Java for Robots

1
Java for Robots

• How to program an
• NXT robot
• with a Java Brain
• Bert G. Wachsmuth
• Seton Hall University

2
The Setup

• Differential drive robot with ultrasound distance
  sensor mounted in front
• Java VM uploaded to NXT
• Connection NXT lt-gt PC via USB or Bluetooth
• Eclipse setup for Java programming
• Eclipse configured for LeJOS

3
Java Program

• A Java program is called a class
• Java is case-sensitive
• Statements are grouped with or end in a
  semicolon
• Every Java program for the NXT has the same
  blueprint
• import lejos.nxt.
• public class Test
• public static void main(String args)

4
Java Basics

• Java programs contain the basic frame work and
• state variables to store global info and data
  (aka fields)
• functions to define particular subtasks (aka
  methods)
• Comments to explain code in plain English
• Single-line comments start with //
• Multi-line comments are enclosed in / /
• Program execution starts at the main method and
  proceeds linearly unless redirected.

5

• import lejos.nxt.
• public class Test
• // global variables go here
• static int heading 0.0
• // function to define subtask
• static void turn(int degrees)
• // subtask code goes here
• public static void main(String args)
• // this is where the program starts
• // code to call the turn function goes
  here

6
Defining Variables

• All variables must have a name, a type, and a
  value compatible with the type.
• All variables must be declared before use
• All variables have a range where they are valid
• State variables are valid everywhere, e.g.
• static int stateVariable -9
• Local variables are valid in the block they are
  defined in
• Variables can have types int, float ( 5.7f),
  double ( 5.7), boolean ( true or false), char
  ( b), or String ( text)
• There is a special type called void and a
  special value named null

7
Defining Functions

• Functions have header and a body
• Header defines name, input variables (and types),
  and return type (often void)
• Body defines what the function actually does
• public static void turn(int degrees)
• // code here
• Functions must be called with values matching the
  input variables as defined in the function header

8
NXT Programs

• To write Java code for the NXT, your first
  statement must be
• import lejos.nxt.
• Then you can use special commands relevant to
  your NXT.
• All possible commands are documented in the LeJOS
  API, which must be available as you program. See
• Robotics Pages -gt Lectures -gt link LeJOS API
• or
• C\JavaNXT\lejos_nxj\docs\api\index.html
• Click on lejos.nxt in top-left frame, then pick
  class in bottom left, to read details in main
  info frame.

9
Sample Programs

• Make some noise
• Show some text
• Move forward for 1 second
• Rotate for 1 second
• Move forward for one 360-degree wheel rotation
• Move forward for x degrees wheel rotation
• Move forward for x cm
• Rotate for x degrees