Home > Articles

This chapter is from the book

Multiple objects can be instantiated from a single class. Each of these objects has its own identity and state. This is an important point. Each object is constructed separately and is allocated its own memory. However, some attributes and methods may be shared by all the objects instantiated from the same class, thus sharing the memory allocated for these class attributes and methods.

A Shared Method

A constructor is a good example of a method that is shared by all instances of a class.

Although methods generally represent the behaviors of an object, the state of the object is normally represented by attributes. There are three types of attributes:

  • Local attributes

  • Object attributes

  • Class attributes

Local Attributes

Local attributes are local to a specific method. Consider the following code:

public class Number {

  public method1() {
    int count;

  }

  public method2() {

  }

}

The method method1 contains a local variable called count. This integer is accessible only inside method1. The method method2 has no idea that the integer count exists.

At this point, we can touch on a very important concept: scope. Attributes exist within a particular scope. In this case, the integer count exists within the scope of method1. In Java, C#, and C++, scope is delineated by curly braces ({}). In the Number class, there are several possible scopes—just start matching the curly braces.

The class itself has its own scope. Each instance of the class (that is, each object) has its own scope. Both method1 and method2 have their own scopes as well. Because count lives within method1's curly braces, when method1 is invoked, a copy of count is created. When method1 terminates, the copy of count is removed.

For some more fun, look at this code:

public class Number {

  public method1() {
    int count;
  }

  public method2() {
    int count;
  }

}

How can this be? There are two copies of an integer count in this class. Remember that method1 and method2 each has its own scope. Thus, the compiler can tell which copy of count to access simply by recognizing which method it is in. You can think of it in these terms:

method1.count;

method2.count;

As far as the compiler is concerned, the two attributes are easily differentiated, even though they have the same name. It is almost like two people having the same last name, but based on the context of their first names, you know that they are two separate individuals.

Object Attributes

There are many design situations in which an attribute must be shared by several methods within the same object. In Figure 3.6, for example, three objects have been constructed from a single class. Consider the following code:

public class Number {

  int count;  // available to both method1 and method2

  public method1() {
    count = 1;
  }

  public method2() {
    count = 2;
  }

}
Figure 3.6Figure 3.6 Object attributes.

In this case, the class attribute count is declared outside the scope of both method1 and method2. However, it is within the scope of the class. Thus, count is available to both method1 and method2. (Basically, all methods in the class have access to this attribute.) Notice that the code for both methods is setting count to a specific value. There is only one copy of count for the entire object, so both assignments operate on the same copy in memory. However, this copy of count is not shared between different objects.

To illustrate, let's create three copies of the Number class:

Number number1 = new Number();
Number number2 = new Number();
Number number3 = new Number();

Each of these objects—number1, number2, and number3—is constructed separately and is allocated its own resources. There are actually three separate instances of the integer count. When number1 changes its attribute count, this in no way affects the copy of count in object number2 or object number3. In this case, integer count is an object attribute.

You can play some interesting games with scope. Take a look at the following code:

public class Number {

  int count;

  public method1() {
    int count;
  }

  public method2() {
    int count;
  }

}

In this case, there are actually three separate memory locations of count for each object. The object owns one copy, and method1() and method2() each own a copy of their own.

To access the object variable from within one of the methods, say method1(), you can use the following code:

public method1() {
    int count;

    this.count = 1;
}

Notice that there is some code that looks a bit weird:

this.count = 1;

The selection of the word this as a keyword is perhaps unfortunate. However, we must live with it. The use of the this keyword directs the compiler to access the object variable count, and not the local variables within the method bodies.

The this Keyword

In Java, the keyword this is a reference to the current object.

Class Attributes

As mentioned earlier, it is possible for two or more objects to share attributes. In Java, C#, and C++, you do this by making the attribute static:

public class Number {

  static int count;

  public method1() {
  }

}

By declaring count as static, this attribute is allocated a single piece of memory for the class. Thus, all objects of the class use the same memory location for count. Essentially, each class has a single copy, which is shared by all objects of that class (see Figure 3.7).

Figure 3.7Figure 3.7 Class attributes.

There are many valid uses for class attributes; however, you must be aware of potential synchronization problems. Let's instantiate two Count objects:

Count Count1 = new Count();
Count Count2 = new Count();

For the sake of argument, let's say that the object Count1 is going merrily about its way and is using count as a means to keep track of the pixels on a computer screen. This is not a problem until the object Count2 decides to use attribute count to keep track of sheep. The instant that Count2 records its first sheep, the data that Count1 was saving is lost.

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