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Today Session 3 · Classes and Objects Rookie Bootcamp · Programming Track

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Rookie Bootcamp · Programming Track · Session 3

Classes &
Objects

One blueprint, as many robot parts as you need

Today's mission

No more mystery words

Last session's Intake.java is back. You can read most of it now.

But some words we said to "just copy for now." They're circled in gold.

Today you learn what every one means.

01

Why objects?

A swerve drive has four identical modules

Without classes, the code gets out of hand

Drivetrain.java0 variables

There has to be a better way.

★ Story time

1960sOslo, Norway

Two Norwegians who wanted to simulate the world

The goalAt the Norwegian Computing Center, Ole-Johan Dahl and Kristen Nygaard wanted to simulate real systems: factories, traffic, customers waiting in line.

The ideaDescribe one kind of thing once (a class), then make as many as you want (objects), each with its own data.

1967Their language, Simula 67, introduced classes and objects. Java's classes come straight from that idea.

2001They won the Turing Award, computing's version of the Nobel Prize.

The big idea

The class is the blueprint. Objects are the real things.

CLASS

SwerveModule

Every module has

driveId

angle

One drawing. Says what every module has and does.

new ×4 →

frontLeftobject

driveId: 1
angle: 0°

frontRightobject

driveId: 3
angle: 45°

backLeftobject

driveId: 5
angle: 90°

backRightobject

driveId: 7
angle: -30°

Same design. Each object has its own motor ID and its own current angle.

Inside a class

What it knows, and what it does

SwerveModule

Knows fieldsdriveIdanglespeed

Does methodsdrive()setAngle()stop()

Intake

Knows fieldsmotorbeamBreak

Does methodsrun()stop()hasGamePiece()

Fields are variables that belong to the object. Methods are what the object can do.

Surprise

You've been using objects since last session

msg.length()Ask the String object msg how many characters it has

System.out.println("Hi")Ask the screen-printing object System.out to print

motor.setThrottle(0.5)Ask the motor object to spin at half power

The dot means: "this object, please do this."

02

Writing a class

Our example for today

Meet the Battery class

Battery.javaa blueprint

public class Battery {
    String name;
    int year;
    double voltage;

    boolean isMatchReady() {
        return voltage >= 12.0;
    }
}

LINE 1The class name: Battery. Capital first letter, in Battery.java.

FIELDSWhat every battery knows: its name, year and voltage. Declared inside the class, outside any method.

METHODWhat every battery can do: say if it's ready for a match.

NOTEThe method uses the field voltage directly. It's this battery's own voltage.

★ Our team's ruleWe name every battery and check its voltage before every match. 12.0 V or more means it's match-ready.

From blueprint to battery

`new` builds an object from the class

Main.javauses the blueprint

Battery a = new Battery();
a.name = "2025-A";
a.year = 2025;
a.voltage = 12.8;

System.out.println(a.isMatchReady());

output

true

TYPEBattery a: a variable named a that holds a Battery. The class is the type.

NEWnew Battery(): build one real battery object from the blueprint.

DOTFill in a's own fields, using the dot.

CALLAsk a whether it's match-ready.

Interactive · object factory

Each object gets its own copy of the fields

CLASS

Battery

name · year · voltage
isMatchReady()

Click a battery's voltage to change it.

Press new Battery() to build one.

Predict, then reveal

Same method, different answers

aBattery

name: "2025-A"
year: 2025
voltage: 12.8

bBattery

name: "2023-C"
year: 2023
voltage: 11.8

a.isMatchReady()true

b.isMatchReady()false

Each call uses that object's own voltage.

Compare

Class vs. object

The blueprint

Class

Written once, in its own file
Capital first letter: Battery
Describes fields and methods
Exists while you write code

VS

The real thing

Object

As many as you want
Made with new
Lower-case variable: spareBattery
Exists while the program runs

Battery is the class. spareBattery is one battery.

Callback to last session

One class per file, and files work together

📁 ex01

.java

Battery.java

public class Battery { ... }

The blueprint

.java

Main.java

public class Main { main ... }

Uses it: new Battery()

Main.java can use Battery without any import, because both files are in the same folder.

