What is class in java definition

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 an instance of the class of objects known as bicycles. A class is the blueprint from which individual objects are created.

The following Bicycle 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 fields cadence , speed , and gear represent the object’s state, and the methods ( changeCadence , changeGear , speedUp etc.) define its interaction with the outside world.

You may have noticed that the Bicycle class does not contain a main method. That’s because it’s not a complete application; it’s just the blueprint for bicycles that might be used in an application. The responsibility of creating and using new Bicycle objects belongs to some other class in your application.

Here’s a BicycleDemo class that creates two separate Bicycle objects and invokes their methods:

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

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

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Java Classes and Objects

Everything in Java is associated with classes and objects, along with its attributes and methods. For example: in real life, a car is an object. The car has attributes, such as weight and color, and methods, such as drive and brake.

A Class is like an object constructor, or a «blueprint» for creating objects.

Create a Class

To create a class, use the keyword class :

Main.java

Create a class named » Main » with a variable x:

Remember from the Java Syntax chapter that a class should always start with an uppercase first letter, and that the name of the java file should match the class name.

Create an Object

In Java, an object is created from a class. We have already created the class named Main , so now we can use this to create objects.

To create an object of Main , specify the class name, followed by the object name, and use the keyword new :

Example

Create an object called » myObj » and print the value of x:

Multiple Objects

You can create multiple objects of one class:

Example

Create two objects of Main :

Using Multiple Classes

You can also create an object of a class and access it in another class. This is often used for better organization of classes (one class has all the attributes and methods, while the other class holds the main() method (code to be executed)).

Remember that the name of the java file should match the class name. In this example, we have created two files in the same directory/folder:

Main.java

Second.java

When both files have been compiled:

You will learn much more about classes and objects in the next chapters.

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What is class in java definition

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