1. Introduction to Java
for FIRST Robotics
Andrew Merrill
Software Mentor, FRC Team
1540
Computer Science Teacher,
Catlin Gabel School
What is Java?
• Java is a Programming Language
o documented by the Java Language Specification
o "Java is object-oriented, strongly typed, with C-
like syntax"
• Java is a Class Library
o documented by the:
§ Java ME CLDC API Specification, or the

2. § Java SE API Specification, or the
§ Java EE API Specification
o "Java has graphics, threads, networking, data
structures, etc..."
• Java is a Runtime Environment
o documented by the Java Virtual Machine
Specification
o "Java is compiled to bytecodes which are
interpreted by a virtual machine
Goldilocks and the Three Javas
• Small - Java Micro Edition (ME)
o designed for mobile and embedded devices
o used for FRC robotics
• Medium - Java Standard Edition (SE)
o designed for regular laptop, desktop, server
applications
o the most common edition
o widely used in computer science courses
(including AP)
• Large - Java Enterprise Edition (EE)
o designed for application servers, distributed
systems

3. Common Java Misconceptions
• Java is not limited to Web Programming or Applets
• Java is not JavaScript, despite the misleadingly similar
name!
Sample Java Program
public class DemoBot extends IterativeRobot
{
Joystick stick;
Jaguar launcher;
public void teleopInit()
{
stick = new Joystick(1); // joystick on USB
port 1
launcher = new Jaguar(2); // speed controller on
PWM port 2
}
public void teleopPeriodic()
{
if (stick.getTrigger()) // when the joystick
trigger is pressed
launcher.set(0.75); // run launcher at 75%
power
else
launcher.set(0.0); // otherwise, stop the
launcher
}
}

4. Classes
• A class defines a new type of object
o example classes: DemoBot, Joystick,
Jaguar
• Each class contains two kinds of members:
o Fields: variables that hold data needed by this
object
§ example fields: stick, launcher
§ fields are nouns
o Methods: functions that perform the object's
actions
§ example methods: getTrigger, set
§ methods are verbs
• By convention, class names begin with a capital letter
Objects
• An object is an instance of a class
• Objects are accessed via variables
• Variables are declared in advance to refer to objects
from a particular class
o Example: Jaguar launcher;

5. • Objects are created with the operator new
o Example: launcher = new
Jaguar(2);
• Members of an object are accessed with the syntax
variable.member
o Example: launcher.set(0.75);
• When no variable refers to an object anymore, it is
automatically "garbage collected"
• By convention, variable and function names begin
with a lower case letter
Inheritance
• A child class can extend a parent class
o alternative terminology: a sub-class can extend a
super-class
• Objects of the child class can do everything that
objects of the parent class can do
o child class objects inherit the fields and methods
of the parent class
o allows code to be shared between similar classes
• Examples:
o class DemoBot extends
IterativeRobot
o class Jaguar extends PWM
o class Victor extends PWM
o class Servo extends PWM
• Child classes can override parent class methods

6. o new method must have the same name and
parameters
o Example: teleopPeriodic()
in IterativeRobot
• Differences from C++
o no multiple inheritance
o all methods can be overridden by default
o all classes extend the built-in Object class
Constructors
• A constructor is a special method in class
• It is automatically run when a new object is created
• Constructors always have exactly the same name as
the class
• Constructors have no return type
• Constructors are often used to initialize the class's
fields
• Example:
public class DemoBot extends SimpleRobot
{
Joystick stick;
Jaguar launcher;
DemoBot()
{
stick = new Joystick(1);
launcher = new Jaguar(2);
}

7. Interfaces
• An interface is like a class, but...
o it has no fields
o its methods have no bodies
• So what does it have?
o method names with their parameters and return
type
• A class can implement an interface (or several
interfaces)
• Think of an interface as a promise to write certain
methods
• Example interface:
interface SpeedController
{
double get();
void set(double speed);
}
• To implement an interface:
o class Jaguar extends PWM
implements SpeedController
Static and Final
• A static field is a class field, not an object field
• A final field is a constant - its value can't be changed

8. • Example: static final int maxAngle =
90;
• Example: Joystick.BUTTON_TRIGGER
• A static method can be run without making an object
first
• Example: time = Timer.getUsClock();
Packages and Importing
• Java classes can be organized into packages
• Each package goes in a separate directory (or folder)
• How to use a class from the package
edu.wpi.first.wpilibj:
o Write the package name every time you use the
class name
§ Example: stick = new
edu.wpi.first.wpilibj.Joystic
k(1);
o or import the class from the package
§ Example: import
edu.wpi.first.wpilibj.Joystic
k;
o or import every class from the package
§ Example: import
edu.wpi.first.wpilibj.*;

