Java Class and Objects

Java Class and Objects

Java class objects: In this tutorial, you will find out about the concept of classes and objects in Java with the help of examples.

Java is an object-oriented programming language. The central concept of the object-oriented methodology is to break complex issues into smaller objects.

An object is any substance that has a state and behavior. For instance, a bike is an object. It has

States: inactive, first apparatus, and so on

Behaviors: slowing down, quickening, and so forth

Before we find out about objects, we should initially think about classes in Java.


Java Class

A class is an outline for the object. Before we make an object, we first need to characterize the class.

We can think about the class as a sketch (prototype) of a house. It contains all the insights regarding the floors, entryways, windows, and so forth In light of these depictions, we manufacture the house. House is the object.

Since numerous houses can be produced using a similar depiction, we can make numerous objects from a class.


Create a class in Java

We can create a class in Java using the class keyword. For example,

class ClassName {
  // fields
  // methods
}

Here, fields (variables) and methods represent the state and behavior of the object respectively.

fields are used to store data
methods are used to perform some operations
For our bicycle object, we can create the class as

class Bicycle {

  // state or field
  private int gear = 5;

  // behavior or method
  public void braking() {
    System.out.println("Working of Braking");
  }
}

In the above example, we have created a class named Bicycle. It contains a field named gear and a method named braking().

Here, Bicycle is a prototype. Now, we can create any number of bicycles using the prototype. And, all the bicycles will share the fields and methods of the prototype.


Java Objects

An object is called an instance of a class. For example, suppose a Bicycle is a class then MountainBicycle, SportsBicycle, TouringBicycle, etc can be considered as objects of the class.

Creating an Object in Java

Here is how we can create an object of a class.

className object = new className();

// for Bicycle class
Bicycle sportsBicycle = new Bicycle();

Bicycles touringBicycle = new Bicycle();

We have used the new catchphrase alongside the constructor of the class to make an object. Constructors are like methods and have a similar name as the class. For instance, Bicycle() is the constructor of the Bicycle class.

Here, sportsBicycle and touringBicycle are the names of objects. We can use them to access fields and methods of the class.

As should be obvious, we have made two objects for the class. We can make various objects of a single class in Java.

Note: Fields and methods of a class are likewise called members of the class.


Access Members of a Class

We can use the name of objects alongside the . operator to get to members from a class. For instance,

class Bicycle {

  // field of class
  int gear = 5;

  // method of class
  void braking() {
    ...
  }
}

// create object
Bicycle sportsBicycle = new Bicycle();

// access field and method
sportsBicycle.gear;
sportsBicycle.braking();

In the above example, we have created a class named Bicycle. It includes a field named gear and a method named braking(). Notice the statement,

Bicycle sportsBicycle = new Bicycle();

Here, we have made an object of Bicycle named sportsBicycle. We at that point use the object to get to the field and method for the class.

sportsBicycle.gear – access the field gear
sportsBicycle.braking() – access the method braking()

We have mentioned the word method quite a few times. You will learn about Java methods in detail in the next chapter.

Now that we understand what is class and object. Let’s see a fully working example.


Example: Java Class and Objects

class Lamp {
  
  // stores the value for light
  // true if light is on
  // false if light is off
  boolean isOn;

  // method to turn on the light
  void turnOn() {
    isOn = true;
    System.out.println("Light on? " + isOn);

  }

  // method to turnoff the light
  void turnOff() {
    isOn = false;
    System.out.println("Light on? " + isOn);
  }
}

class Main {
  public static void main(String[] args) {

    // create objects led and halogen
    Lamp led = new Lamp();
    Lamp halogen = new Lamp();

    // turn on the light by
    // calling method turnOn()
    led.turnOn();

    // turn off the light by
    // calling method turnOff()
    halogen.turnOff();
  }
}

Output

Light on? true
Light on? false

In the above program, we have created a class named Lamp. It contains a variable: isOn and two methods: turnOn() and turnOff().

Inside the Main class, we have created two objects: led and halogen of the Lamp class. We then used the objects to call the methods of the class.

led.turnOn() – It sets the isOn variable to true and prints the output.
halogen.turnOff() – It sets the isOn variable to false and prints the output.
The variable isOn defined inside the class is also called an instance variable. It is because when we create an object of the class, it is called an instance of the class. And, each instance will have its own copy of the variable.

That is, led and halogen objects will have their own copy of the isOn variable.


Example: Create objects inside a similar class

Note that in the past example, we have made objects inside another class and got to the members from that class.

In any case, we can likewise make objects inside a similar class.

class Lamp {
  
  // stores the value for light
  // true if light is on
  // false if light is off
  boolean isOn;

  // method to turn on the light
  void turnOn() {
    isOn = true;
    System.out.println("Light on? " + isOn);

  }

  public static void main(String[] args) {
    
    // create an object of Lamp
    Lamp led = new Lamp();

    // access method using object
    led.turnOn();
  }
}

Output

Light on? true

Here, we are creating the object inside the main() method of the same class.


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