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First fare 2011 frc-java-introduction


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First fare 2011 frc-java-introduction

  1. 1. Introduction to Java for FIRST Robotics Andrew MerrillSoftware Mentor, FRC Team 1540 Computer Science Teacher, Catlin Gabel SchoolWhat 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. 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 machineGoldilocks 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. 3. Common Java Misconceptions • Java is not limited to Web Programming or Applets • Java is not JavaScript, despite the misleadingly similar name!Sample Java Programpublic class DemoBot extends IterativeRobot{ Joystick stick; Jaguar launcher; public void teleopInit() { stick = new Joystick(1); // joystick on USBport 1 launcher = new Jaguar(2); // speed controller onPWM port 2 } public void teleopPeriodic() { if (stick.getTrigger()) // when the joysticktrigger is pressed launcher.set(0.75); // run launcher at 75%power else launcher.set(0.0); // otherwise, stop thelauncher }}
  4. 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 objects actions § example methods: getTrigger, set § methods are verbs • By convention, class names begin with a capital letterObjects • 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. 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 letterInheritance • 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. 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 classConstructors • 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 classs fields • Example: public class DemoBot extends SimpleRobot { Joystick stick; Jaguar launcher; DemoBot() { stick = new Joystick(1); launcher = new Jaguar(2); }
  7. 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 SpeedControllerStatic and Final • A static field is a class field, not an object field • A final field is a constant - its value cant be changed
  8. 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. 9. • The Java library package java.lang is always imported automaticallyClass 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 classprivate yes no no no(default) yes yes no noprotected yes yes yes nopublic yes yes yes yesJava 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. 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, CharacterNote: No unsigned numbers, unlike C/C++Math • + for addition • - for subtraction • * for multiplication • / for division (warning: if you divide two integer types, youll 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. 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. 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( • You can catch and handle Exceptions yourself: try { // do possibly dangerous stuff here // keep doing stuff } catch (Exception e) {
  13. 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. 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. 15. Using Java for FRC RoboticsInstalling Java for FRC • You do not need LabView or WindRiver Workbench (C++) installed • In fact, you dont 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 o NetBeans is available from (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 " dates.xml"
  16. 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. Youre 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 codeCreating 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. 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 programSimpleRobot 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. 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 actionsDisplaying 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. 19. o NetConsole requires the National Instruments LabView libraries (installed from the FIRST DVD) o Download NetConsole from o More info is at (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. 20. § 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 WPIs Java for FRC page: § § Read Getting Started With Java For FRC and WPI Library Users Guide o FirstForge site for WPILib: § o FRC 2012 Beta Test Forums: § =1525 o Java Forum at Chief Delphi
  21. 21. § splay.php?f=184 o Java Forum at FIRST Forums: § =1334 o Official Sun/Oracle Java Tutorial §• Books o Java in a Nutshell by David Flanagan (OReilly) o Effective Java by Joshua Bloch