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Ruby Programming
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Ruby Is an Object-Oriented Language

Let's say it again. Ruby is a genuine object-oriented language. Everything you manipulate is an object, and the results of those manipulations are themselves objects. However, many languages make the same claim, and they often have a different interpretation of what object-oriented means and a different terminology for the concepts they employ.

So, before we get too far into the details, let's briefly look at the terms and notation that we'll be using.

When you write object-oriented code, you're normally looking to model concepts from the real world in your code. Typically during this modeling process you'll discover categories of things that need to be represented in code. In a jukebox, the concept of a ``song'' might be such a category. In Ruby, you'd define a class to represent each of these entities. A class is a combination of state (for example, the name of the song) and methods that use that state (perhaps a method to play the song).

Once you have these classes, you'll typically want to create a number of instances of each. For the jukebox system containing a class called Song, you'd have separate instances for popular hits such as ``Ruby Tuesday,'' ``Enveloped in Python,'' ``String of Pearls,'' ``Small talk,'' and so on. The word object is used interchangeably with class instance (and being lazy typists, we'll probably be using the word ``object'' more frequently).

In Ruby, these objects are created by calling a constructor, a special method associated with a class. The standard constructor is called new.

song1 = Song.new("Ruby Tuesday")
song2 = Song.new("Enveloped in Python")
# and so on

These instances are both derived from the same class, but they have unique characteristics. First, every object has a unique object identifier (abbreviated as object id). Second, you can define instance variables, variables with values that are unique to each instance. These instance variables hold an object's state. Each of our songs, for example, will probably have an instance variable that holds the song title.

Within each class, you can define instance methods. Each method is a chunk of functionality which may be called from within the class and (depending on accessibility constraints) from outside. These instance methods in turn have access to the object's instance variables, and hence to the object's state.

Methods are invoked by sending a message to an object. The message contains the method's name, along with any parameters the method may need.[This idea of expressing method calls in the form of messages comes from Smalltalk.] When an object receives a message, it looks into its own class for a corresponding method. If found, that method is executed. If the method isn't found, ... well, we'll get to that later.

This business of methods and messages may sound complicated, but in practice it is very natural. Let's look at some method calls. (Remember that the arrows in the code examples show the values returned by the corresponding expressions.)

"gin joint".length 9
"Rick".index("c") 2
-1942.abs 1942
sam.play(aSong) "duh dum, da dum de dum ..."

Here, the thing before the period is called the receiver, and the name after the period is the method to be invoked. The first example asks a string for its length, and the second asks a different string to find the index of the letter ``c.'' The third line has a number calculate its absolute value. Finally, we ask Sam to play us a song.

It's worth noting here a major difference between Ruby and most other languages. In (say) Java, you'd find the absolute value of some number by calling a separate function and passing in that number. You might write

number = Math.abs(number)     // Java code

In Ruby, the ability to determine an absolute value is built into numbers---they take care of the details internally. You simply send the message abs to a number object and let it do the work.

number = number.abs

The same applies to all Ruby objects: in C you'd write strlen(name), while in Ruby it's name.length, and so on. This is part of what we mean when we say that Ruby is a genuine OO language.
Ruby Programming
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