why we use abstract class code example

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: is it necessary for abstract class to have abstract method

No, abstract class can have zero abstract methods.

Example 3: when we should use abstract class

When to use an abstract class
An abstract class is a good choice if we are using the inheritance concept since it provides a common base class implementation to derived classes.
An abstract class is also good if we want to declare non-public members. In an interface, all methods must be public.
If we want to add new methods in the future, then an abstract class is a better choice. Because if we add new methods to an interface, then all of the classes that already implemented that interface will have to be changed to implement the new methods.
If we want to create multiple versions of our component, create an abstract class. Abstract classes provide a simple and easy way to version our components. By updating the base class, all inheriting classes are automatically updated with the change. Interfaces, on the other hand, cannot be changed once created. If a new version of an interface is required, we must create a whole new interface.
Abstract classes have the advantage of allowing better forward compatibility. Once clients use an interface, we cannot change it; if they use an abstract class, we can still add behavior without breaking the existing code.
If we want to provide common, implemented functionality among all implementations of our component, use an abstract class. Abstract classes allow us to partially implement our class, whereas interfaces contain no implementation for any members.

Example 4: can you declare an abstract method in a non abstract class

No. A normal class(non-abstract class) cannot have abstract methods.