Home > Articles > Programming > General Programming/Other Languages

  • Print
  • + Share This
This chapter is from the book

The @interface Section

When you define a new class, you have to do a few things. First, you have to tell the Objective-C compiler where the class came from. That is, you have to name its parent class. Second, you have to specify what type of data is to be stored in the objects of this class. That is, you have to describe the data that members of the class will contain. These members are called the instance variables. Finally, you need to define the type of operations, or methods, that can be used when working with objects from this class. This is all done in a special section of the program called the @interface section. The general format of this section looks like this:

@interface NewClassName: ParentClassName
{
    memberDeclarations;
}

methodDeclarations;
@end

By convention, class names begin with an uppercase letter, even though it's not required. This enables someone reading your program to distinguish class names from other types of variables by simply looking at the first character of the name. Let's take a short diversion to talk a little about forming names in Objective-C.

Choosing Names

In Chapter 2, "Programming in Objective-C," you used several variables to store integer values. For example, you used the variable sum in Program 2.4 to store the result of the addition of the two integers 50 and 25.

The Objective-C language allows you to store data types other than just integers in variables as well, as long as the proper declaration for the variable is made before it is used in the program. Variables can be used to store floating-point numbers, characters, and even objects (or, more precisely, references to objects).

The rules for forming names are quite simple: They must begin with a letter or underscore (_), and they can be followed by any combination of letters (upper- or lowercase), underscores, or the digits 09. The following is a list of valid names:

  • sum
  • pieceFlag
  • i
  • myLocation
  • numberOfMoves
  • sysFlag
  • ChessBoard

On the other hand, the following names are not valid for the stated reasons:

  • sum$value $—is not a valid character.
  • piece flag—Embedded spaces are not permitted.
  • 3Spencer—Names can't start with a number.
  • int—This is a reserved word.

int cannot be used as a variable name because its use has a special meaning to the Objective-C compiler. This use is known as a reserved name or reserved word. In general, any name that has special significance to the Objective-C compiler cannot be used as a variable name.

Always remember that upper- and lowercase letters are distinct in Objective-C. Therefore, the variable names sum, Sum, and SUM each refer to a different variable. As noted, when naming a class, start it with a capital letter. Instance variables, objects, and method names, on the other hand, typically begin with lowercase letters. To aid readability, capital letters are used inside names to indicate the start of a new word, as in the following examples:

  • AddressBook—This could be a class name.
  • currentEntry—This could be an object.
  • current_entry—Some programmers use underscores as word separators.
  • addNewEntry—This could be a method name.

When deciding on a name, keep one recommendation in mind: Don't be lazy. Pick names that reflect the intended use of the variable or object. The reasons are obvious. Just as with the comment statement, meaningful names can dramatically increase the readability of a program and will pay off in the debug and documentation phases. In fact, the documentation task will probably be much easier because the program will be more self-explanatory.

Here, again, is the @interface section from Program 3.2:

//---- @interface section ----

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

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

@end

The name of the new class is Fraction, and its parent class is NSObject. (We talk in greater detail about parent classes in Chapter 8, "Inheritance.") The NSObject class is defined in the file NSObject.h, which is automatically included in your program whenever you import Foundation.h.

Instance Variables

The memberDeclarations section specifies what types of data are stored in a Fraction, along with the names of those data types. As you can see, this section is enclosed inside its own set of curly braces. For your Fraction class, these declarations say that a Fraction object has two integer members, called numerator and denominator:

int  numerator;
int  denominator;

The members declared in this section are known as the instance variables. As you'll see, each time you create a new object, a new and unique set of instance variables also is created. Therefore, if you have two Fractions, one called fracA and another called fracB, each will have its own set of instance variables. That is, fracA and fracB each will have its own separate numerator and denominator. The Objective-C system automatically keeps track of this for you, which is one of the nicer things about working with objects.

Class and Instance Methods

You have to define methods to work with your Fractions. You need to be able to set the value of a fraction to a particular value. Because you won't have direct access to the internal representation of a fraction (in other words, direct access to its instance variables), you must write methods to set the numerator and denominator. You'll also write a method called print that will display the value of a fraction. Here's what the declaration for the print method looks like in the interface file:

-(void) print;

The leading minus sign (-) tells the Objective-C compiler that the method is an instance method. The only other option is a plus sign (+), which indicates a class method. A class method is one that performs some operation on the class itself, such as creating a new instance of the class.

An instance method performs some operation on a particular instance of a class, such as setting its value, retrieving its value, displaying its value, and so on. Referring to the car example, after you have manufactured the car, you might need to fill it with gas. The operation of filling it with gas is performed on a particular car, so it is analogous to an instance method.

Return Values

When you declare a new method, you have to tell the Objective-C compiler whether the method returns a value and, if it does, what type of value it returns. You do this by enclosing the return type in parentheses after the leading minus or plus sign. So this declaration specifies that the instance method called currentAge returns an integer value:

–(int) currentAge;

Similarly, this line declares a method that returns a double precision value. (You'll learn more about this data type in Chapter 4, "Data Types and Expressions.")

–(double) retrieveDoubleValue;

A value is returned from a method using the Objective-C return statement, similar to the way in which we returned a value from main in previous program examples.

If the method returns no value, you indicate that using the type void, as in the following:

–(void) print;

This declares an instance method called print that returns no value. In such a case, you do not need to execute a return statement at the end of your method. Alternatively, you can execute a return without any specified value, as in the following:

return;

Method Arguments

Two other methods are declared in the @interface section from Program 3.2:

–(void) setNumerator: (int) n;
–(void) setDenominator: (int) d;

These are both instance methods that return no value. Each method takes an integer argument, which is indicated by the (int) in front of the argument name. In the case of setNumerator, the name of the argument is n. This name is arbitrary and is the name the method uses to refer to the argument. Therefore, the declaration of setNumerator specifies that one integer argument, called n, will be passed to the method and that no value will be returned. This is similar for setDenominator, except that the name of its argument is d.

Notice the syntax of the declaration for these methods. Each method name ends with a colon, which tells the Objective-C compiler that the method expects to see an argument. Next, the type of the argument is specified, enclosed in a set of parentheses, in much the same way the return type is specified for the method itself. Finally, the symbolic name to be used to identify that argument in the method is specified. The entire declaration is terminated with a semicolon. Figure 3.1 depicts this syntax.

Figure 3.1

Figure 3.1 Declaring a method

When a method takes an argument, you also append a colon to the method name when referring to the method. Therefore, setNumerator: and setDenominator: is the correct way to identify these two methods, each of which takes a single argument. Also, identifying the print method without a trailing colon indicates that this method does not take any arguments. In Chapter 7, "More on Classes," you'll see how methods that take more than one argument are identified.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020