That folder is called a package. More in section 7.

★ Today's exercisesUnzip today's pack into C:\bootcamp, then open the C:\bootcamp\session3 folder in WPILib VS Code. Each exercise is its own folder inside it: ex01, ex02... The first line of each file says package ex01;. Leave it alone for now.

▶ Try it · 6 min

Your first class

Open the ex01 folder

In Battery.java, add a method describe() that prints the name, year and voltage.
In Main.java, build three batteries with new and fill in their fields.
Call describe() and isMatchReady() on each one.
Stretch: add a method int age(int thisYear).

example output

2025-A (2025): 12.8 V
  match-ready: true
2024-B (2024): 11.8 V
  match-ready: false
2023-C (2023): 12.6 V
  match-ready: true

★ Hintdescribe() doesn't return anything, so its return type is void. Inside it, just use name, year and voltage.

No more mystery words

Find the class parts

The class: Intake
Two fields: motor and beamBreak
Each one built with new: a motor object and a sensor object
Three methods: run, stop, hasGamePiece

03

Constructors

The problem

What if you forget to fill in the fields?

Battery c = new Battery();
System.out.println(c.name);
System.out.println(c.isMatchReady());

output

null
false

cBattery

name: null
year: 0
voltage: 0.0

Fields you don't set get default values: 0, 0.0, false, or null for objects.

What if Java made you fill in the important parts when you build it?

Constructor

A constructor sets up each new object

Battery.javawith a constructor

public class Battery {
    String name;
    int year;
    double voltage;

    public Battery(String name, int year, double voltage) {
        this.name = name;
        this.year = year;
        this.voltage = voltage;
    }
    // ...isMatchReady() as before
}

NAMESame name as the class: Battery.

NO TYPENo return type, not even void. That's how Java knows it's a constructor.

JOBCopy the values handed in into this object's fields.

WHENRuns once, automatically, every time you say new.

now in Main.java

Battery a = new Battery("2025-A", 2025, 12.8);

this

`this` means "my own"

1 · The call

new Battery("2025-A", 2025, 12.8)

→

2 · Parameters

name"2025-A"

year2025

voltage12.8

Temporary. Gone when the constructor ends.

→

3 · this object's fields

new Batteryobject

this.name: "2025-A"
this.year: 2025
this.voltage: 12.8

Kept as long as the object lives.

⚠ The classic bugvoltage = voltage; copies the parameter into itself. The field is never set and stays 0.0. Always write this.voltage = voltage;

The error everyone hits

Once you add a constructor, `new Battery()` stops working

problems

Main.java:5: error: constructor Battery in class Battery
cannot be applied to given types;
    Battery spare = new Battery();
                    ^
  required: String,int,double
  found:    no arguments

MEANS"Battery's constructor needs three things and you gave it none."

REQUIREDWhat it wants: a String, an int, a double.

FIXnew Battery("Spare", 2024, 12.6)

Read the error: "required" tells you exactly what to hand in.

Constructors on our robot

What if the robot had two intakes?

public class Intake {
    private final PWMSparkMax motor;
    private final DigitalInput beamBreak;

    public Intake(int motorChannel, int sensorChannel) {
        motor = new PWMSparkMax(motorChannel);
        beamBreak = new DigitalInput(sensorChannel);
    }
}

Intake front = new Intake(0, 1);
Intake back  = new Intake(2, 3);

The channel numbers move into the constructor. One class, two separate intakes.

The four-module problem, solved

new SwerveModule(1, 2);
new SwerveModule(3, 4);
new SwerveModule(5, 6);
new SwerveModule(7, 8);

Write the class once. Build it four times with different IDs.

▶ Try it · 5 min

Add a constructor

Open the ex02 folder

Run Main.java. Every battery says 0.0 V. Why?
Find the bug in Battery's constructor and fix it.
In Main.java, add a fourth battery ("2022-D", 2022, 11.9) in one line.
Stretch: add a second constructor that takes only a name (year 2026, voltage 12.0).

before the fix

2025-A: 0.0 V  ready: false
2024-B: 0.0 V  ready: false
2023-C: 0.0 V  ready: false

★ HintLook for a line where a name is assigned to itself.

