Object-Oriented Programming

Principles of OOP


  • Implementation hiding
  • Providing a minimal interface that offers all the behavior and attributes the intended user expects, without revealing the internal implementation details
  • Allows the implementation to vary without changes having to be made to the external usage of the class.
  • Allows the external usage of the class to vary without changes having to be made to the implementation of the class.
  • A design that violates abstraction is said to be intrusive.


  • Combining data and the functions that act on it, within a class.
  • Data is maintained by the class.
  • Data hiding
  • Data within the class is hidden from the rest of the program and is only accessible through methods of the class via the public interface - getters / setters; inspectors / mutators.
  • Information hiding; implementation details are kept hidden .


  • Heirarchical reuse of behavior and attributes between similar classes.
  • Deriving specialized versions of general classes.
  • Overriding some of the behavior of a class.
Weaknesses of inheritance
  • Inheritance violates abstraction.
  • Code reuse (via inheritance) is intrusive.
  • Polymorphism (via inheritance) is intrusive.
  • Using inheritance for code reuse requires knowledge of the implementation details of the parent class; the child class cannot be designed and maintained independently of the parent class.
  • Polymorphism is only possible for the child class if the parent class supports it; the parent class cannot be designed and maintained independently of the child class.
  • Multiple inheritance is especially problematic due to occasional naming overlaps between the methods or attributes of the parent classes.
Prefer delegation to inheritance
  • Delegation supports abstraction.
  • Code reuse (via delegation) is non-instrusive.
  • Polymorphism (via delegated interfaces) is non-intrusive.
  • Inheritance is falling out of favor, much as goto statements did in earlier decades.
  • An alternative to inheritance is for the child class to have an instance of the parent class as a data member and to access the behavior and attributes of the parent class through the data member (via the public interface of the parent class).
  • This changes the relationship between the child class and the parent class from "is-a" to "has-a", from generalization to composition.
  • The design approach for this, delegation, helps make composition as powerful for code reuse as inheritance.
  • Delegation is more flexible; you can delegate to a different object at run-time, whereas the inheritance of a class is fixed at compile-time.
  • Interfaces, combined with delegation, make it easier to avoid inheritance.
  • The child class implements the public interface of the parent class.
  • The relevant method calls are delegated to the parent object.
  • In this way, the instance of the child class can be used as if it were an instance of the parent class (which is one of the benefits of inheritance).
  • Multiple interfaces can be used in place of multiple inheritance.
  • Polymorphism is more natural with interface delegation than with inheritance; rather than instrusively declaring virtual methods in the parent class, as with inheritance, the child class independently performs whatever actions needed in addition to or instead of delegating to the parent object.
  • (via inheritance) An object decides what method to apply to itself at run-time depending on its place in the inheritance hierarchy.
  • (via delegation) One or more objects are accessed using the same public interface.  The objects are of different classes or change to different classes over time, causing them to respond differently to the same actions.

Data Members

  • The attributes (data) of a class
  • aka member variables, object variables, class variables


  • The behavior (functions) of a class.
  • aka member functions, object functions, class functions


  • Initializes an object when it's created.


  • Returns the value of an attribute (data) of the class.
  • aka getters / accessor methods
  • GetTime( )
  • GetTemperature( )


  • Changes the value of an attribute (data) of the class.
  • aka setters
  • SetTime( SomeTime )
  • SetTemperature( SomeTemperature )


  • Causes the object to perform an action or service.

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