In this page we will all Class and Object concepts.

  • Object in Java

    Object is a instance of class, object has state and behaviors.
    An Object in java has three characteristics:

    1) State
    2) Behavior
    3) Identity

    • State/Property/data: Represents data (value) of an object.

    • Behavior/Methods: Represents the behavior (functionality) of an object such as deposit, withdraw etc.

    • Identity: Object identity is typically implemented via a unique ID. The value of the ID is not visible to the external user. But,it is used internally by the JVM to identify each object uniquely.

  • Class in java

    Class is a blueprint of an object which have certain properties and behavior. Class is template defined so that we can create the object of this class type.

    Class is a logical entity.

    No memory is allocated for class.

    A class in java contain:

    • Data Member/Data /fields

    • Member Methods

    • Constructors

    • Blocks

    • Nested Classes and Interfaces


    Syntax to declare class

    
    class Class_Name
    {
    //body of class
    }

    class keyword is used to create class in java.

  • Real life example of object and class

    In real world many examples of object and class like dog, cat, and cow are belong to animal's class. Each object has state and behaviors. For example a dog has state:- color, name, height, age as well as behaviors:- barking, eating, and sleeping.

    Real life example of object and class
  • Simple Example of Creating Class and Object

    In this example, we are going to create Student class that have two data members id and name. We are creating the object of the Student class by new keyword and printing the object values


    Create Student Class.

    
    class Student {
        int id;
        String name;
    }


    Create Class which contain main method so that we can create the object of Student class.

    
    class Example {
        public static void main(String[] args) {
            //create the object of student class
            Student s = new Student();
            //assign values for s
            s.id = 34;
            s.name = "durgesh";
            //now printing the values of object
            System.out.println("ID :" + s.id);
            System.out.println("Name :" + s.name);
        }
    
    }

    Output:
    ID :34
    Name :durgesh

    new keyword is used to create object of any class.

  • new keyword

    new is a keyword which is used to dynamically allocate the memory for an objects at run time .Memory for objects are allocated at run time in Heap Area of Ram.

  • Let us take another simple example to understant class and object.

    In this example we are going to create Person class which have name , id and address .


    Create Person class and declare name , id and address

    
    //creating student class
    class Person {
    
        String name;//instance member
        int id;//instance member
        String address;//instance member
    
    }


    Create another class which contain main() method.

    
    
    class Example {
    
        public static void main(String[] args) {
            //create the object of Person class
            Person p1 = new Person();
            //assign values for p1
            p1.name = "John";
            p1.id = 34343;
            p1.address = "USA";
            //now printing the values of object
            System.out.println("ID :" + p1.id);
            System.out.println("Name :" + p1.name);
            System.out.println("Address :" + p1.address);
            System.out.println("===============================");
            //creaet another object of Person class
            Person p2 = new Person();
            //assign values for p1
            p2.name = "Petter";
            p2.id = 3242;
            p2.address = "India";
            //now printing the values of object
            System.out.println("ID :" + p2.id);
            System.out.println("Name :" + p2.name);
            System.out.println("Address :" + p2.address);
    
        }
    
    }
    

    Output:
    ID :34343
    Name :John
    Address :USA
    ===============================
    ID :3242
    Name :Petter
    Address :India

    We can also create these two classes in a single file .
    Execute the class which contain main() method.

  • Understanding above example.

    In the above example we have created 2 person object . so each of two person object have separate copy of instance variable.

    Understanding above example.
  • Anonymous object

    Anonymous simply means nameless. An object which has no reference variable is known as anonymous object. It can be used at the time of object creation only.


    If you have to use an object only once, anonymous object is a good approach. For example:

    
    new Calculation();//anonymous objec


    Calling method using reference variable.

    
        Person c=new Person();  
        c.print();  


    Calling method through anonymous object

    
    new Person().print(5); 


    Example of Anonymous Object creation.

    
    class Person {
    
        public void print() {
            System.out.println("Hello....");
        }
        public static void main(String[] args) {
            new Person().print();//calling print method on anonymous object
        }
    }
    

    Output:
    Hello....

What next?

The next topic is Methods in Java

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