Home > Articles > Software Development & Management

This chapter is from the book

This chapter is from the book



Readers of collection-based code are looking for answers to different questions when they look at the interfaces you have declared for your variables and the implementations you chose for those variables. The interface declaration tells the reader about the collection: whether the collection is in a particular order, whether there are duplicate elements, and whether there is any way to look up elements by key or only through iteration.

The interfaces described below are:

  • Array—Arrays are the simplest and least flexible collection: fixed size, simple accessing syntax, and fast.
  • Iterable—The basic collection interface, allowing a collection to be used for iteration but nothing else.
  • Collection—Offers adding, removing, and testing for elements.
  • List—A collection whose elements are ordered and can be accessed by their location in the collection (i.e., “give me the third element”).
  • Set—A collection with no duplicates.
  • SortedSet—An ordered collection with no duplicates.
  • Map—A collection whose elements are stored and retrieved by key.


Arrays are the simplest interface for collections. Unfortunately, they don’t have the same protocol as other collections, so it’s harder to change from an array to a collection than from one kind of collection to another. Unlike most collections, the size of an array is fixed when it is created. Arrays are also different as they are built into the language, not provided by a library.

Arrays are more efficient in time and space than other collections for simple operations. The timing tests I did to accompany writing this suggest that array access (i.e. elements[i]) is more than ten times faster than the equivalent ArrayList operation (elements.get(i)). (As these numbers vary substantially in different operating environments, if you care about the performance difference you should time the operations yourself.) The flexibility of the other collection classes makes them more valuable in most cases, but arrays are a handy trick to be able to pull out when you need more performance in a small part of an application.


Declaring a variable Iterable only says that it contains multiple values. Iterable is the basis for the loop construct in Java 5. Any object declared as Iterable can be used in a for loop. This is implemented by quietly calling the method iterator().

One of the issues to be communicated when using collections is whether clients are expected to modify them. Unfortunately, Iterable and its helper, Iterator, provide no way to state declaratively that a collection shouldn’t be modified. Once you have an Iterator, you can invoke its remove() method, which deletes an element from the underlying Iterable. While your Iterables are safe from having elements added, they can have elements removed without the object that owns the collection being notified.

As described in “Collection Accessor Method” on page 91, there are a few ways to ensure that a collection is not modified: wrapping it in a unmodifiable collection, creating a custom iterator that throws an exception when a client tries to modify the collection, or returning a safe copy.

Iterable is simple. It doesn’t even allow you to measure the size of instances; all you can do is iterate over the elements. Sub-interfaces of Iterable provide more useful behavior.


Collection inherits from Iterable, but it adds methods to add, remove, search for and count elements. Declaring a variable or method as a Collection leaves many options for an implementation class. By leaving the declaration as vaguely specified as possible, you retain the freedom to change implementation classes later without having the change ripple through the code.

Collections are a bit like the mathematical notion of sets, except that the operations performing the equivalent of union, intersection, and difference (addAll(), retainAll(), and removeAll()) modify the receiver instead of returning newly allocated collections.


To Collection, List adds the idea that elements are in a stable order. An element can be retrieved by providing its index to the collection. A stable sequence is important when the elements of a collection interact with each other. For example, a queue of messages that should be processed in their arrival order should be stored in a list.


A Set is a collection that contains no duplicates (elements that would report that they are equal() to each other). This corresponds closely to the mathematical notion of set, although the metaphor is thin because adding an element to a Set modifies the collection rather than returning a new collection including the added element.

A Set discards information that most collections keep—the number of times an element appears. This is not a problem in cases where the presence or absence of an element is interesting but the number of times the element appears is not. For example, if I want to know who all the authors of books are in a library, I don’t care how many books each author wrote. I just want to know who they are. A Set is an appropriate way to implement such a query.

The elements in a Set are in no particular order. Just because you iterate through them in a certain order once does not mean that the elements will appear in the same order the next time. This lack of predictable order is not a limitation in cases where the elements don’t interact with each other.

Sometimes you want to store duplicates in a collection but remove them for a particular operation. Create a temporary Set and pass it to the operation:

printAuthors(new HashSet<Author>(getAuthors());


The ordering and uniqueness attributes of collections are not mutually exclusive. At times you’d like to keep a collection in order but eliminate duplicates. SortedSet stores ordered-but-unique elements.

Unlike the ordering of a List, which is related to the order in which elements were added or by explicit indexes passed to add(int, Object), the ordering in a SortedSet is provided by a Comparator. In the absence of an explicit order, the “natural order” of the elements is used. For example, strings are sorted in lexicographical order.

To compute the authors contributing to a library, you could use a SortedSet:

public Collection<String> getAlphabeticalAuthors() {
	SortedSet<String> results= new TreeSet<String>();
	for (Book each: getBooks())
	return results;

This example uses the default sorting of strings. If a Book had its author represented by an object, the code above might look like this:

public Collection<String> getAlphabeticalAuthors() {
	Comparator<Author> sorter= new Comparator<Author>() {
		public int compare(Author o1, Author o2) {
			if (o1.getLastName().equals(o2.getLastName()))
				return o1.getFirstName().compareTo(o2.getFirstName());
			return o1.getLastName().compareTo(o2.getLastName());
	SortedSet<String> results= new TreeSet<String>(sorter);
	for (Book each: getBooks())
	return results;


The final collection interface is Map, which is a hybrid of the other interfaces. A Map stores values by key, but unlike a List, the key can be any object and not just an integer. The keys of a Map must be unique, a bit like sets, although the values can contain duplicates. The elements of a Map are in no particular order, also like a Set.

Because Map is not completely like any of the other collection interfaces, it stands alone, not inheriting from any of them. Maps are two collections at the same time; a collection of keys connected to a collection of values. You can’t simply ask a Map for its iterator, because it is not clear whether you want an iterator over the keys, over the values, or over the pairs of keys-and-values.

Maps are useful for implementing two of the implementation patterns: extrinsic state and variable state. Extrinsic state suggests storing special-purpose data related to an object separately from the object itself. One way to implement extrinsic state is with a Map whose keys are the objects and whose values are the related data. In variable state, different instances of the same class store different data fields. To implement this, have the object hold a map which maps from strings (representing the names of the virtual fields) to the values.

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