Data Encapsulation


A central tenet of object-oriented programming is that the objects act as “black boxes”—that is, they hide, or encapsulate, their contents. When you call a method, you don’t care how the object does what you are asking it to do; you simply expect that the object will get the task done. The same concept applies to instance variables. The use of properties, and the accessor methods in particular, help to hide the instance variables themselves. You are indirectly accessing them; you get to control their access (such as making them readonly). By using methods, you also don’t have to worry about how the instance variables themselves are set up; you simply access the methods that are provided. An additional benefit of data encapsulation is that the creator of the class can change the internal workings (change the way the instance variables are set up, or the way a method operates) without breaking code that relies on the class. It’s like a car—most people don’t care how the car works, whether it burns gas or runs on gerbil power, as long as it gets them to the grocery store and back.

 

To be honest, it’s a difficult concept to explain; perhaps this page over at Stack Overflow (great site, BTW) does a better job.

Objects (Part 1): Splitting Classes Into Multiple Files


This is the first in a multi-part series that will talk about the fundamentals of objects. If you’re still keeping track, I suppose these would all fall under Lesson 6: Objects.

In Lesson 3, we began a simple overview to object-oriented programming. Now, we’ve covered the major portions of the basic language—what is “plain C.” There are other topics, such as structs, arrays, and a slew of other rather obscure topics. Some of these are enclosed or remade in the code the Apple provides for free; others will be discussed as necessary. For now, we will start venturing into the exciting world of objects—and they really are exciting.

As part of our calculator, we will allow the calculator to operate on regular values, as well as fractions. For the sake of demonstration, we will begin by creating Fraction objects.

Creating the Test Code

Create a new Xcode project. In the New Project window, choose “Application” under Mac OS X from the left side, then choose “Command Line Tool” from the top right, and make sure the “Type” is set to Foundation. Click Choose, and save it as “FractionDemo” anywhere you choose. In the File List, load FractionDemo.m, and enter the following code:

#import "Fraction.h"
#import 

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    Fraction *myFraction = [[Fraction alloc] init];

	// Set myFraction to value of 2/5
	[myFraction setNumerator: 2];
	[myFraction setDenominator: 5];

	// Display the value of myFraction
	NSLog(@"myFraction has a value of: ");
	[myFraction display];

	[myFraction release];
    [pool drain];
    return 0;
}

Here, you can see that in our main code body, we create an object of type Fraction, and call it myFraction. Ignore the asterisk (* for now; it is a sign that you are actually creating a pointer…that’s an advanced topic that will be further explored later). You set myFraction to the value returned from the standard alloc/init method call.

Next, we call the methods setNumerator: and setDenominator: and pass both integer values; these methods will set the appropriate values of the Fraction object we’ve created. We then send another message call for myFraction to display itself (or so we assume, based on the method name—make sure to write descriptive method names). Then we release our object—don’t worry about the memory management for now.

Now, we will create the actual Fraction object itself.

Creating the Fraction Class

In Xcode, Go to File > New File (⌘N). From the left, choose Cocoa Class under Mac OS X, select Objective-C class from the top left, and make it a Subclass of NSObject. Click Next, and set the following parameters:

  1. Set the File Name to be “Fraction.m”. Note that the “.m” part is automatically added; just type “Fraction”, without the quotes, making sure that the “F” is capitalized. By convention, class names are capitalized.
  2. Make sure that ‘Also create “Fraction.h”‘ is selected.
  3. Leave the default settings for the other fields. Add to Project should display “FractionDemo,” the name of your project, and in your Targets window, the checkbox to the left of the “FractionDemo” entry should be selected.

Click “Finish”. In your File List, you will see two new files: “Fraction.h” and “Fraction.m.” Select Fraction.h, and enter the following code:

#import <Foundation/Foundation.h>

@interface Fraction : NSObject {
	NSInteger numerator;
	NSInteger denominator;
}
- (void)setNumerator:(NSInteger)value;
- (void)setDenominator:(NSInteger)value;
- (void)display;

@end

Here we define two instance variables, called “numerator” and “denominator”. They are defined as type NSInteger; this is identical to the standard int type; there’s really no difference. My personal preference is NSInteger.

Next, we define the methods that we call in the main() method. There should not be anything new here, if you’ve read through Lesson 3.

In Fraction.m, enter the following code:

#import "Fraction.h"

@implementation Fraction
- (void)setNumerator:(NSInteger)value {
	numerator = value;
}

- (void)setDenominator:(NSInteger)value {
	denominator = value;
}

- (void)display {
	NSString *numeratorString = [[NSString alloc] initWithFormat:@"%d", numerator];
	NSString *denominatorString = [[NSString alloc] initWithFormat:@"%d", denominator];
	NSLog(@"%@/%@", numeratorString, denominatorString);
	[denominatorString release];
	[numeratorString release];
}

@end

Chewing through this code, the first two methods should not present anything new. The last method does warrant some discussion, however.

