Differentiate Abstract class and Interface with suitable examples. What is a collection in JAVA? Difference HashMap and Hashtable with suitable examples. in Programming in Java

Example 1: abstract class vs interface

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.

Example 2: class vs interface

A class can be instantiated by 
creating its objects.
An interface is
never instantiated as the methods 
declared inside an interface are abstract
and does not perform any action, 
  so there is no use of instantiating any
interface.
A class is declared using a keyword class. 
In the same way, an interface is 
created using a keyword interface.
The members of a class can have 
access modifier like public, private, protected.
But the members of an interface
are always public as they have to be accessed 
by the classes implementing them.
The methods inside a class are 
  defined to perform an action on the fields
declared in the class. 
The methods inside an interface are purely abstract.
A class can implement any number 
of interfaces but can extend only one 
super class.
An interface can extend any number
of interfaces but cannot implement any
interface.
A class has constructors defined inside
it to get the variable initialized.
But, an interface does not have any
constructors as there are no fields 
to be initialized. The fields of an
interface are initialized 
at the time of their declaration only.