Break

Stretch!

Back in 10 minutes. Next: arrows, and the most famous crash in Java.

04

References and null

Callback to last session's memory stepper

Every object variable holds an arrow

countint

1

A primitive (int, double, boolean, char...) sits right inside its box.

msgString

Count:space1

A String box holds an arrow.

aBattery

Battery object

name: "2025-A"
voltage: 12.8

So does every object variable. The arrow is called a reference.

Interactive · press Step, predict first

Two arrows, one battery

output

MEMORY

Explained at last

`==` compares arrows. `.equals()` compares contents.

modeString

auto

from the dashboard

targetString

auto

typed in your code

mode == targetfalse

Same letters, but two different objects in two places. The arrows are different.

mode.equals(target)true

.equals() looks inside and compares the characters.

That's why we said last session: always compare Strings with .equals().

null

null: a box with an arrow to nothing

spareBattery

no object

Battery spare = null;

The variable exists. The battery doesn't.

Object fields you never set start as null too (remember c.name printing null?).

Primitives can't be null. An int always holds a number.

★ Story time

1965London, England

The billion-dollar mistake

1965British computer scientist Tony Hoare added the null reference to a language called ALGOL W, "simply because it was so easy to implement."

DecadesAlmost every language copied it, Java included. Arrows to nothing caused countless crashes, bugs and security holes.

2009At a conference he publicly apologized, calling it his "billion-dollar mistake."

BonusHe also invented Quicksort, one of the most-used ways to sort data in the world.

Even legends make mistakes. The good ones admit it.

Interactive · the most famous crash in Java

The NullPointerException

output

MEMORY

NullPointerExceptions on the robot

It deploys fine. Then it crashes when you enable.

public class Intake extends SubsystemBase {
    private PWMSparkMax motor;   ⟦// no new!⟧

    public void stop() {
        motor.setThrottle(0.0);
    }
}

driver station console

Unhandled exception: java.lang.NullPointerException:
Cannot invoke "PWMSparkMax.setThrottle(double)"
because "this.motor" is null
  at frc.robot.subsystems.Intake.stop(Intake.java:5)

✓

Build succeeds

✓

Deploy succeeds

✕

Enable: crash, robot code restarts

Rule: every object field gets its new, where it's declared or in the constructor.

▶ Try it · 5 min

Null hunt

Open the ex03 folder

Run Main.java. It crashes with a NullPointerException.
Read the message: which variable is null? Which file and line?
Fix it, run again. A second, sneakier null is waiting.
Both fixed? The robot drives and prints "All systems go".

first crash

Exception in thread "main"
java.lang.NullPointerException:
Cannot invoke "ex03.Motor.setThrottle(double)"
because "this.rightMotor" is null
  at ex03.Drivetrain.drive(Drivetrain.java:15)
  at ex03.Main.main(Main.java:10)

★ Hint for the second oneLook closely at setup(). Is it filling in the field, or making a brand-new variable with the same name?

No more mystery words

Why Intake can't crash with null

Both object fields get their new right where they're declared
So motor is never null when run() or stop() uses it
beamBreak is never null either

05

static vs. instance

Interactive · static means "shared by the whole class"

One clock on the wall, a backpack for everyone

public class Battery {
!!    static int batteriesMade = 0;
    double voltage;

    public Battery(double voltage) {
        this.voltage = voltage;
        batteriesMade++;
    }
}

STATICOne copy, owned by the class. Like the wall clock.

INSTANCEOne per object. Like each student's backpack.

Battery class · staticbatteriesMade0

A mystery from last session, solved

Why is `main` static?

!!public static void main(String[] args) {

When the program starts, no objects exist yet.

So main has to work without an object. It belongs to the class: static.

Static methods you've already used

Math.max(3, 7) → 7

Math.abs(-0.4) → 0.4

No new Math() needed. You call them on the class name, because they don't need any object's data.

