Java – Class and Objects with Example

Java is an Object-Oriented Language. And classes and objects are the fundamental components of OOP’s.

Objects are real-life entity and classes are the blueprint for creating an object. For example: consider a class for a car, it has color, some weight, size, etc, these are its attributes and drive, gear, brake, etc are its behaviour.

CLASS

A class is a user-defined blueprint or prototype for creating Objects. It is a logical entity. We can say that classes are categories, and objects are items within each of those categories.

A class in Java contain:

• Methods.
• Fields.
• Constructors.
• Interfaces.

For the declaration of a class following components are included:

1. Modifiers:
A class can be public or default access.

2. Class name:
A class should have a name with an initial letter Capitalized.

3. SuperClass(if any)/ Interfaces:
If required to include SuperClass then proceed by the keyword ‘extends’. Using the keyword implements, a class can have more than one interface.

4. Body:
A class boundary is within the curly braces'{}’.

Syntax for class:

class ClassName
{
  // field;  
  //method;
}

Syntax example:

public class Dog
 {
  //variables
  int age;
  String color;

   //Methods
  void eat()
  {
        
  }
  void bark()
  {
    
  }
}

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OBJECTS:

It can be defined as real-world entities that have state and behavior. An object is a runtime entity. Eg. bike, Lamp, pen, table, etc. An Object is an instance of a class. It is a physical as well as a logical entity.

Objects has three characteristics:

• State: It is represented by attributes of an object that is the data value.
• Behaviour: It represents the functionality that is the behaviour of an object. Example withdraw, deposit, etc.
• Identity: It is a unique ID given to each Object created so that JVM can identify all the objects uniquely.

Example:
Name of an Object: dog
Identity: Name of the dog.
State: age, color, breed, size, etc.
Behaviour: eat, sleep, bark, run, etc.

Creating an Object:

As mentioned above that class is a blueprint for objects. So, Object is created from a class. And ‘new’ Keyword is used to create an Object.Three steps to create Object:

• Declaration: Declaration of a variable with the variable name with an object type.
• Instantiation: When an object of a class is created, it is said to be instantiated. And the ‘new‘ keyword is used to create an object.
• Initialization: The ‘new’ operator allocates memory for a new object that is being instantiated. Also invokes the class constructor.

Syntax of Object:

ClassName ReferenceVariableName = new ClassName(); 

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Following Example shows the creation of an Object:

 public class Dog 
 {
    //constructor having  one parameter 'name'
    public Dog(String dogName) 
    {
      System.out.println("My Dog Name is :" + dogName );
    }

    public static void main(String []args) {
      // Creating an Object
      Dog myDog = new Dog( "Bruno" );
    }
 }

Output:

 My Dog Name is :Bruno

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Creating Multiple Objects:

Multiple Objects can be created for the same class.

The following example shows that Objects are created for One class, invoking different variables.

class MyClass 
 {
  int x = 10;
  int y = 20;
     
 }
  public class Main
  {

    public static void main(String[] args) 
    {
     MyClass Obj1 = new MyClass();
     MyClass Obj2 = new MyClass();
     System.out.println("Value of x:" +Obj1.x);
     System.out.println("Value of y:" +Obj2.y);
    }
 }

Output:

 Value of x:10
 Value of y:20

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Example of class and objects in Java:

public class Dog 
 {
   // Instance Variables 
   String name;
   int age; 
   String color; 

  // Constructor Declaration of Class 
 public Dog(String name,int age, String color) 
 { 
   this.name = name;
   this.age = age;
   this.color = color;
 } 
 public void display()
 {
        
    System.out.println("Hi my name is "+ name); 
    System.out.println("My age is "+ age);
    System.out.println("My body color is "+ color);
 }


 public static void main(String[] args) 
 { 
    Dog myDog = new Dog("Bruno",1,"white"); 
    myDog.display();
 } 
 }

Output:

 Hi my name is Bruno
 My age is 1
 My body color is white

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