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61. 'class' concept






The class is the logical construct upon which the entire Java language is built because it defines the shape and nature of the object. Any concept you wish to implement in a Java program must be encapsulated within a class.

Perhaps 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 the class is a template for an object, and an object is an instance of a class.

On a high level, a Class consists of Variables and Methods. Variables store data and Methods will contian the code to use the data.

'class' syntax                                                                                                                                                                                                                                                          

  class classname
  {
     
      type instance-variable1;
      type instance-variable2;
      -
      -
      type instance-variableN;


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

       type methodname2(parameter-list)
      {
         //body of the method
       }


       -
       -

       type methodnameN(parameter-list)

      {
         //body of the method
       }

}


Instance Variables -  The data or variables defined in the class are called Instance variables

Example  

int i;   (Here we've defined the instance variable 'i' as int)

Methods - The code is contained inside the methods.

Example 

public static void main(String args[])
{
     i=0;
    
     if(i==1)
       System.out.println("The value of i is 1");
     else 
       System.out.println("The value of i is "+i);
 }

i.e. The statements inside the method 'main( )' is nothing but code


Collectively, the variables and methods defined within a class are called members of the class.

Simple example to demonstrate the 'class'

   class Box
   {
        double width;
        double height;
        double  depth;
 
        public static void main(String args[])
        {
             double volume;
              volume = width*height*depth;
              System.out.println("The volume of the Box is "+volume);
         }  
   
    }


Going forward lets work on multiple classes to understand how classes work in real. In order to do this we've to stop executing our programs in TextPad editor and switch to Eclipse IDE





Please comment below to feedback or ask questions.

How to install Eclipse IDE will be explained in next post.




2 comments:

jenny howard said...

Hi,

class Box
{
double width;
double height;
double depth;

public static void main(String args[])
{
double volume;
volume = width*height*depth;
System.out.println("The volume of the Box is "+volume);
}

}

This program stated by yu it contains some members declared outside the method scope so so how they are acessible by method?

Arun Motoori said...

There are two types of variables.
1. Local Variables - Variables declared inside the methods (These variables can only be accessible inside the method where they are declared)
2. Global Variables - Variables declared inside the Class but outside the methods (The variables can be accessible by all the methods in the Class)

Global variables are used when we need to use the same variable in different methods.