9. • The Java library package java.lang is always
imported automatically
Class Member Access Control
• Java restricts who can access the members of a class
Can be Can be Can be
Can be
accessed accessed accessed
accessed
from the from the from
from
same same any child
any class
class package class
private yes no no no
(default) yes yes no no
protected yes yes yes no
public yes yes yes yes
Java Data Types
• Number Types
o Integers
§ byte (8 bits, range from -128 to 127)
§ short (16 bits, range of _ 32767)
§ int (32 bits, range of about _ 2 billion)

10. § long (64 bits, range of about _ 9 quintillion
or 1019)
o Floating Point
38
§ float (32 bits, range of about _ 10 ,
precision of about 7 decimal digits)
308
§ double (64 bits, range of about _ 10 ,
precision of about 16 decimal digits)
• Other Types
o boolean (true or false)
o char (one Unicode character)
• String (standard library class)
• wrapper classes: Byte, Short, Integer, Long,
Float, Double, Boolean, Character
Note: No unsigned numbers, unlike C/C++
Math
• + for addition
• - for subtraction
• * for multiplication
• / for division (warning: if you divide two integer
types, you'll get an integer type result)
• % for remainder after division (for example, 10 % 3 is
2)
• Math.sqrt(x) for square root
• Math.abs(x) for absolute value
• Math.min(a,b), Math.max(a,b) for
minimum and maximum

11. • Math.sin(x), Math.cos(x),
Math.tan(x) for trigonometry
• If you need more math functions: import
com.sun.squawk.util.MathUtils
o MathUtils.pow(a,b),
MathUtils.log(a),
MathUtils.atan2(y,x)
Randomness
• The Java library provides a class called Random
• It is in the java.util package, so you should
import java.util.Random;
• A Random object is a random number generator
• Only create one random number generator per
program!
o Example: public static Random
generator = new Random();
• Example: How to generate a random integer in the
range 0...359:
o int spinDirection =
generator.nextInt(360);
• Example: How to generate a random floating point
number in the range 0...1:
o double probability =
generator.nextDouble();

12. Casting
• To force a double into an int (losing the decimal
part):
o int x = (int) 3.7;
• To force an int to be treated as a double:
o double average = ((double) total)
/ count;
• To tell Java that an Object is really a Jaguar:
o Jaguar launcher = (Jaguar)
getSpeedController();
Exceptions
• When your program crashes, Java throws an
Exception
• Example:
java.lang.ArithmeticException: / by zero
at DemoBot.teleopPeriodic(DemoBot.java:15)
• You can catch and handle Exceptions yourself:
try {
// do possibly dangerous
stuff here
// keep doing stuff
}
catch (Exception e) {

13. launcher.set(0);
System.out.println("launcher
disabled");
System.out.println("caught: "
+ e.getMessage());
}
Java ME Data Structures
• Arrays
o Fixed number of elements
o All elements of the same type (or compatible
types)
o Random access by element index number
o Useful array utilities in the
package com.sun.squawk.util.Arrays
§ sort, copy, fill, binarySearch
o Example:
int data[] = new int[100];
data[0] = 17;
data[5] = data[0] + 1;
System.out.println(data[17]);
• Vector
o Variable number of elements
o All elements must be objects
o Random access by element index number
o import java.util.Vector;
o Example:

14. Vector speedControllers = new
Vector();
speedControllers.addElement(new
Jaguar(1));
Jaguar controller = (Jaguar)
speedControllers.elementAt(0);
• Hashtable
o Otherwise known as a dictionary, map,
associative array, lookup table
o Given a key, can quickly find the associated
value
o Both the key and value must be objects
o import java.util.Hashtable;
o Example:
Hashtable animals = new
Hashtable();
animals.put("cow", "moo");
animals.put("chicken", "cluck");
animals.put("pig", "oink");
String chickenSound = (String)
animals.get("chicken");
System.out.println("a chicken
goes" + chickenSound);
• Other Data Structures:
o SortedVector in edu.wpi.first.wpili
bj.util
o Stack in java.util
o IntHashtable in com.sun.squawk.util

15. Using Java for FRC Robotics
Installing Java for FRC
• You do not need LabView or WindRiver Workbench
(C++) installed
• In fact, you don't need anything from the FIRST DVD
• Works on Windows (including Windows 7), Mac OS
X, and Linux
1. Download the Java SE JDK and the NetBeans IDE
o The Java SE JDK is available from
http://java.sun.com
o NetBeans is available from
http://netbeans.org/downloads (get the Java SE
version)
o OR, you can download the Java JDK/NetBeans
Bundle from Sun/Oracle
2. Install the JDK and NetBeans
3. Follow the instructions in Getting Started With Java
for FRC to download and install FRC plugins:
A.Select Tools -> Plugins -> Settings,
and click Add
B. Type the Name "FRC Java"
C. Type the URL
"http://first.wpi.edu/FRC/java/netbeans/update/up
dates.xml"