The error everyone hits

"cannot be referenced from a static context"

public static void main(String[] args) {
    ⟦describe()⟧;
}

problems

error: non-static method describe() cannot be
referenced from a static context

public static void main(String[] args) {
    Battery a = new Battery("2025-A", 2025, 12.8);
    a.describe();
}

TRANSLATION"describe() needs a battery. Which battery? You didn't make one."

FIXMake an object with new, then call the method on it.

Interactive · the tempting wrong fix

Don't "fix" it by making everything static

SwerveModule.javatry both

public class SwerveModule {
    double angle;
}

angle is an instance field

Where static is exactly right

Constants: `static final`

private static final int kMotorChannel = 0;
private static final int kSensorChannel = 1;
private static final double kIntakeSpeed = 0.75;

◆ Robot projectsMost teams keep these in one file, Constants.java, so every number lives in one place.

STATICOne copy for the class. Every Intake uses the same speed.

FINALCan't change after it's set. Try, and the compiler refuses.

kWPILib style: constants start with k.

▶ Try it · 4 min

Fix it the right way

Open the ex04 folder

Main.java has three "static context" errors.
Fix each one by making a SwerveModule object and calling the method on it.
Do not add the word static to SwerveModule.java.
Run it. A built-in check warns you if the modules are secretly sharing an angle.

when it's right

FL angle: 45.0
FR angle: 90.0
FL is driving at 0.5
CHECK PASSED: each module has its own angle

★ HintSwerveModule fl = new SwerveModule("FL"); then fl.setAngle(45);

Break

Recharge!

Back in 10 minutes. Next: private, packages, and lots of objects.

06

private and public

The problem

If anyone can change a field, anyone can break it

Shooter.javasomeone else's file

// "I'll just make it faster"
motor.speed = ⟦5.0⟧;

A motor only understands -1.0 to 1.0. Nothing stops 5.0 from getting in.

And a teammate's code could change it in the middle of a match.

5.0 ?!

⚡

Interactive · private

`private`: only this class can touch it

SafeMotor.javaclick to switch

speed is private

🔒

0.0

⚡

compiler

Ready.

The front door

public methods are the safe way in

public class SafeMotor {
    private double speed;

    public void setSpeed(double speed) {
        this.speed = Math.clamp(speed, -1.0, 1.0);
    }

    public double getSpeed() {
        return speed;
    }
}

HIDDENThe field is private. Nobody outside can touch it.

SETTERThe only way to change it. It can clamp, check, or refuse.

GETTERLets others read the value without changing it.

Hide the inside. Offer safe buttons on the outside. This is called encapsulation.

The robot rule of thumb

Fields private, useful methods public

Every field in Intake is private
run(), stop(), hasGamePiece() are public: other code needs them
A helper only this class uses? Make it private too.

▶ Try it · 5 min

Build a SafeMotor

Open the ex05 folder

In SafeMotor.java, make speed private.
Main.java stops compiling on one line. Read the error, then delete that line: that's the point!
Make setSpeed clamp the value to -1.0 to 1.0.
Run it until every check says PASS.

goal

setSpeed(0.5) -> 0.5   PASS
setSpeed(3.0) -> 1.0   PASS
setSpeed(-7.0) -> -1.0   PASS
setSpeed(-0.25) -> -0.25   PASS

★ HintOne line: Math.clamp(speed, -1.0, 1.0). An if / else if works too.

07

Packages and imports

package

A package is a folder

package frc.robot.subsystems;

📁frc

📁robot

📁subsystems

📄Intake.java

Each dot is a folder. The package line must match where the file lives, or it won't compile.

import

import: borrowing a class from another package

package frc.robot.subsystems;

import org.wpilib.drivers.motor.PWMSparkMax;

"I'm using the PWMSparkMax class from WPILib's org.wpilib.drivers.motor package."

forgot the import?

error: cannot find symbol
  symbol:   class PWMSparkMax
  location: class Intake

PWMSparkMax motor;

VS Code's light bulb (Ctrl + .) adds the import for you.

★ Story time

2016March · the internet

Eleven lines that broke the internet

The codeA tiny package called left-pad: about 11 lines that add spaces to the front of a piece of text.

