Example for explaining object-oriented concepts

Off the top of my head: You're making a game, something like SimCity.

You've got a base class called building, and subclasses called residential, commercial, utility, or whatnot.

All buildings have a location, value, and land-tax (you're mayor remember).
Residential buildings provide living space.
Commercial buildings provide working space.
Utility buildings provide services.

The calculateValue and calculateLandTax functions would be handled differently for residential and commercial buildings.

I once had to teach children these concepts. The projects were mostly simple video-games which lent itself very nicely to basic OOD/OOP. You had Sprite which was a root of a hierarchy. It would have attributes such as size, speed, and health and these would often be overriden to have different values in different subclasses. Common methods that all Sprites had (but sometimes overrode) would be things like move. Then you had subclasses such as GoodGuy, BadGuy, and sometimes we'd have a Scenery class (for sprites that just moved around in the background). Sometimes BadGuy would have further subclasses such as BossBadGuy and BigBossBadGuy which would override some of the BadGuy methods (such as attack, die, respawn).

The best example I know of is actors on a stage in a play - it also helps you avoid ending up in the inheritance tar pit.

Say we have the play Julius Caesar from William Shakespeare. You could implement it with a bunch of Actor classes, from which e.g. the Calpurnia class is derived. You could have multiple Calpurnia classes (say for different actresses knowing the role of Calpurnia, each with different strength an weaknesses, but all of them able to play - more or less - properly their role).

When during a play of Julius Caesar Calpurnia says the line:

What mean you, Caesar? think you to walk forth? You shall not stir out of your house to-day.

The instance of the Calpurnia class (= object) sends a message (= calls a method on) an instance of the Julius Caesar class. Which reacts according to the script (= algorithm implemented in the called method) with the line:

The things that threaten'd me ne'er look'd but on my back; when they shall see The face of Caesar, they are vanished.

The instance of the Calpurnia class though has no idea how the Caesar object creates this answer (= encapsulation) and everyone is perfectly aware, that this is not the real Julius Caesar (or Calpurnia) standing on the stage, but actors representing reasonably romanticized versions of the historic persons (= abstraction).

Another participating class in this scenario is the Spectator class. They make heavy use of the fact, that all roles belong to the Actor class and as such instances of them can be watched, listened too or applauded – or booed – irrespective of their concrete subclass (= polymorphism).

And of course there are thousands of different ways on how'd you be able to represent a play(house) in an object oriented fashion...

I always use musicians and their instruments. A flute player has a flute. A flue player is a musician. A band is a collection of musicians. Each musician can play their respective instrument when asked to. But I think I like the metaphor of actors in a play better.

my favorite example for illustrating the concepts of object-oriented programming is POS system case study in Larman's Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design...