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In this chapter, we ve covered the essentials of object orientation and the features Ruby provides to make object-oriented code a reality. You ve looked at the concepts that apply to object orientation in most languages, such as inheritance, encapsulation, class methods, instance methods, and the various types of variables that you can use. Lastly, you developed a basic set of classes to produce a simple dungeon. Let s reflect on some of the concepts we covered in this chapter: Classes: A class is a collection of methods and data that are used as a blueprint to create multiple objects relating to that class. Objects: An object is a single instance of a class. An object of class Person is a single person. An object of class Dog is a single dog. If you think of objects as real-life objects, a class is the classification, whereas an object is the actual object or thing itself. Local variable: A variable that can only be accessed and used from the current scope. Instance/object variable: A variable that can be accessed and used from the scope of a single object. An object s methods can all access that object s instance variables. Global variable: A variable that can be accessed and used from anywhere within the current program. Class variable: A variable that can be accessed and used within the scope of a class and all of its child objects. Encapsulation: The concept of allowing methods to have differing degrees of visibility outside of their class or associated object.

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Let s look at another couple of useful functions: reversed and sorted. They re similar to the list methods reverse and sort (with sorted taking similar arguments as sort), but they work on any sequence or iterable object, and instead of modifying the object in place, they return reversed and sorted versions: >>> sorted([4, 3, 6, 8, 3]) [3, 3, 4, 6, 8] >>> sorted('Hello, world!') [' ', '!', ',', 'H', 'd', 'e', 'l', 'l', 'l', 'o', 'o', 'r', 'w'] >>> list(reversed('Hello, world!')) ['!', 'd', 'l', 'r', 'o', 'w', ' ', ',', 'o', 'l', 'l', 'e', 'H'] >>> ''.join(reversed('Hello, world!')) '!dlrow ,olleH' Note that although sorted returns a list, reversed returns a more mysterious iterable object. You needn t worry about what this really means; you can use it in for loops or methods such as join without any problems. You just can t index or slice it or call list methods on it directly; in order to do that you have to convert it, using the list type, as shown in the previous example.

Polymorphism: The concept of methods being able to deal with different classes of data and offering a more generic implementation (as with the area and perimeter methods offered by your Square and Triangle classes). Module: An organizational element that collects together any number of classes, methods, and constants into a single namespace. Namespace: A named element of organization that keeps classes, methods, and constants from clashing. Mix-in: A module that can mix its methods in to a class to extend that class s functionality. Enumerable: A mix-in module provided as standard with Ruby that implements iterators and list-related methods for other classes such as collect, map, min, and max. Ruby uses this module by default with the Array and Hash classes. Comparable: A mix-in module provided as standard with Ruby that implements comparison operators (such as <, >, and ==) on classes that implement the generic comparison operator <=>. Throughout the next several chapters, I ll assume you have a knowledge of how classes and objects work, and how the different scopes of variables (including local, global, object, and class variables) work.

Now that you have a rst draft with a graphic on every slide, go back and review what you have with an eye toward re ning your presentation and making it better.