Who used itThousands of JavaScript projects imported it, often without knowing, through other packages they imported.

One dayIts author removed it from npm, the big public library of JavaScript code. Builds failed all over the world, including tools used by major websites.

Modern programs are built from other people's classes. Know what you import.

Interactive · click a file

Tour of a robot project

📁 src/main/java

📁 frc/robot

📁 subsystems

📁 commands

Click a file

Every file is one class, in a folder that matches its package.

08

Many objects: arrays and lists

Arrays · callback to last session's char slots

An array is a row of numbered slots

@@1SwerveModule[] modules = new SwerveModule[4];
@@2modules[0] = new SwerveModule(1, 2);
@@3modules[1] = new SwerveModule(3, 4);
@@3modules[2] = new SwerveModule(5, 6);
@@3modules[3] = new SwerveModule(7, 8);

Four slots, numbered 0 to 3. Each one starts as null.

Last slot is length - 1. modules[4] crashes: ArrayIndexOutOfBoundsException.

modulesSwerveModule[]

null

0

null

1

null

2

null

3

(1, 2)

(3, 4)

(5, 6)

(7, 8)

for-each loop

One loop talks to every object

for (SwerveModule m : modules) {
    m.stop();
}

Read it as: "for each module m in modules, stop it."

★ The four-module problem, fully solvedOne class, one array, one loop. Fix a bug once and all four modules get the fix.

modules[0]m.stop() ✓

modules[1]m.stop() ✓

modules[2]m.stop() ✓

modules[3]m.stop() ✓

ArrayList

ArrayList: a list that grows

import java.util.ArrayList;

ArrayList<Battery> cart = new ArrayList<>();
cart.add(new Battery("2025-A", 2025, 12.8));
cart.add(new Battery("2024-B", 2024, 12.3));

System.out.println(cart.size());      // 2
System.out.println(cart.get(0).name); // 2025-A

<Battery>"A list of Batteries only." Anything else won't compile.

ADDGrows as you add. Arrays are a fixed size; lists aren't.

SIZE / GETHow many, and the one at a position. Still numbered from 0.

⚠ Off by one, againcart.get(cart.size()) crashes. The last one is size() - 1.

▶ Try it · 6 min

The battery cart

Open the ex06 folder

Make an ArrayList<Battery> called cart.
Add the five batteries listed in the comments.
Loop over the cart and print each battery's status.
Count how many are match-ready and print the total.
Stretch: find and print the battery with the highest voltage.

example output

2025-A  12.8 V  ready
2025-B  11.8 V  charge me
2024-A  12.7 V  ready
2024-B  11.9 V  charge me
2023-A  12.5 V  ready
3 of 5 batteries are match-ready

★ HintMake an int ready = 0; before the loop and add 1 inside an if.

09

A first look at extends

★ Story time

1970sXerox PARC, California

A computer language for kids

The nameAlan Kay coined the term "object-oriented programming." He pictured objects like living cells: each does its own job and sends messages to the others.

SmalltalkHis team built Smalltalk, a language he hoped kids could use to build their own programs.

DynabookHe also sketched the Dynabook: a thin, portable computer for children. That was 1972, decades before tablets existed.

The ideas you're learning today were designed with students in mind.

extends

extends means "is a kind of"

SubsystemBase written by WPILib

periodic()setDefaultCommand(...)... and more

is a kind of

Intake written by us

run()stop()hasGamePiece()

+ everything above, for free

public class Intake extends SubsystemBase {

    @Override
    public void periodic() {
        // runs every 20 ms
    }
}

@Override: "I'm replacing a method I inherited." The robot calls periodic() every 20 ms (the loop from our first session).

The details come in a later session on command-based code. For now: an Intake is a Subsystem.

10

Putting it together

The missing piece

Where does the Intake object come from?

RobotContainer imports our Intake class
and builds one Intake object with new
plus a controller object for the driver
Hold A: the intake's run method. Let go: stop.

Class written in Intake.java → object built in RobotContainer → methods called every loop.

