OOP

• Following are the four pillars of OOP principle,

  • Encapsulation

  • Abstraction

  • Polymorphism

  • Inheritance

Encapsulation

• Encapsulation is a language construct that facilitates the bundling of data with the methods operating on that data.

• Encapsulation doesn't guarantee data protection, cohesive design or information hiding.

  Information Hiding

  • Information hiding is a design principle that strives to shield client classes from the internal workings of a class.

  • Encapsulation facilitates, but does not guarantee, information hiding.

  • Read more about relation between information hiding and encapsulation.

Abstraction

• Abstraction is preserving information that is relevant in a given context, and forgetting information that is irrelevant in that context.

Polymorphism

• The provision of a single interface to entities of different types or the use of a single symbol to represent multiple different types is known as polymorphism.

  Types of polymorphism

  • Major classes of polymorphism

    • Ad hoc polymorphism

      • Polymorphic functions that can be applied to arguments of different types, but that behave differently depending on the type of the argument to which they are applied.

    • Parametric polymorphism

      • A function or a data type to be written generically, so that it can handle values uniformly without depending on their type.

      • Parametric polymorphism is a way to make a language more expressive while still maintaining full static type-safety.

    • Subtype polymorphism

      • Subtyping allows a function to be written to take an object of a certain type T, but also work correctly, if passed an object that belongs to a type S that is a subtype of T (according to the Liskov substitution principle).

      • This type relation is sometimes written S <: T. Conversely, T is said to be a supertype of S—written T :> S. Subtype polymorphism is usually resolved dynamically.

    • Coercion polymorphism

      • Implicit type conversion by compiler aka type juggling is also a type of polymorphism.

  • Polymorphism can be distinguished by when the implementation is selected.

    • Static polymorphism typically occurs in ad hoc and parametric polymorphsim.

    • Dynamic poymorphism usually occurs in subtype polymorphism. But subtype polymorphism can be achieved through static polymorphism as well.

Inheritance

• The mechanism of basing an object or class upon another object (prototype-based inheritance) or class (class-based inheritance), retaining similar implementation.

• Also defined as deriving new classes (sub classes) from existing ones such as super class or base class and then forming them into a hierarchy of classes.

• Inheritance only reuses implementation and establishes a syntactic relationship, not necessarily a semantic relationship (inheritance does not ensure behavioral subtyping).

• The inheritance hierarchy of an object is fixed at instantiation when the object's type is selected and does not change with time.

• Types of inheritance

  • Single

  • Multiple

  • Multilevel

  • Hierarchical

  • Hybrid

Subtyping

• Subtyping enables a given type to be substituted for another type or abstraction, and is said to establish an is-a relationship between the subtype and some existing abstraction, either implicitly or explicitly, depending on language support.

• Subtyping refers to compatibility of interfaces.

  Important readings

  • Implementation Inheritance v/s Interface Implementation

  • Inheritance v/s subtyping

  • Implementation inheritance is evil