Variables and Data Types

The building blocks of every Java program

Overview

Every piece of information in a Java program is stored in a variable. Before you can use a variable, you must declare it — give it a name and a data type that defines what kind of value it holds.

See table of contents for a breakdown of this section.

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Primitive Data Types

Java has several built-in data types. The four you will use most in FRC programming are:

Type	Stores	Example
int	Whole numbers	int motorPort = 1;
double	Decimal numbers	double speed = 0.75;
boolean	True or false	boolean isRunning = false;
String	Text	String subsystemName = "Drivetrain";

Note

String starts with a capital letter because text behaves differently from numbers in Java — don't worry about why yet, just remember to capitalize it. You will learn more in Java Classes.

Java is case sensitive

Int and int are different. Boolean and boolean are different. Always use lowercase for primitive type names.

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Declaring and Assigning Variables

Declaration creates the variable. Assignment gives it a value. You can do both at once or separately:

Declaring and assigning variables

        // Declare only — value is unset
        int motorPort;

        // Assign a value
        motorPort = 1;

        // Assign a new value later
        motorPort = 2;

FRC Example

FRC variable declarations

        double driveSpeed = 0.5;          // half speed
        boolean isShooterRunning = false;
        int leftMotorPort = 1;

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Constants and final

If a variable should never change after it is first set, declare it with final. This prevents accidental reassignment and makes your intent clear.

Local final variable

        final int LEFT_MOTOR_PORT = 1;

In FRC code you will often see public static final together:

Class constant (public static final)

    public static final int LEFT_LEADER_ID = 1;

• public — any class can read this value
• static — shared across all uses of the class; accessible without creating an object first
• final — the value cannot be changed after assignment

Naming convention

Java constants are written in ALL_CAPS_WITH_UNDERSCORES. Regular variables use lowerCamelCase.

Naming conventions

    public static final int MAX_SPEED = 100;   // constant
        double currentSpeed = 0.0;  // variable

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Scope

Where you declare a variable determines where it can be used — this is called scope.

• Field (class-level variable) — declared outside any method, available to all methods in the class. Fields are covered in depth in Java Classes.
• Local variable — declared inside a method, only exists while that method is running

Drivetrain.java

public class Drivetrain extends SubsystemBase {

    private double topSpeed = 1.0;  // field — available in all methods

    public void setSpeed(double speed) {
        double adjusted = speed * topSpeed;  // local — only exists in this method
        motor.set(adjusted);
    }
}

Tip

Declare hardware objects (motors, sensors) as fields so every method in the class can reach them. Use local variables for temporary calculations inside a single method.

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Operators

Arithmetic

Operator	Meaning	Example
+	Add	score + 1
-	Subtract	distance - offset
*	Multiply	speed * 0.5
/	Divide	ticks / 2048.0
%	Remainder (leftover after division)	7 % 2 → 1

Comparison (result is always boolean)

Operator	Meaning
==	Equal to
!=	Not equal to
<	Less than
>	Greater than
<=	Less than or equal
>=	Greater than or equal

Logical (combine boolean values)

Operator	Meaning	Example
&&	AND — both must be true	isRunning && !isStopped
\|\|	OR — at least one must be true	buttonA \|\| buttonB
!	NOT — flips true to false and vice versa	!isFinished

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Comments

Comments are ignored by the compiler and are used to explain code to other programmers (including your future self).

Code comments

    // Single line comment

    /*
       Multi-line comment —
       spans multiple lines
    */

    /**
     * Doc comment — appears when you hover over this item in VSCode.
     * Supports HTML formatting.
     */

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Naming Conventions

Consistent naming makes code readable across a team.

What	Convention	Example
Variables and methods	lowerCamelCase	driveSpeed, setMotorOutput
Classes	UpperCamelCase	Drivetrain, ShooterSubsystem
Constants	ALL_CAPS_UNDERSCORES	MAX_SPEED, LEFT_LEADER_ID
Packages	all lowercase	frc.robot.subsystems

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Knowledge Check

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#

Which data type stores whole numbers without a decimal point?

int

double

boolean

String

int holds integers like 1, 42, or -7. Use double when you need decimal precision (e.g., 0.75).

#

What naming convention should Java constants follow?

lowerCamelCase

UpperCamelCase

ALL_CAPS_UNDERSCORES

kebab-case

Constants use ALL_CAPS_WITH_UNDERSCORES to visually distinguish them from regular variables. Example: public static final int MAX_SPEED = 100;

#

A variable declared inside a method is called a:

Field

Constant

Parameter

Local variable

Local variables exist only while the method runs. Fields are declared outside methods and are available to all methods in the class.

#

Which arithmetic operator gives the remainder after division?

/

*

-

%

% is the modulo operator. For example, 7 % 3 equals 1 because 3 goes into 7 twice with 1 left over.

#

Which is the correct way to declare a motor port constant that any class can read and that can never change?

int LEFT_MOTOR_PORT = 1;

private final int leftMotorPort = 1;

public static final int LEFT_MOTOR_PORT = 1;

public int LEFT_MOTOR_PORT = 1;

public makes it readable by other classes, static means it belongs to the class itself (no object needed), and final prevents reassignment. Together they form the standard Java constant pattern.

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