If you refer back to Lesson 3, you’ll notice that we defined the display method (we called it showResults there) as

- (void)showResults {
	NSLog(@"This is a fraction with a value of %d/%d", numerator, denominator);
}

Our display method shows a different way to do this. First, we highlight NSString’s initWithFormat: method, which works like NSLog() does, except that instead of outputting, it is saved as a string. In the actual NSLog(), we use the %@ format specifier, which takes an NSString as an argument. Finally, we have to release the NSStrings that we’ve created…again, don’t worry about the memory management at the moment.

If we Build and Run now, we get the following output:

myFraction has a value of:
2/5

This is exactly what we’re looking for.

You’ll notice that we haven’t really done anything new since Lesson 3. In fact, that is true, except that we have created a Fraction class in separate files. Looking back over the code entries, the bold lines show where the dependencies lie—in a class’s .m file, it #imports the correspond .h (known as the ‘header’) file. In any file that uses the class you’ve created, you also need to import the header. Here, you’ve made it easier to maintain a larger project; as we add more classes, you won’t be working with one gigantic file.

Objective-C Lesson 3: Object-Oriented Programming


When object-oriented programming arrived in the programming world, it was considered a savior of software by some and yet another foolish experiment by others. Regardless of the perspective, we need to define some basic concepts and terminology before we begin

A Thing

An object is a thing. In the real world, you have “things,” all of which have certain properties. For example, you probably have a car—a “thing.” The car has properties, such as paint color, total mileage, make, model, and year. You can also do things with the car, or have something done to it—for example, you can drive twenty miles in the car, or wash the car.

In an object-oriented program, your specific car is an instance of a class, generally called Car. A class is the definition of the object, and in most cases can be thought of as a “factory” for its instances. A class can create many instances of the same type of object, although they have to have different names. An object is simply a variable, but it can be (and usually is) of a custom type.

Instance Variables and Methods

As mentioned before, your car, and any other car, has certain properties that are potentially unique to that specific instance. These properties are known as instance variables, or ivars for short. Instance variables are variables, of either the standard data types, or of any other object, but they are private to the object—that is, every single instance of Car has its own identical set of instance variables; although the variables may be of the same type and number, they can (and usually do) contain different values across different instances.

A method is the proper term for a function, or simply a piece of code that is called upon to perform some action. In Objective-C, there are two types of methods—class methods and instance methods. Class methods are performed directly on the class that an object belongs to—for example, a class methods might ask the car factory how many cars it produced the previous year. Instance methods are performed on specific instances of a class—for example, you might ask one specific car to drive twenty miles, or you might wash one specific car; you wouldn’t do those things to the entire factory. Instance methods affect the state or condition of the object. Just like two identical cars adopt different characteristics over their lifetime, different instances of Car can take on different traits as well.

Message Objects

In Objective-C, there is a specific syntax for calling methods:

[ClassOrInstance method];

The method call begins with an open bracket. This is followed by the name of the class or instance, depending on whether the method belongs to a class or instance. This is followed by a space, and then the name of the method. This is followed by a closing bracket, and the all-important semicolon.
In more formal terminology, the following message call might be described as

[receiver message];

The meaning is the same.

Sample Program

Now, with the theory cleared up, we can move on to our first program involving classes.

Program 3.1

// A simple implementation of a Fraction
#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
	NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

	int numerator = 2;
	int denominator = 5;
	NSLog(@"This is a fraction with a value of %d/%d", numerator, denominator);

	[pool drain];
	return 0;
}

The output is

This is a fraction with a value of 2/5

In Program 3.1, we defined a numerator and denominator variable, and set them to integer values. We then printed the result of this simple fraction.

This program is short and condensed, and seems to be pretty simple—and it is. But what if we had a hundred fractions that we wanted to store? We’d have to keep track of two hundred variables. And performing operations on those variables would not be any easier.

As a matter of fact, Program 3.1 is not a true object-oriented program. It is still written using C-style code. We have not created any objects in the code; our Fraction object still exists only psychologically.

Program 3.2 presents the exact same functionality, except that we actually define this object in code. We then refer to the object as Fraction throughout the program. Don’t worry if it looks daunting; an explanation follows.

