What does "data abstraction" exactly mean?

Question

What does data abstraction refer to? Please provide real life examples alongwith.

Answer 1

Abstraction has two parts:

• Hide details that don't matter from a certain point of view
• Identify details that do matter from a certain point of view and consider items to be of the the same class if they possess those details.

For example, if I am designing a program to deal with inventory, I would like to be able to find out how many items of a certain type the system has in stock. From the perspective of the interface system, I don't care if I am getting this information from a database, a csv file, a remote repository via a SOAP interface or punch cards. I just care that I can can say widget.get_items_in_stock() and know that it will return an integer.

If I later decide that I want to record that number in some other way, the person designing the interface doesn't need to know, care or worry about it as long as widget still has the get_items_in_stock() method. Like wise, the interface doesn't need to care if I subclass the widget class and add a get_square_root_of_items_in_stock() method. I can pass an instance of the new class to it just as well.

So in this example, we've hidden the details of how the data is acquired and decided that anything with a get_items_in_stock() method is an instance of the same class (or a subclass thereof) for certain purposes.

Answer 2

Data abstraction is any device that allows you to treat data as humans encounter it rather than as it is stored on machine.

At the lowest level, all primitive data types are abstractions -- as programmers, we don't usually have to deal with data at the bit level (which is how it is ultimately stored) but as integers, floating point numbers, characters, etc.

We then add layers onto that abstraction -- maybe two integers represents a Point, or we and enumerations to represent the months of the year, days of the week, etc.

With each abstraction layer, we move further from the machine and (hopefully) closer to human understanding of the data. This can extract a performance penalty -- it may not always be the case that points can be most efficiently represented by two integers. This is compensated for by the shorter development (and maintenance) time when abstractions are used.