data hiding, encapsulation

Scenario: You go to an ATM machine, swipe in using your atm card, and ask for your bank balance.

Before we take this scenario apart, let’s add another word to our dictionary:

A type specification is a named collection of defined responsibilities. In object oriented terms, a class is a type specification.

Now let us analyze this scenario.
We will need an ATM object to represent our ATM machine. The objects type specification is a class that defines two responsibilities (in our scenario):

  1. login – takes an ATM card and a pin number, and throws an exception if they do not match
  2. getBalance – returns the balance in the account corresponding to the ATM card

Look at the login responsibility. It involves the following steps:

  1. Read the ATM card number
  2. Call a validating service passing the card number and pin number entered by the user
  3. Receive the result from the service – valid login or not
  4. If validity is confirmed, start an ATM session
  5. Return the result to the caller

These steps are fairly complex. But they do not concern the entity that invokes the login responsibity of the ATM object. The invoking entity (human user) merely has an ATM card number and pin number, and needs to start an ATM session.

It’s the responsibility of the ATM object to validate these numbers before starting a new session, by performing the 5 steps that I mentioned above. So we encapsulate these 5 steps into a responsibility called login, which we define in the type specification (i.e. class) of the ATM object. Any entity that needs to login to a new session on an ATM object, may now invoke the login responsibility of the ATM object, passing valid card and pin numbers.

So we can explain encapsulation as defining a responsibility.

Now, let’s examine the getBalance responsibility. It could go like this:

  1. Make a call to some service, passing in the ATM card number
  2. Receive the balance amount from the service
  3. Return this balance to the caller

getBalance is a responsibility all by itself. It’s also a good example of data hiding. Data hiding is a form of encapsulation in which we define access to data as a responsibility.

So all data hiding is encapsulation, but not all encapsulation is data hiding. Go figure (think this through for yourself, it’s quite simple).

The getBalance responsibility is an example of data hiding in which information can only be retrieved. But often we need to control how the data is stored as well. So let us add two responsibilities to the ATM type specification: deposit and withdraw. The deposit responsibility would involve incrementing the balance by some amount. The withdraw responsibility is more work: decrementing the balance by some amount unless the balance is already zero; in which case, no amount would be decremented since a negative balance is meaningless. We need not worry about the business logic involved when depositing or withdrawing some amount, as these responsibilities belong to the ATM object, as defined in it’s type specification.

What is the difference between data hiding and returning status messages? In our ATM example, both login and getBalance return data. How can we tell that getBalance involves data hiding and login does not? Simple. The job of the getBalance responsibility is to retrieve data. Being a responsibility defined for retrieving data, it is an example of data hiding. But the login responsibility involves starting an authenticated session on an ATM object. The data it returns is only to let the caller know whether the responsibility was fulfilled or not. Since it’s primary responsibility isn’t retrieval of data, the login responsibility is not an example of data hiding.

The main advantage of any form of encapsulation (including data hiding) is that an entity need not know how to perform a given task. It only needs to know whose responsibility it is to perform that task.

For example, to retrieve my account balance, I don’t need to know the steps to do it. I only need to know that the ATM object is responsible for fetching my balance. If the steps to fetch account balances are changed, only the ATM type specification is affected; it’s getBalance responsibility must be modified to reflect the new procedure. Since no one else knows how to get an account balance, no other type specifications are affected. Only a small part of the system had to changed, so the chances of something breaking unexpectedly in our software model is greatly reduced.

Lastly, I’m always open to criticism. The comments section is open to the house.

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14 responses to “data hiding, encapsulation

  1. A good attempt to make the distiction between Data Hiding and Encapsulatin clearer!
    I however have a question regarding data hiding i.e.,
    data hiding means that hide your implementation details from users, right? If so, aren’t the implementation details (the steps to be followed during login are not being exposed to users? If answer to this question is YES , tell me if it’s not an example of data hiding too (as well as of Encapsulation.)

  2. Hi Vikas,

    You said: “data hiding means that [you] hide your implementation details from users”

    That isn’t quite right. When we “hide implementation details”, we can say that we are “encapsulating” a set of steps as a “responsibility”, and we define this responsibility within a type specification. In object oriented terms, this means that we are placing a set of steps inside a method, and defining this method in a class.

    Data hiding is one kind of encapsulation. Data hiding is what happens when we define a responsibility whose primary objective is to retrieve data. The retrieval can take place by performing a set of steps, but data retrieval is the primary objective. Any example of data hiding is an example of encapsulation as well. But data hiding is just one kind of encapsulation, so not all instances of encapsulation involve data hiding.

    The primary purpose of the “login” responsibility in the example is not to retrieve data. It does involve data retrieval, which is performed as one of the required steps. But it’s primary purpose is to start an authenticated session. The value it returns is just a status value, which tells the user whether or not the session was started successfully.

  3. I need to know what are ATM object
    (Name the ATM object)please

    Thanks

  4. Hi shiva kumar,

    An ATM object is being used in this example to represent an ATM machine. ATM stands for Automated Teller Machine, as described in the link below:

    http://en.wikipedia.org/wiki/Automatic_teller_machine

  5. Hi !

    Nice information. I understood the difference between the 2 concepts very well. Thanks !

  6. the session is very informative. implementation of data hiding in oops for the same example is required to understand the advantage of oops concept completely.

    thank u.

  7. Got clear about the concept of encapsulation and data hiding. Really appreciable as providd good example.

  8. thanks
    bfor that i hv always in confusion but u clearlly wit very ggod example
    exclent

  9. Hi
    Thanks. I am always confusing about this two but now i am clear about it and this is because of you. So again Thank you.

  10. Though data hiding is reffered to encapsulation. But what is the difference between data hiding and data encapsulation?

  11. its really too good, example given is excellent and too informative.

  12. I’m totally confused with the OO-Paradigm “Abstraction”
    Grady Booch once said “The world would never had reached the current stature had abstraction not been in the foreplay”
    Plz someone help me with this concept

  13. very good example on encapsulation and datahiding

  14. kindly explain
    1. Encapsulation
    2. Data hiding
    3. abstraction

    in one example
    thanks
    ajay

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