What is the use of Abstract class

Contract-first design and development is not something new. It was formally introduced by Bertrand Meyer as a part of his Eiffel Programming Language design and has appeared in various technical publications since 19861,2. Therefore, understanding contracts from an old-school tool/technology perspective might be helpful to grasp why it it's useful.

oops concepts with examples

Static classes and class members are used to create data and functions that can be accessed without creating an instance of the class. Static class members can be used to separate data and behavior that is independent of any object identity: the data and functions do not change regardless of what happens to the object. Static classes can be used when there is no data or behavior in the class that depends on object identity