Study the object orientation lecture notes to prepare for this quiz.

Be able to answer all of the following questions (in English, not code):

• What are some benefits of object-oriented programming?

• How would you describe what a “class” is to somebody who knows how to program computers but does not know anything about object-oriented programming?

• Why do we use inheritance?

• Why is object-oriented programming a good way to write software that simulates some aspect of the world?

What are some benefits of object-oriented programming?

All the data and code that makes sense for some type of thing can live in one place and basically act as a black box. This has huge advantages for mental models (having an idea about what the program is doing) and imposes a good organization of the actual code. For example, a Person class can encapsulate all the data and methods that make sense for Person things, so that in some part of code one can write john.changeClothes() and not worry about how it’s actually done or what data is read or altered. Object-oriented programming allows a programmer to work with “objects” on which “actions” can be performed; and the results of actions are typically invisible to the programmer: instead of passing data into a function, internal object data is utilized. Applying actions to objects is often exactly how a programmer wants to think about a problem.

How would you describe what a “class” is to somebody who knows how to program computers but does not know anything about object-oriented programming?

A class is a kind of definition that says what data and actions are available to members of that class. So a Person is a class, Siddharth is a member (or instance or object) of that class, and Siddharth, like all other members of the same class, has certain data and potential actions: Siddharth has a name, an age, a home address, etc. and Siddarth can sit down, stand up, change clothes, buy a house, etc. More specifically, a class defines the data and actions that operate on that data; once you have a class you can create instances (variables) of that class, and set the data and perform the actions.

Why do we use inheritance?

Sometimes distinct classes share common data and/or common actions. For example, rectangles, triangles, and ellipses are all distinct classes, but have common “shape” properties and actions. Each of those three classes have an x,y position in a 2D space and should be able to compute an area, or rotate, or display themselves graphically. Those common data and actions can be pulled out of the three classes and put into a new Shape class, from which Rectangle, Triangle, and Ellipse inherit. When one class inherits from another, we say the inheritting class is a subclass, and that the subclass “is a” member of the parent class. So Rectangle “is a” Shape because Rectangle inherits from Shape. Having a superclass or inherited class allows us to extract common features, which not only reduces our code but also documents a hierarchical relationship among classes.

Why is object-oriented programming a good way to write software that simulates some aspect of the world?

Often, we carve the world into hierarchical categories. For example, an elephant is an African mammal which is further more a Mammal. If we know something about Mammals (such as that they have hair) then we can infer that Elephants do, too. Of course, this is true, as far as mammals and elephants go, because Mammal is a category that was created for exactly this purpose (inferring the properties of the members of the class). In another example, we might simulate planetary motions by creating a “Moving Body” class which has mass (and therefore gravitational force), velocity, etc. Asteroids, planets, and other such things would be members (or maybe subclasses) of the “Moving Body” class. If we can take a real-world phenomenon and categorize like we have done with the animal kingdom or planetary motions, then we can easily use object-oriented programming to simulate or model the phenomenon.