simple robot movement using LeJOS?

To program a robot movement, you can use the Motor class in LeJOS. You can control the speed of the motor using the setSpeed method, and make it move forward or backward using the forward and backward methods. You can also use the rotate method to make the robot rotate a specific number of degrees.

What are the key components of the LeJOS API?

The key components of the LeJOS API include classes for controlling the motors and sensors, classes for data logging and debugging, and classes for navigation and localization.

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What you’re looking for early on

Does the candidate show a strong understanding of Java?

Has the candidate demonstrated experience with LeJOS or similar systems?

Is the candidate able to troubleshoot and solve problems?

Does the candidate have experience with Lego Mindstorms?

Next 20 minutes

Specific LeJOS development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

Describe the difference between the Motor and RegulatedMotor classes in LeJOS.

The Motor class provides basic motor operations, while the RegulatedMotor class includes additional features such as the ability to control the motor's acceleration and deceleration.

How would you program a robot to follow a line using LeJOS?

To program a robot to follow a line, you can use a light sensor to detect the line. You can then use a control loop to adjust the robot's direction based on the sensor's readings. If the robot is on the line, it should go straight, and if it is off the line, it should turn back towards the line.

What are the different types of sensors supported by LeJOS?

LeJOS supports a variety of sensors, including touch sensors, light sensors, sound sensors, ultrasonic sensors, and gyro sensors.

Describe the difference between the SensorPort and SensorModes classes in LeJOS.

The SensorPort class represents the physical sensor ports on the Lego brick, while the SensorModes class represents the different modes that a sensor can operate in, such as touch, light, and ultrasonic.

How would you program a robot to avoid obstacles using LeJOS?

To program a robot to avoid obstacles, you can use an ultrasonic sensor to detect the obstacles. You can then use a control loop to adjust the robot's direction based on the sensor's readings. If the sensor detects an obstacle, the robot should turn to avoid it.

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At this point, a skilled LeJOS engineer should demonstrate strong problem-solving abilities, proficiency in LeJOS programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with LeJOS.

What does this simple LeJOS program do?
import lejos.nxt.*;

public class HelloWorld {

    public static void main(String[] args) {
        LCD.drawString("Hello, World", 0, 0);
        Button.waitForAnyPress();
    }
}
This program displays the text 'Hello, World' on the LCD of a LEGO Mindstorms NXT robot. It then waits for any button on the robot to be pressed.
What will be the output of this LeJOS program?
import lejos.nxt.*;

public class MotorTest {

    public static void main(String[] args) {
        Motor.A.forward();
        Motor.B.forward();
        Button.waitForAnyPress();
    }
}
This program will cause the A and B motors of a LEGO Mindstorms NXT robot to start moving forward. It will then wait for any button on the robot to be pressed.
What does this LeJOS code snippet do?
import lejos.nxt.*;

public class SensorTest {

    public static void main(String[] args) {
        LightSensor light = new LightSensor(SensorPort.S1);
        int lightValue = light.getLightValue();
        LCD.drawInt(lightValue, 4, 0, 0);
        Button.waitForAnyPress();
    }
}
This program reads the light value from a light sensor connected to sensor port S1 of a LEGO Mindstorms NXT robot. It then displays this light value on the robot's LCD and waits for any button to be pressed.
What does this LeJOS code snippet related to threading do?
import lejos.nxt.*;

public class ThreadTest extends Thread {

    public void run() {
        Motor.A.forward();
    }

    public static void main(String[] args) {
        (new ThreadTest()).start();
        Button.waitForAnyPress();
    }
}
This program starts a new thread that causes the A motor of a LEGO Mindstorms NXT robot to start moving forward. The main thread then waits for any button on the robot to be pressed.

Wrap-up questions

Final candidate for LeJOS Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle LeJOS application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

What are the different types of motors supported by LeJOS?

LeJOS supports a variety of motors, including the standard Lego motors, as well as third-party motors.

Describe the difference between the NXT and EV3 versions of LeJOS.

The NXT and EV3 versions of LeJOS are designed for different versions of the Lego Mindstorms programmable bricks. The EV3 version includes additional features and supports more sensors and motors than the NXT version.

How would you program a robot to navigate a maze using LeJOS?

To program a robot to navigate a maze, you can use a combination of sensors to detect the walls of the maze. You can then use a search algorithm, such as depth-first search or breadth-first search, to find a path through the maze.

LeJOS application related

Product Perfect's LeJOS development capabilities

Beyond hiring for your LeJOS engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in LeJOS projects, and can engage in multiple capacities.

LeJOS Consultive assessments

LeJOS Application Conversion Assessment LeJOS Application Integration Assessment LeJOS Data & Database Integration Assessment

Custom LeJOS development projects

Custom LeJOS App Conversion LeJOS Custom App Development Process App UI Design for LeJOS Facelifts

Hiring guide for LeJOS Engineers

LeJOS Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

What you’re looking to see on the LeJOS engineer at this point.

LeJOS Consultive assessments

Custom LeJOS development projects