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Classes and Objects

Object-Oriented Programming (OOP) is a way of organizing code around objects, bundles of data and behavior that model real things. Almost everything you've written so far in Java has secretly been part of a class, so let's finally unpack what that actually means.

Classes: The Blueprint​

A class is a blueprint that describes what an object will look like and how it will behave. A class by itself doesn't take up memory for actual data, it just defines the structure: what fields (data) and methods (behavior) any object built from it will have.

cookie-cutter-class

Think of a class like a cookie cutter. The cutter itself isn't a cookie, it just defines the shape. You can use the same cutter to stamp out as many cookies as you want, and each cookie is separate from the others, even though they all share the same shape.

Class Formula​

class [ClassName] {
    [fields]
    [methods]
}
class Flywheel {
    double voltage;

    void spin() {
        System.out.println("Spinning at " + voltage + " volts");
    }
}

Here, Flywheel is the class (the cookie cutter). It defines a field, voltage, and a method, spin(), but on its own, it isn't an actual flywheel you can use yet.

Objects: The Instance​

An object is an actual instance of a class, one of the "cookies" stamped out by the cutter. Each object gets its own copy of the fields defined by its class, so changing one object's data doesn't affect any other object made from the same class.

The new Keyword​

Objects are created using the new keyword, which tells Java to allocate memory for a new instance of a class.

Flywheel shooterWheel = new Flywheel();
shooterWheel.voltage = 12.0;
shooterWheel.spin();

Output:

Spinning at 12.0 volts

You can create as many independent objects from the same class as you want:

Flywheel shooterWheel = new Flywheel();
Flywheel intakeWheel = new Flywheel();

shooterWheel.voltage = 12.0;
intakeWheel.voltage = 6.0;

System.out.println(shooterWheel.voltage);
System.out.println(intakeWheel.voltage);

Output:

12.0
6.0

Even though shooterWheel and intakeWheel were both built from the Flywheel class, they're independent objects, each with their own voltage field.

Everything is (Kind of) an Object​

Remember from the Variables section that String is a reference type, since it's technically an object of the String class? This is true for nearly everything in Java that isn't a primitive data type. When you write:

String frcYear = "2026";

You're really creating an object of the String class, which happens to have its own fields (the characters it stores) and methods (like .length() or .equals()) defined for you already, by the creators of Java itself. The only difference is that Java gives String objects a shorthand syntax (string literals) so you don't have to type new String("2026") every time.

For more information about defining your own methods and how objects are created step by step, see the Constructors section.