define|use abstract|concrete class and interface in java with|of example

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  Classes in java:

                         Class is collection of associated data members and methods is known as Class. We know  that if we want to write any java program then it must starts with concept called class i.e; without classes concept there is no java program.

 Types Of classes in java:

 Concrete classes:

                   A Concrete class is one in which contains fully defined methods or implemented or concrete methods.Once the class is concrete we can create its object directly.

   Syntax:

    Class  <class-name> // fully defined class

    {

      Return-type method()

      {

       -----------------

      ------------------

      }

    };

  For Example:

     Class Demo

   {

       void method1()

     {

       System.out.println(“this is fully defined method of method1()”);

     }

      void method2()

    {

      System.out.println(“this is fully defined method of method2()”);

    }

      public  static void main(String args[])

    {

      Demo D=new Demo();// this  is Demo class object  “D” created directly because Demo is                  

                                        //Concrete class

       D.method1();

       D.method2();

     }

  }

Here Class Demo is containing two defined methods. Hence it is known as Concrete class, so that whose object is created directly.

Limitation of Concrete Class:

1)      In real industry concrete classes are highly recommended to use / to deal with specific which are  requirements  suitable to only one programmer.

Industry no recommended to use concrete classes to deal with common requirements which are suitable for all java programmers or universally.

2)      While dealing  common requirements  with concrete classes then we get limitations as

a)      More amount of memory space.

b)      Less performance.

Abstract Class:

    Abstract class comes into picture when one of the requirements are  not possible with concrete class.  Requirements are to deal with common requirements for all java programmers universally ,Concrete class does not support this requirement.

Definition of Abstract Method:

    An abstract method is one which contains only method declaration/prototype but not body.

In order to make any un defined method as abstract method, that declaration of undefined method must be preceded by a keyword “Abstract”.

Syntax for Abstract Method:

     abstract Return-type Method-Name(list of formal type if any);

For Example:

     abstract void demo_method();

Definition of Abstract Class:

      An  abstract class in which it contains some defined methods and some undefined methods / unimplemented methods / abstract methods.

   We know that every method of java belongs to class including abstract method. If a class contains abstract method then the class is known as abstract class.

 Programmatically In order to make a class as abstract ,whose definition must be preceded by a keyword called Abstract.

Point To Remember:

                An abstract class object cannot create directly but we can it indirectly.

Indirect creation of object.

 An object of abstract class is equal to an object of its sub class. Or       

 An object of abstract class is equal to an object of that class which extends that abstract class. Or

 An object of sub class of abstract class is nothing but an object of abstract class.

  Syntax of Abstract class:

    abstract class <class-name>

  {
      abstract return-type method-name(list of formal parameters if required);

  };

  For Example:

   abstract class Demo

 {

    abstract void demo-test1();

    abstract void demo-test2();

  };

    Class Derived-Demo

  {

     void demo-test1()

   {

      System.out.println(“this is defined method  of Derived-Demo class”);

    }

     void demo-test2()

   {

     System.out.println(“this defined method of Derived-Demo class”);

   }

   

      public static void main(String args[])

   {

      Demo D=new Demo();// INVALID

      Demo D=new Derived-Demo();// VALID and creation of object to abstract class is indirect.

      D.demo-test1();

      D.demo-test2();

    }

  }

     From the above program / example is abstract class such that we cannot create object this class 
demo directly.

  Demo  D=new Demo();// Invalid

  But we can create object of this abstract class indirectly.

  Demo D=new Derived-Demo();//valid and it is indirect creation of object.

  Points To Remember Regarding Abstract class:

            1)      Abstract class are always reusable because abstract classes contains common reusable features.

            2)      The definition of abstract class should not be final because abstract class must always 
                  participates in  inheritance process.

            3)      Abstract class concept of java makes use of polymorphism along with method 
                  overriding for business logic development and uses dynamic  binding concept for 
                  execution logic.

            4)      The advantages of abstract classes are

a)      Use less memory space for application

b)      More performance

In java programming an object of abstract can be declared but it cannot be referenced 
indirectly w.r.t concrete class.

           5)      An abstract class of java is not recommended to contain parameterized constructors.

                 But it default contains system defined default constructor.

           6)      An object of abstract class contains details about those features which are available
            in the same class but it never contains details about those features which are specially
            available in its derived class.

      Abstract Base Class And Abstract Derived Class:

            An abstract base class is one in which contains physical representation of abstract
  methods.An abstract derived class is one in which contains logical representation of abstract
  methods which are inherited from abstract base class.

    Both abstract base class and abstract derived class are reusable by the derived class.

For Example:

     abstract Class  Base

   {

      abstract void f1();

     abstract void f2();

  };

  Class Derived extends Base

{

      Void f1()   // f1() method is defined here but f2() still is abstract (not defined) but 

   {                     //there is exists of  f2() method logical .  

     System.out.println(“this is f1() method”);

  }

}