16. D.On the Available Plugins tab, select the 5
FRC Java plugins, and click Install
E. Accept all of the agreements, and ignore the
validation warning
F. Restart NetBeans
G.Select Tools -> Options (for Windows)
or NetBeans -> Preferences (for Mac)
H.Select Miscellaneous -> FRC
Configuration and enter your team number
4. You're done!
• After installing the plugins, you should have
a sunspotfrcsdk folder in your home directory
o sunspotfrcsdk/doc/javadoc/index.h
tml has the class library documentation
o sunspotfrcsdk/lib/WPILibJ/src has
the class library source code
Creating and Running a Java Program
• From the File menu, select New Project
• Select the "FRC Java" category
• Select a template:
o IterativeRobotTemplateProject or Si
mpleRobotTemplateProject
• Click Next
• Give your project a Project Name
• Change the Project Location if you want
• Click Finish

17. • To run your program, either:
o Select Run Main Project from the Run
menu; or
o Click the green triangle in the toolbar; or
o Press F6
• Whichever means you choose, this will:
o Compile and build your project
o Download your program to the cRio
o Reboot the cRio to run your program
• Wait for it to say it is waiting for the cRio to reboot
• Then move to the Driver Station
• Wait for the "Communication" and "Robot Code"
lights to go from red to green
• Click "Enable" to start the program
SimpleRobot vs. IterativeRobot
• Your robot class will extend either SimpleRobot
or IterativeRobot
• The SimpleRobot class provides two methods you
should override:
o autonomous(), which is called when
autonomous mode starts
o operatorControl(), which is called when
teleoperated mode starts
o You need to write your own loops in these
functions to keep them running
o You can execute a sequence of actions easily

18. o make sure to run getWatchdog().feed()
inside your loop
• The IterativeRobot classes provides more
methods to overide:
o disabledInit(), autonomousInit(),
teleopInit()
§ called when the robot enters the given mode
o disabledPeriodic(),
autonomousPeriodic(),
teleopPeriodic()
§ called approx. every 10 ms
o disabledContinuous(),
autonomousContinuous(),
teleopContinuous()
§ called as fast as the robot can
o The IterativeRobot provides the main loop for
you, and calls the appropriate functions
o You need to design your own state machine to
execute a sequence of actions
Displaying Diagnostic Output
• Option 1: System.out.println()
o This is the usual way to display text output from
a Java program
o Example: System.out.println("curren
t speed is " + speed);
o To view the output, install the NetConsole viewer

19. o NetConsole requires the National Instruments
LabView libraries (installed from the FIRST
DVD)
o Download NetConsole from
http://first.wpi.edu/Images/CMS/First/NetCons
oleClient_1.0.0.4.zip
o More info is at
http://first.wpi.edu/FRC/frccupdates.html (Team
Update 4.0)
• Option 2: Driver Station User Messages
o There is a six line area for User Messages on the
Driver Station
o You can display text in it using
the DriverStationLCD class
o Example (displays "Hello" on line 2, column 1):
DriverStationLCD dsLCD =
DriverStationLCD.getInstance();
dsLCD.println(DriverStationLCD.Line.
kUser2, 1, "Hello");
dsLCD.updateLCD();
• Option 3: SmartDashboard
o Convenient way to log diagnostic info to a
remote display
o New Java client that runs on desktop/laptop
computer
o Client program automatically displays logged
fields without needing configuration
o Download the client from:

20. §http://firstforge.wpi.edu/sf/frs/do/viewSumm
ary/projects.smartdashboard/frs
o Example:
SmartDashboard.init();
SmartDashboard.log("Disabled",
"System State");
SmartDashboard.log(leftDrive.get(),
"Left Drive");
SmartDashboard.log(rightDrive.get(),
"Right Drive");
SmartDashboard.log(rollerAvg, "Roller
Avg. Value");
SmartDashboard.log(basket.hasBall(),
"Ball in Robot");
Resources
• Websites
o WPI's Java for FRC page:
§ http://first.wpi.edu/FRC/frcjava.html
§ Read Getting Started With Java For
FRC and WPI Library Users Guide
o FirstForge site for WPILib:
§ http://firstforge.wpi.edu/sf/projects/wpilib
o FRC 2012 Beta Test Forums:
§ http://forums.usfirst.org/forumdisplay.php?f
=1525
o Java Forum at Chief Delphi

21. §http://www.chiefdelphi.com/forums/forumdi
splay.php?f=184
o Java Forum at FIRST Forums:
§ http://forums.usfirst.org/forumdisplay.php?f
=1334
o Official Sun/Oracle Java Tutorial
§ http://download.oracle.com/javase/tutorial/
• Books
o Java in a Nutshell by David Flanagan (O'Reilly)
o Effective Java by Joshua Bloch