Program 3.2

// A Fraction class
#import <Foundation/Foundation.h>

//———————— Interface ————————

@interface Fraction : NSObject
{
	int numerator;
	int denominator;
}

- (void)showResults;
- (void)setNumerator:(int)n;
- (void)setDenominator:(int)d;

@end

//———————— Implementation ————————

@implementation Fraction

- (void)showResults {
	NSLog(@"This is a fraction with a value of %d/%d", numerator, denominator);
}

- (void)setNumerator:(int)n {
	numerator = n;
}

- (void)setDenominator:(int)d {
	denominator = d;
}

@end

//———————— Main Program ————————

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    // Create an instance of Fraction
	Fraction *myFraction;

	myFraction = [[Fraction alloc] init];

	// Set myFraction to 2/5
	[myFraction setNumerator:2];
	[myFraction setDenominator:5];

	// Display the value of myFraction
	[myFraction showResults];

	// Clear up memory
	[myFraction release];

    [pool drain];
    return 0;
}

The output is

This is a fraction with a value of 2/5

Code, Demystified

Program 3.2 is divided into three logical sections. These are known as the interface, implementation, and program section, respectively.

The interface section (delineated by @interface in the code) provides basic program-wide definitions, the implementation section (delineated by @implementation) provides the implementation of the methods, and the program section contains the main code that does something.

The @interface Section

Every class in Objective-C has a single parent, or superclass, from which the class inherits. Next, you have to declare the instance variables that belong to the class. Finally, you declare the methods, or actions, that the class can perform. This is all done in the @interface section, which looks like this:

@interface ClassName : ParentClassName
{
	instanceVariableDeclarations;
}
methodDeclarations
@end

Remember that the compiler ignores spaces, so you may put line breaks and spaces anywhere within the code above.
By convention, the name of a class begins with a capital letter, as in the Fraction class.

You can tell that our Fraction class is a subclass of NSObject, which is the root object that almost everything else inherits from, directly or indirectly. NSObject is declared in the Foundation files which you imported.

Fraction contains two instance variables; both are ints. They are called instance variables because every instance of Fraction creates a new set of these variables, unique to that specific instance. So, FractionA’s numerator is a different variable than FractionB’s numerator. Objective-C automatically keeps track of this for you.

Currently, our Fraction class declares (and later implements) three methods. All are instance methods, as noted by the minus sign () preceding each method name. A class method is delineated by a plus sign (+).

Before we discuss the method declaration, we must remember than an object is considered a “black box.” Ideally, objects encapsulate, or hide, their instance variables, from anything except itself. The object exists as a single entity, and anything that works with the object does not need to know, and should not know, about the object’s inner workings. Therefore, we need to declare specific methods to set the variables, because the program should not directly manipulate them. We also need a separate method to print the values, because, once again, the program should not be able to directly access the variables. This is a fundamental concept in Objective-C, and is observed throughout the libraries and code.

The Fraction class defines three methods, all of which return nothing. The value in parentheses before the method name specifies the return type of the method. void is programming parlance for “nothing,” or nil. Any data type or object can be a return value; a method might return a float, int, or another Fraction, as illustrated by the following methods:

- (int)numerator;
- (float)divide;
- (Fraction *)randomFraction;

The asterisk after the Fraction is used to delineate a pointer; this is not an important concept at the moment. For now, simply remember that anytime you create an instance of a custom object, the name of the instance should be prefixed with an asterisk.

All of these methods must return a value somewhere in the method. Just as main returns 0 at the end, these methods must return an int value, a float value, and another instance of Fraction, respectively. The compiler will complain if you do not return a value.

The latter two methods declared in Fraction return no value, but they do each take an integer value as an argument. In the case of setNumerator:, the name of the argument is n. This name is arbitrary, as long as it follows the standard naming conventions, and is used only within the method to refer to the argument. The argument itself exists only within the method; you cannot access n anywhere else in Fraction.

Arguments are values that are passed into a method, which the method would not be able to obtain otherwise. For example, setNumerator: requires a value to set to the numerator, but it would not receive a custom value without an argument.

Note the syntax of these method declarations. The method name begins with the minus or plus, signifying the method type. This is followed by the return type in parentheses, the name of the method, and a colon. This colon is followed by an argument type in parentheses, a name for the argument, and a semicolon. In a method that takes an argument, the colon is considered part of the method name.

A method can take multiple arguments; they are delineated by a space, a new section of the method name, another colon, an argument type, and name. This will be covered in a later chapter.

The @implementation Section

In the implementation section, you define the code that each method implements. The general appearance of the @implementation section is similar to this:

@implementation ClassName
	methodDefinitions;
@end

In the implementation of Fraction, you also declare the method types and names, followed by an open brace. Between the opening and closing brace, you insert the code that the method will execute when it is called.

The program Section

In this program, the main program section contains the all-important main function that is the first thing to be executed.

In Program 3.2, we first create an instance of Fraction:

 Fraction *myFraction;

Again, don’t worry about the asterisk. From a technical perspective, it says that myFraction is a pointer to a Fraction.

Now that you’ve created an object “container” to store a Fraction, you have to actually create a Fraction to fill that space. You do that in the next line,

myFraction = [[Fraction alloc] init];

In this line, you call the alloc method on Fraction; alloc is a class method. Although you never defined the alloc method, it is inherited from NSObject. Inheritance will be discussed in a later lesson.

