问题
As part of my learning i think the best answer(with meaning) for definition of abstraction that i found is from stackoverflow:
What is abstraction?
Besides that,
As part of current online course cs61B Fall 2006, Berkeley, i learnt the similar below definition of ADT close to above definition but added an extra word 'invariant'. Shall i consider this word as corollary to the above definition? or Is this word part of the definition?
An _Abstract_Data_Type_ (ADT) is a class that has a well-defined interface, but its implementation details are firmly hidden from other classes. That way, you can change the implementation of a class without jeopardizing the programs that depend on it. The Date class is an ADT.
ADT allows you to enforce invariants.
An invariant is a fact about a data structure that is always true (assuming the code is bug-free), no matter what methods are called by external classes. For example, the Date ADT enforces the invariant that a Date object always represents a valid date. An invariant is enforced by allowing access to certain fields only through method calls.
回答1:
One of my favourite definitions of abstraction is Robert C. Martin's, from the book Agile Principles, Patterns, and Practices:
Abstraction is the elimination of the irrelevant and the amplification of the essential.
Based on that definition, an abstraction can take many forms, on many different levels:
- It can be an entire system
- It can be a subsystem
- It can be a definition of a message format (akin to a protocol)
- It can be a function (as in Functional Programming)
- It can be an abstract base class
- It can be an interface
- It can be a concrete class
- etc.
In Object-Oriented Software Construction, Bertrand Meyer describes classes as Abstract Data Types. The idea in this book is that the description of an Abstract Data Type includes pre- and post-conditions. In Eiffel (the language used in the book), these are called assertions, but we also know them as invariants.
Meyer's view on Object-Oriented Design includes invariants. On a detailed (class/object/function) level, I agree that it makes sense to view invariants as a way to describe an abstraction.
On the other hand, if your abstraction is a description of a high-level architecture, those kind of invariants doesn't make sense. Still, even at a high level, it would often be valuable to define pre- and post-condition (e.g. "System A must be responding in order for System B to work correctly"), so it does makes sense to think about invariants in relation to abstractions.
来源：https://stackoverflow.com/questions/22186528/is-invariant-property-part-of-the-definition-of-abstraction