abstract classes and interfaces in java

                                    abstract class have no implementation of methods functions inside it. the classes which extending abstract class have to implement it
  

                                    Interfaces specify what a class must do and not how. 
It is the blueprint of the class.
It is used to achieve total abstraction. 

We are using implements keyword for interface.

Abstract=
Sometimes we may come across a situation
where we cannot provide implementation to
all the methods in a class. We want to leave the 
implementation to a class that extends it.
  In that case we declare a class
as abstract by using abstract keyword on method
signature.In my framework I have created my
PageBase class as super
class of the all page classes. 
I have collected all common elements
and functions into PageBase class and
all other page classes extent PageBase class.
By doing so, I don't have to locate very
common WebElements and it provides
reusability in my framework.
Also
1)Abstract classes cannot be instantiated
2)An abstarct classes contains abstract method,
concrete methods or both.
3)Any class which extends abstarct class must
  override all methods of abstract class
4)An abstarct class can contain either
  0 or more abstract method.  

                                    // abstract class
abstract class Shape
{
   // abstract method
   abstract void sides();
}
class Triangle extends Shape
{
   void sides()
   {
      System.out.println("Triangle shape has three sides.");
   }
}
class Pentagon extends Shape
{
   void sides()
   {
      System.out.println("Pentagon shape has five sides.");
   }
   public static void main(String[] args)
   {
      Triangle obj1 = new Triangle();
      obj1.sides();
      Pentagon obj2 = new Pentagon();
      obj2.sides();
   }
}

                                    An abstract method is the method which does’nt have any body. 
Abstract method is declared with
keyword abstract and semicolon in place of method body.

  public abstract void <method name>();
Ex : public abstract void getDetails();
It is the responsibility of subclass to provide implementation to 
abstract method defined in abstract class

                                    Interface 
1) Interface contains only abstract methods 
2) Access Specifiers for methods in interface
must be public
3) Variables defined must be public , static ,
final
4) Multiple Inheritance in java is implemented
using interface
5) To implement an interface we use
implements keyword

Abstract Class
1) Abstract class can contain abstract methods,
concrete methods or both
2) Except private we can have any access
specifier for methods in abstract class.
3) Except private variables can have any access
specifiers
4)We cannot achieve multiple inheritance using
abstract class.
5)To implement an interface we use implements
keyword