Object Oriented Programming I

See csse2002 for staff, assessment, and course logistics.

Today’s outline

1. Objects, Classes, & Object-Oriented Programming
2. Encapsulation
3. Inheritance
4. Abstraction
5. Polymorphism (next week)

Why OOP?

A single bank account example, grown in stages, motivates the four pillars below.

Stage 1 — one global balance. A single static bankBalance shared by the whole “bank” — fine for one account, but there’s no way to represent more than one account or customer at once.

Stage 2 — parallel arrays of accounts. Adding more accounts naively means adding a parallel array for every piece of account data (accountNumbers, balances, …) plus a manually-maintained accountCount, and a findAccountIndex helper to keep them in sync by position.

Stage 3 — parallel arrays of account holders too. Adding a holder’s name/id alongside each account means yet more parallel arrays (holderIds, holderNames) and another manually-maintained linkedHolderIds array to associate accounts with holders — the bookkeeping to keep every array in sync by index grows with every new piece of data.

This is the motivation for OOP: instead of scattering related data across parallel arrays and free functions, bundle a variable and the functions that act on it together — e.g. instead of a free function f(x), a method that belongs to x. This bundling is exactly what a class does.

The four pillars of object orientation

1. Encapsulation — see java-encapsulation
2. Inheritance — see java-inheritance
3. Abstraction — see java-abstraction
4. Polymorphism — next week