Home > Articles > Programming > Java

This chapter is from the book

This chapter is from the book

Why Enumerate a Type?

Here's the older approach of simulating enumerations:

class Bread {
    static final int wholewheat = 0;
    static final int ninegrain = 1;
    static final int rye = 2;
    static final int french = 3;
}

You would then declare an int variable and let it hold values from the Bread class, e.g.

int todaysLoaf = Bread.rye;

Drawbacks of using ints to enumerate

Using final ints to represent values in an enumeration has at least three drawbacks.

  • All the compiler tools (debugger, linker, run-time, etc.) still regard the variables as ints. They are ints. If you ask for the value of todaysLoaf, it will be 2, not "rye". The programmer has to do the mapping back and forth mentally.

  • The variables aren't typesafe. There's nothing to stop todaysLoaf getting assigned a value of 99 that doesn't correspond to any Bread value. What happens next depends on how well the rest of your code is written, but the best case is that some routine notices pretty quickly and throws an exception. The worst case is that your computer-controlled bakery tries to bake "type 99" bread causing an expensive sticky mess.

  • Use of integer constants makes code "brittle" (easily subject to breakage). The constants get compiled into every class that use them. If you update the class where the constants are defined, you must go and find all the users of that class, and recompile them against the new definitions. If you miss one, the code will run but be subject to subtle bugs.

How enums solve these issues

Enumerated types were introduced with JDK 1.5 to address these limitations. Variables of enumerated types

  • Are displayed to the programmer or user as Strings, not numbers

  • Can only hold values defined in the type

  • Do not require clients to be recompiled when the enumeration changes

Enumerated types are written using a similar syntax to class declarations, and you should think of them as being a specialized sort of class. An enumerated type definition can go in a file of its own, or in a file with other classes. A public enumeration must be in a file of its own with the name matching the enumeration name.

You might create an enumerated type to represent some bread flavors. It would be defined like this:

enum Bread { wholewheat, ninegrain, rye, french }

That lets you declare variables of type Bread in the usual way:

Bread todaysLoaf;

You can assign an enum value to a variable of Bread type like this:

todaysLoaf = Bread.rye;

All the language tools know about the symbolic names. If you print out a Bread variable, you get the string value, not whatever numeric constant underlies it internally.

System.out.println("bread choice is: " + todaysLoaf);

This results in output of:

bread choice is: rye

How enums are implemented

Under the covers, enum constants are static final objects declared (by the compiler) in their enum class. An enum class can have constructors, methods, and data. Enum variables are merely pointers to one of the static final enum constant objects.

You'll understand enums better if you know that the compiler treats them approximately the same way as if it had seen this source code:

class Bread extends Enum {
   // constructor
   public Bread(String name, int position) { super(name, position); }

   public static final Bread wholewheat = new Bread("wholewheat",0);
   public static final Bread ninegrain = new Bread("ninegrain", 1);
   public static final Bread rye = new Bread("rye", 2);
   public static final Bread french = new Bread("french", 3);

   // more stuff here
}

This is an approximation because the parent class, java.lang.Enum, uses a generic parameter, and we cover generics later. Bringing generics into the definition of Enum was an unnecessary complication aimed at improving the type-safety of enums. The work could and should have been moved into the compiler. The previous code should give you a good idea of how enums are represented.

Namespaces in Java and in enumerations

Namespace isn't a term that occurs in the Java Language Specification. Instead, it's a compiler term meaning "place where a group of names are organized as a whole." Some older languages only have one global namespace that holds the names of all methods and variables. Along with each name, the compiler stores information about what type it is, and other details.

Java has many namespaces. All the members in a class form a namespace. All the variables in a method form a namespace. A package forms a namespace. Even a local block inside a method forms a namespace.

A compiler will look for an identifier in the namespace that most closely encloses it. If not found, it will look in successively wider namespaces until it finds the first occurrence of the correct identifier. You won't confuse Java if you give the same name to a method, to a data field, and to a label. It puts them in different namespaces. When the compiler is looking for a method name, it doesn't bother looking in the field namespace.

Each enumeration has its own namespace, so it is perfectly valid for enumeration values to overlap with other enums or other variables, like this:

enum Fruit { peach, orange, grapefruit, durian }
enum WarmColor { peach, red, orange }

Some more Java terminology: the enumeration values apple, red, peach, plum and orange are known as enum constants. The enumeration types Fruit and WarmColor are enum types.

Enum constants make software more reliable

Here's an amazing thing: the constants that represent the enumeration values are not compiled into other classes that use the enumeration. Instead, each enum constant (like Bread.rye previously) is left as a symbolic reference that will be linked in at run-time, just like a field or method reference.

If you compile a class that uses Bread.rye, and then later add some other bread flavors at the beginning of the enumeration (pumpernickel and oatmeal), Bread.rye will now have a different numeric value. But you do not need to recompile any classes that use the enumeration. Even better, if you remove an enum constant that is actually being used by some other class (and you forget to recompile the class where it's used), the run-time library will issue an informative error message as soon as you use the now-removed name. This is a significant boost to making Java software more reliable.

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