Objects

• An object is similar to an abstract datatype:
  • Contains some hidden data with a set of operations on them.
  • All interactions with this hidden data is though certain operations → methods, member-functions,…
• Examples:
  • A counter → an object which holds a single integer value.
  • A filesystem or database → an object with large number of data with a number of operations on them.

Features of Object Oriented Programming:

1. Abstraction.
2. Subtyping.
3. Dynamic Lookup.
4. Inheritance.

Abstraction

<aside> 💡 Main idea: Display only useful details and hiding unnecessary ones.

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• Objects are similar to abstract datatypes:
  • They have a public interface visible to outside world.
  • But their implementation is private an hidden from outside world or abstracted.
  • Changing implementations should not affect the interactions with the object.

Subtyping

<aside> 💡 Idea is to have a queue that can hold objects of different types. Subtyping is the way to achieve this.

</aside>

• Solution is to arrange the type of the objects in a hierarchy.

  • A subtype is a specialization of a type. (e.g. e-ticket is a subtype of ticket→ Has all features of a ticket and more)

  <aside> 💡 If A is a subtype of B , wherever an object of type B is needed, an object of type A can be used.

  </aside>

  • Think of it like this: Every object of type A is also an object of type B
  • Or like subsets: If $X\\sube Y$, every $x \\in X$ is also in $Y$.

If f() is a method of B and A is a subtype of B , every object of A also supports f() Note: Implementation of f() can be different in A.