Java-Abstract keyword

abstract
--------
An abstract keyword is declared with

i)methods.
ii)classes.

abstract method
---------------
A method declared using the keyword abstract is known as abstract method.

An abstract method doesn't have any body.

we should terminate an abstract method using semicolon;

syntax
------
       abstract accessmodifier returntype methodname();

abstract classs
---------------
   A class declared using the keyword abstract is known as abstract class.

we should an abstract method in an abstract class,we can't declare an abstract method in a concrete class.

we can declare an abstract as well as concerete methods in an abstract class.

It is not compulsory to declare an abstract method in an abstract class.

we cannot create object for abstract class.

syntax
------
 accessmodifier abstract class ClassName
 {
   //
 }

  ex:
  abstract class A
  {
     public abstract void m1();
     public abstract void m2();

     public abstract void m3()
     {

     }
  }


abstract methods are used as a method contract in a project.

An abstract is used if we want partial implementation of a class.

we implement abstract methods of  an abstract in a subclass.

If a subclass extends abstract class then the subclass must implement all the abstract methods of abstract class.

Program
-------
abstract class A
{
    abstract public void m2();
    abstract public void m3();
   
    public void m1()
    {
      System.out.println("m1");
    }
 


    }
  class B extends A{

     public void m2()
     {
      System.out.println("m2");
     }
     
      public void m3()
      {
        System.out.println("m3");
      }
      public static void main(String args[])
      {
        B b=new B();
        b.m1();
        b.m2();
        b.m3();
      }
  }

If a subclass extends abstract class then the subclass must implement all the abstract methods of abstract class otherwise the subclass also must be declared as abstract because the abstract method of a super class is inherited as abstract method only,but we know a concrete class doesn't allow abstract methods so we should mark the subclass as abstract.

Program
-------
abstract class A
{
   public abstract void m1();
   public abstract void m2();
   public abstract void m3();
   public void m4()
   {
     System.out.println("m4");
   }
}
abstract class B extends A
{

   public void m1()
   {
     System.out.println("m1");
   }
   public void m2()
   {
     System.out.println("m2");
   }
 

}
class C extends B
{
public void m3()
{
  System.out.println("m3");
}
public static void main(String args[])
   {
      C c=new C();
      c.m1();
      c.m2();
      c.m3();
      c.m4();
   
   }
}
 
In the above program we are declaring 4 methods m1,m2,m3,m4 in class A

m1,m2,m3-->abstract
m4-->concrete method

2--we are extending A to B
3--implementing methods of A in B,but we are not overriding all the abstract methods in subclass.
4--Marked subclass B as abstract,we if don't implement all the abstract methods in a subclass we should declare subclass also as abstract
5--Extending B -->C
6--Implementing m3()
7--Testing Program  using main() in C class.
8--Here C is a concrete subclass so we create an object for C class and invoke properties of C,as well as  its abstract super classes A,B.

Concrete method
---------------
A method with body is known as concrete method.

ex:
 void m1()
 {
 }

Concrete class
--------------
A class which consists of only  concrete methods is known as concrete class.


ex:
 class A
 {
 void m2()
 {
 }
 void m3()
 {
 }
 void m1()
 {
 }
}


abstract keyword is declared with ----and -----.
A concrete class allows abstract methods
true
false
An abstract class consists of both ---and ---methods.
we can create object for abstract class
Abtract methods of super class are implemented in a -------.
we should override --------methods of abstract super class in a subclass otherwise the subclass also must be declared as--------.
we should use abstract class if we want -------implementation.
An abstract method is a method---------body.