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INTRODUCTION TO CLASS IN JAVA



Class
            Class is a collection of data members and member functions.
Data members
            Data members are nothing but simply variables that we declare inside the class so it called data member of that particular class.
Member functions
                        Member functions are the function or you can say methods which we declare inside the class so it called member function of that particular class. The most important thing to understand about a class is that it defines a new data type. Once defined, this new type can be used to create objects of that type. Thus, a class is a template for an object, and an object is an instance of a class. Because an object is an instance of a class, you will often see the two words object and instance used interchangeably.
Syntax of class:
class classname
{
     type instance-variable1;
     type instance-variable2;
     //....
     type instance-variableN;

     type methodname1(parameter-list)
     {
        // body of method
     }

     type methodname2(parameter-list)
     {
        // body of method
     }
     // ...
     type methodnameN(parameter-list)
     {
       // body of method
     }
}
When we define a class, you declare its exact form and nature. We do this by specifying the data that it contains and the code that operates on that data. The data, or variables, defined within a class are called instance variables. The code is contained within methods.
NOTE : C++ programmers will notice that the class declaration and the implementation of the methods are stored in the same place and not defined separately.


Syntax:
accessing data member of the class: objectname.datamember name;
accessing methods of the class: objectname.method name();
So for accessing data of the class: we have to use (.) dot operator.
NOTE: we can use or access data of any particular class without using (.) dot operator from inside that particular class only.

How to declare object of class in java?

Syntax of object:
classname objectname;         \\ declaration of object.
objectname = new classname();      \\ allocate memory to object (define object).
or we can directly define object like this
classname objectname = new classname();
 

Assigning Object Reference Variables :
Suppose 

Box b1 = new Box();
Box b2 = b1;
 
Here b1 is the object of class Box. And we assign b1 to b2 by b2=b1.

Here we did not use new keyword for b2 so b1 and b2 will both refer to the same object. The assignment of b1 to b2 did not allocate any memory or copy any part of the original object. It simply makes b2 refer to the same object as does b1.Thus, any changes made to the object through b2 will affect the object to which b1 is referring, since they are the same object.
NOTE: When you assign one object reference variable to another object reference variable, you are not creating a copy of the object, you are only making a copy of the reference.

 Anurag

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