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What Is a Class?

In the real world, you'll often find many individual objects all of the same kind. There may be thousands of other bicycles in existence, all of the same make and model. Each bicycle was built from the same set of blueprints and therefore contains the same components. In object-oriented terms, we say that your bicycle is aninstance of theclass of objects known as bicycles. Aclass is the blueprint from which individual objects are created.

The followingBicycle class is one possible implementation of a bicycle:

class Bicycle {    int cadence = 0;    int speed = 0;    int gear = 1;    void changeCadence(int newValue) {         cadence = newValue;    }    void changeGear(int newValue) {         gear = newValue;    }    void speedUp(int increment) {         speed = speed + increment;       }    void applyBrakes(int decrement) {         speed = speed - decrement;    }    void printStates() {         System.out.println("cadence:" +             cadence + " speed:" +              speed + " gear:" + gear);    }}

The syntax of the Java programming language will look new to you, but the design of this class is based on the previous discussion of bicycle objects. The fieldscadence,speed, andgear represent the object's state, and the methods (changeCadence,changeGear,speedUp etc.) define its interaction with the outside world.

You may have noticed that theBicycle class does not contain amain method. That's because it's not a complete application; it's just the blueprint for bicycles that might beused in an application. The responsibility of creating and using newBicycle objects belongs to some other class in your application.

Here's aBicycleDemo class that creates two separateBicycle objects and invokes their methods:

class BicycleDemo {    public static void main(String[] args) {        // Create two different         // Bicycle objects        Bicycle bike1 = new Bicycle();        Bicycle bike2 = new Bicycle();        // Invoke methods on         // those objects        bike1.changeCadence(50);        bike1.speedUp(10);        bike1.changeGear(2);        bike1.printStates();        bike2.changeCadence(50);        bike2.speedUp(10);        bike2.changeGear(2);        bike2.changeCadence(40);        bike2.speedUp(10);        bike2.changeGear(3);        bike2.printStates();    }}

The output of this test prints the ending pedal cadence, speed, and gear for the two bicycles:

cadence:50 speed:10 gear:2cadence:40 speed:20 gear:3