.NET/Java PDF, Tiff, Barcode SDK Library

The last major type of variable is the class variable. The scope of a class variable is within the current class, as opposed to within specific objects of that class. Class variables start with two @ symbols (@@) as opposed to the single @ symbol of object variables. Class variables are particularly useful for storing information relevant to all objects of a certain class. For example, you could store the number of objects created so far in a certain class using a class variable like so:

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class Square def initialize if defined (@@number_of_squares) @@number_of_squares += 1 else @@number_of_squares = 1 end end end

FIGURE 1-5 The next slide shows the same photograph with the background stripped away to indicate that something unexpected had happened.

Because @@number_of_squares is a class variable, it s already defined each time you create a new object (except for the first time, but that s why you check to see if it s defined, and if not, give it an initial value of 1).

x = 2 x += 1 x *= 2 x It also works with other data types:

method down to @@number_of_squares = defined (@@number_of_squares)

Clicking again, Mark displayed a new version of the photograph, except in this one, Bob was missing from the photograph. In his place was a thick black line like the chalk outline from a crime scene, as shown in Figure 1-6. You see, Bob Ernst is dead today, Mark said. One of my witnesses that I want to bring in the case I cannot bring you. Bob Ernst cannot come in here today. He is no longer here. He didn t know he was going to need to be here. He didn t leave us anything in video. He didn t leave us anything in writing that would talk about the issues that we need to talk about. This striking photograph visually communicated to the jurors that the worst had happened that Bob and Carol s happy marriage ended abruptly and unexpectedly when Bob died of a heart attack. The black outline where Bob had been in the photo visually brought home the point that Bob s death suddenly had left a hole in Carol s life, and in her heart.

@@number_of_squares + 1 : 1

In your Square class you defined two methods: initialize and area. Both are object methods, as they relate to, and operate directly upon, an object. Here s the code again:

>>> fnord = 'foo' >>> fnord += 'bar' >>> fnord 'foobar' Augmented assignments can make your code more compact and concise, and in many cases, more readable.

FIGURE 1-6 This slide shows Bob missing from the photograph, with only a thick black line to indicate

class Square def initialize(side_length) @side_length = side_length end def area @side_length * @side_length end end

Once you ve created a square with s = Square.new(10), you can use s.area to get back the area of the square represented by s. The area method is made available in all objects of class Square, so it s considered to be an object method. However, methods are not just useful to have available on object instances. It can be useful to have methods that work directly upon the class itself. In the previous section you used a class variable to keep a count of how many square objects had been created, and it would be useful to access the @@number_of_squares class variable in some way other than through Square objects. Here s a simple demonstration of a class method:

class Square def self.test_method puts "Hello from the Square class!" end def test_method puts "Hello from an instance of class Square!" end end Square.test_method Square.new.test_method

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Hello from the Square class! Hello from an instance of class Square!

Tip In general, you should not use += with strings, especially if you are building a large string piece by piece in a loop (see the section Loops later in this chapter for more information about loops). Each addition and assignment needs to create a new string, and that takes time, making your program slower. A much better approach is to append the small strings to a list, and use the string method join to create the big string when your list is finished.

This class has two methods. The first is a class method, and the second is an instance method, although both have the same name of test_method. The difference is that