Mission complete

No more mystery words

Click a layer to turn it on or off.

Structure Data Control Methods Objects (new!)

▶ Try it · 10 min · in pairs

Build a subsystem from scratch

Open the ex07 folder

Motor.java and BeamBreak.java are fake robot parts. Don't change them.
In Intake.java: two private fields (a Motor and a BeamBreak).
A constructor that takes the two channel numbers and builds both with new.
run(): spin at 0.75 unless there's a game piece, then stop. stop(). hasGamePiece().
Run Main.java: a 50-loop simulated match checks your Intake.

goal

loop  0: motor 0.75  piece: false
loop 19: motor 0.75  piece: false
loop 20: motor 0.0   piece: true
PASS  motor runs before the piece
PASS  motor stops when the piece arrives
PASS  hasGamePiece() is true at the end
3 / 3 checks passed

★ HintThe beam is blocked when a piece is inside, so hasGamePiece() returns !beamBreak.get(). Same as Intake.java.

Recap

Five things to remember

01

A class is a blueprint. new builds an object.

02

Constructors set up new objects. Use this.

03

Object variables hold arrows. null means no object.

04

static belongs to the class. Never use it to silence errors.

05

Fields private, useful methods public.

Quick check · tap an answer

How much do you remember?

1 · What does `new` do?

The class is the blueprint; new builds one real object from it

2 · `b = a; b.voltage = 11.9;` Now `a.voltage` is...

a and b are two arrows to the same battery

3 · Why is `main` static?

main runs before any object is built, so it belongs to the class

4 · What's wrong with `voltage = voltage;` in a constructor?

It copies the parameter into itself. Use this.voltage = voltage;

Up next · Session 4

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Classes &Objects

No more mystery words

Why objects?

Without classes, the code gets out of hand

Two Norwegians who wanted to simulate the world

The class is the blueprint. Objects are the real things.

What it knows, and what it does

You've been using objects since last session

Writing a class

Meet the Battery class

new builds an object from the class

Each object gets its own copy of the fields

Same method, different answers

Class vs. object

Class

Object

One class per file, and files work together

Battery.java

Main.java

Your first class

Open the ex01 folder

Find the class parts

Constructors

What if you forget to fill in the fields?

A constructor sets up each new object

this means "my own"

Once you add a constructor, new Battery() stops working

What if the robot had two intakes?

The four-module problem, solved

Add a constructor

Open the ex02 folder

Stretch!

References and null

Every object variable holds an arrow

Two arrows, one battery

== compares arrows. .equals() compares contents.

null: a box with an arrow to nothing

The billion-dollar mistake

The NullPointerException

It deploys fine. Then it crashes when you enable.

Null hunt

Open the ex03 folder

Why Intake can't crash with null

static vs. instance

One clock on the wall, a backpack for everyone

Why is main static?

Static methods you've already used

"cannot be referenced from a static context"

Don't "fix" it by making everything static

Constants: static final

Fix it the right way

Open the ex04 folder

Recharge!

private and public

If anyone can change a field, anyone can break it

private: only this class can touch it

public methods are the safe way in

Fields private, useful methods public

Build a SafeMotor

Open the ex05 folder

Packages and imports

A package is a folder

import: borrowing a class from another package

Eleven lines that broke the internet

Tour of a robot project

Many objects: arrays and lists

An array is a row of numbered slots

One loop talks to every object

ArrayList: a list that grows

The battery cart

Open the ex06 folder

A first look at extends

A computer language for kids

extends means "is a kind of"

Putting it together

Where does the Intake object come from?

No more mystery words

Build a subsystem from scratch

Open the ex07 folder

Classes &
Objects

`new` builds an object from the class

`this` means "my own"

Once you add a constructor, `new Battery()` stops working

`==` compares arrows. `.equals()` compares contents.

Why is `main` static?

Constants: `static final`

`private`: only this class can touch it

1 · What does `new` do?

2 · `b = a; b.voltage = 11.9;` Now `a.voltage` is...

3 · Why is `main` static?

4 · What's wrong with `voltage = voltage;` in a constructor?

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