Classes and Objects

Classes and objects help us keep related information and methods together.

Classes Are Blueprints

A class is like a blueprint. A blueprint describes how to build something, but it is not the finished object.

Here is a class that describes a simple robot motor:

public class RobotMotor {
  private double speed;

  public void setSpeed(double newSpeed) {
    speed = newSpeed;
  }

  public void stop() {
    speed = 0.0;
  }
}

This class has:

• A variable named speed that remembers the motor's speed.
• A method named setSpeed that changes the speed.
• A method named stop that sets the speed to zero.

A variable inside a class is called a field. Fields store information about an object.

Objects

An object is something created from a class. If a class is a blueprint for a house, an object is a house built from that blueprint.

We use new to create an object:

RobotMotor intakeMotor = new RobotMotor();

• RobotMotor is the type of object.
• intakeMotor is the name we give the object.
• new RobotMotor() creates it.

We use a dot (.) to call one of the object's methods:

intakeMotor.setSpeed(0.6);
intakeMotor.stop();

We can create more than one object from the same class:

RobotMotor leftMotor = new RobotMotor();
RobotMotor rightMotor = new RobotMotor();

leftMotor.setSpeed(0.5);
rightMotor.setSpeed(-0.5);

The two motors are separate objects, so each one can have a different speed.

Constructors

A constructor sets up an object when it is created. It has the same name as its class.

public class RobotMotor {
  private int deviceId;
  private double speed;

  public RobotMotor(int newDeviceId) {
    deviceId = newDeviceId;
    speed = 0.0;
  }

  public void setSpeed(double newSpeed) {
    speed = newSpeed;
  }
}

The constructor needs a device ID, so we provide one when we create each motor:

RobotMotor intakeMotor = new RobotMotor(10);
RobotMotor feederMotor = new RobotMotor(11);

The constructor does not use void or another return type.

Public and Private

public and private control where something can be used:

• public means other classes can use it.
• private means only its own class can use it.

Fields are usually private. Public methods give other code a safe way to use the object:

public class RobotMotor {
  private double speed;

  public void setSpeed(double newSpeed) {
    speed = newSpeed;
  }

  public double getSpeed() {
    return speed;
  }
}

Other code can call getSpeed() to read the private field:

RobotMotor motor = new RobotMotor();
motor.setSpeed(0.5);

System.out.println(motor.getSpeed());

tip

For now, make fields private and make the methods that other code needs public.

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A class is a blueprint, and an object is something created from that blueprint. Next, we will learn how to organize groups of values.