After you allocate a Fraction, you have to actually initialize it. Therefore, you pass the value of the alloc method to NSObject’s init method, and the resulting Fraction is assigned to myFraction. This nested sequence of messages is used almost every time you create a new object.

Next, you set the values of myFraction to 2 and 5 by using the proper method calls.

[myFraction setNumerator:2];
[myFraction setDenominator:5];

You pass the integers 2 and 5 as arguments to the method. The method then takes these arguments and sets the numerator and denominator variables of myFraction.

You then call myFraction’s showResults method to print the value of the fraction.

We then free up the memory that Fraction uses by calling the release method:

[myFraction release];

This is a critical part of programming for iOS. Whenever you create a new object through alloc/init, you have to release it—in other words, every alloc/init call is balanced by a release call. Otherwise, the now-useless object will still sit in memory, taking up space; in a memory-constrained environment such as the iPhone, you want to use as little memory as you can.

Messaging Receivers

In every message, you send the message to a specific receiver—the object or class which implements the method. However, instance methods are sent to specific instances, so that the same method call to two different Fractions will result in two Fractions with potentially different values. For example,

[fraction1 setNumerator:2];
[fraction1 setDenominator:5];

[fraction2 setNumerator:3];
[fraction2 setDenominator:4];

Will result in two separate fractions, one with a value of 2/5 and another with a value of 3/4. Although you are calling the same methods on both, they are received by different objects, with their own set of numerator and denominator variables.

Data Encapsulation

As mentioned before, you shouldn’t directly access the instance variables of an object. Data encapsulation provides some security against people working with the class, to prevent them from tinkering with the inner workings. So in the main function, how would you access these values?

Generally, you access these values by writing special methods, known as getter methods (the set methods that we’ve already wrote for Fraction are known as setter methods; they are generally written in pairs).

The getter methods for the Fraction class look something like this:

- (int)numerator;
- (int)denominator;

- (int)numerator {
	return numerator;
}
- (int)denominator {
	return denominator;
}

It doesn’t matter if the method name is the same as the variable name; in fact, it is common practice.

Program 3.3 contains the main function that tests these new methods.

Program 3.3

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    // Create an instance of Fraction
	Fraction *myFraction;

	myFraction = [[Fraction alloc] init];

	// Set myFraction to 2/5
	[myFraction setNumerator:2];
	[myFraction setDenominator:5];

	// Display the value of myFraction
	NSLog (@"The value of myFraction is %d/%d", [myFraction numerator], [myFraction denominator]);

	// Clear up memory
	[myFraction release];    [pool drain];
    return 0;
}

The output is

The value of myFraction is 2/5

Objective-C 2.0 introduces properties, which provide an easy way to create getter and setter methods. This will also be covered in a later lesson.

Shameless Self-Promotion


To get the word out, I’d generally rely on search engines. And I can’t afford to pay Google to get listed (and most of those ads aren’t exactly reputable anyway). So, here’s some shameless self promotion. I promise I won’t really do this much more. Feel free to not read this. You may want to entertain yourself elsewhere (I’d recommend Uncyclopedia).


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😀

Language Concepts


C was conceived and created as a procedural programming language, whereas Objective-C was to be object-oriented, hence the name. In a procedural language, the code is focused around variables, data, and functions—how to store data and what to do with the data. In contrast, an object-orientated language focuses on creating objects, which are then used to do certain things, just like objects, or “things”, do in real life.

Object-orientated code seems to involve more work initially—there is a lot of “boilerplate” code for even the simplest objects. Fortunately, most of this code is already provided in Xcode’s templates, and the objects will quickly become more useful.

So what is an object? Put simply, it is a “thing.” Throughout this book, one of the objects that we will be creating will be a Die—the kind you might find in a board game. From the program’s perspective, the die is a “black box”—it hides its inner workings; the object performs any task that is asked of it (assuming that the object has been programmed to actually perform the task), and when it finishes, the object is no longer used. How the object performs its task is irrelevant, as far as the program itself is concerned.

Once you create an object, you can then tell your program to produce as many of them as you need. Therefore, your die object can create a pair of itself—a pair of dice.

These dice have traits, such as color, size, or the number of faces. You can also perform actions with these dice—you can roll one, or you can roll both of them. After rolling both of them, you would then add, or perhaps multiply, the resulting numbers.

From a higher-level viewpoint, all the program has to do is ask the dice to roll themselves, and report a total. The program does not have to know how the dice do that. In fact, if you were not the original creator of the die object, you wouldn’t either—and that’s perfectly fine.

Object-orientated programs allow developers to hide the inner workings of their program, while also making the program more efficient to run, as well as maintain. It has become the de facto programming language convention for most large programs, and likely will remain as such for years to come.

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