Home > Articles > Software Development & Management

Implementation Patterns: Collections

Kent Beck discusses the metaphors behind collections, the issues to be expressed through the use of collections, the collection interfaces and what they mean to the reader, the collection implementations and what they say, an overview of functions available in the Collections class, and finally a discussion of extending collections through inheritance.
This chapter is from the book

This chapter is from the book

I must say that I didn’t expect this chapter to amount to much. When I started writing it, I thought I would end up with an API document—types and operations. The basic idea is simple: a collection distinguishes between objects in the collection and those not in the collection. What more was there to say?

What I discovered is that collections are a far richer topic than I ever suspected, both in their structure and the possibilities they offer for communicating intent. The concept of collections blends several different metaphors. The metaphor you emphasize changes how you use collections. Each of the collection interfaces communicates a different variation on the theme of a sack of objects. Each of the implementations also communicates variations, mostly with regard to performance. The result is that mastering collections is a big part of learning to communicate well with code.

Collection-like behavior used to be implemented by providing links in the data structure itself: each page in a document would have links to the previous and next pages. More recently, the fashion has swung to using a separate object for the collection which relates elements. This allows the flexibility to put the same object in several different collections without modifying the object.

Collections are important because they are a way of expressing one of the most fundamental kinds of variation in programming, the variation of number. Variation in logic is expressed with conditionals or polymorphic messages. Variation in the cardinality of data is expressed by putting the data into a collection. The precise details of that collection reveal much about the intention of the original programmer to a reader.

There is an old (by computer terms) saying that the only interesting numbers are 0, 1 and many (this saying was not written by a numerical analyst). If the absence of a field expresses “zero” and the presence of a field expresses “one”, then a field holding a collection is a way of expressing “many”.

Collections hover in a strange world halfway between a programming language construct and a library. Collections are so universally useful, and their use is so well understood, that it almost seems time to have a mainstream language that allows statements like plural unique Book books; instead of the current Collection<Book> books= new HashSet<Book>();. Until collections are first-class language elements, it is important to know how to use the current collection library to express common ideas in straightforward ways.

The remainder of the chapter is divided into six parts: the metaphors behind collections, the issues to be expressed through the use of collections, the collection interfaces and what they mean to the reader, the collection implementations and what they say, an overview of functions available in the Collections class, and finally a discussion of extending collections through inheritance.


As suggested above, collections blend different metaphors. The first is that of a multi-valued variable. There is a sense in which a variable that refers to a collection is really a variable referring to several objects at the same time. Looked at this way, the collection disappears as a separate object. The collection’s identity is not interesting, only the objects to which it refers. As with all variables, you can assign to a multi-valued variable (add and remove elements), retrieve its value, and send the variable messages (with the for loop).

The multi-valued variable metaphor breaks down in Java because collections are separate objects with identity. The second metaphor mixed into collections is that of objects—a collection is an object. You can retrieve a collection, pass it around, test it for equality, and send it messages. Collections can be shared between objects, although this creates the possibility of aliasing problems. Because collections are a set of related interfaces and implementations, they are open to extension, both with expanded interfaces and new implementations. So, just as collections “are” multi-valued variables, they also “are” objects.

The combination of the two metaphors makes for some strange effects. Because a collection is implemented as an object that can be passed around, you get the equivalent of call-by-reference, where instead of passing a variable’s contents to a routine, you pass the variable itself. Changes to the variable’s value are reflected in the calling routine. Call-by-reference went out of fashion in language design a couple of decades ago because of the possibility for unintended consequences. It was hard to debug programs when you couldn’t be certain of all the places where a variable could be modified. Some of the conventions for programming with collections exist to avoid situations where it is hard to read the code and predict where a collection could be modified.

A third metaphor useful for thinking about collections is that of mathematical sets. A collection is a sack of objects just like a mathematical set is a sack of elements. A set divides the world into things in the set and things not in the set. A collection divides the world of objects into objects that are in the collection and objects that are not. Two basic operations on mathematical sets are finding their cardinality (the size() method of collections) and testing for inclusion (represented by the contains() method).

The mathematical metaphor is only approximate for collections. The other basic operations on sets—union, intersection, difference, and symmetric difference—are not directly represented by collections. Whether this is because these operations are intrinsically less useful or because they aren’t used because they aren’t available makes for an interesting debate.

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.


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.


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.


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.


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


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


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.


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.


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