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

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

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[]) { }

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.

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 }

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”

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

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 -> Lectures -> 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.

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