Click here to close now.

Welcome!

Java Authors: Elizabeth White, Esmeralda Swartz, Brian Vandegrift, Liz McMillan, Pat Romanski

Related Topics: Java

Java: Article

Java Collections

Managing collections

As Jason Bell pointed out in his editorial "A Modern Day Cinderella" (JDJ, Vol. 8, issue 9), the spotlight is on J2EE and as a result many programmers are ignoring the foundation of the JDK. J2SE is the Java equivalent of C/C++ standard libraries. Here we deal with the lower-level entities, like the Number types, Integer, Long, Float, and Double.

The Java Collections Framework (JCF) should be your first choice when faced with the task of managing any type of collection. The Collections API is one of the most useful parts of the JDK. Looking back at the projects I've worked on over the past 13 years, to some degree all of them involved managing collections of data structures.

In this article I'll review the collections architecture. I'll also point out some of the useful features of the collections API (sorting and searching). To begin I'll go over the class categories, followed by a more detailed explanation of each. I encourage you to review Sun's documentation at http://java.sun.com/products/jdk/1.2/docs/guide/collections/reference.html.

There are explicit class categories in the Collections Framework. The J2SE Collections Framework consists of interfaces, abstract base classes, and concrete implementations that provide a rich set of functionality for us. The implementations are the classes your application should be utilizing behind the scenes. There are implementations based on maps and others that are backed by arrays. You can make your collection read-only or you can add support for multithreaded access. How is a programmer to decide which entity to use? There are two main criteria: thread safety and usage semantics.

Usage semantics can be further broken down into collection- or map-based access. The library makes a distinction. The Map interface is not related to any of the Collection interfaces, because its main purpose in life is to map a key to an object, while the collection is just a loosely associated group of objects.

Interfaces
Figure 1 provides a class diagram showing the interfaces that make up the Collections API. The interfaces represent the ideal types you should be passing around in your application.

 

I strongly urge you to expose only the interfaces to clients of your classes. If you don't do this and instead pass around references to the concrete implementations, your code will become brittle due to the number of changes required to swap out one interface implementation for another. You should strive to expose the most general interface. For example, if a method is to return an ArrayList, first look and see if the methods exposed by the Collection interface will meet the needs of the intended usage (see Table 1). By doing this, you give yourself the opportunity to modify your method to return a LinkedList or any other type supporting the Collection interface. Who knows? You may even want to provide your own implementation of a Balanced Tree, and if you are instantiating and passing around references to a TreeMap, you'll have to alter the code at each reference.

 

Sets
The semantics of Sets are close to those of Lists. However, Sets lack the notion of direct random access. A Set is just a collection of objects that you may iterate over. A useful feature of Sets is that they do not allow duplicates as long as you override hashCode() and equals() from Object. Listings 1-3 provide a short program that will illustrate this effect. There is the main HashSetExample and two Person classes: one that does not override Object.equals()/hashCode() and one that does.

Running this program produces the follow output.

[000-11-1111, 222-23-1234, 000-11-1111]
[000-11-1111, 222-23-1234]

To remove duplicates your classes must override equals() from java.lang.Object. According to the Javadoc, overriding Object.hashCode() has more to do with performance. Interestingly, Sun's "Introduction to the Collections Framework Short Course" mentions overriding only the Object.hashCode(). Beware that if you follow the tutorial to the letter, you'll still have duplicate entries. You must override Object.equals(), as I've done in Listing 2, to prevent duplicates in your Sets.

How about sorting this list? TreeSet can do that for us, but we still have a choice to make. Will we be sorting by the natural order or do we want an ad hoc sort? For this article we'll examine the ad hoc sort (to implement your own natural order, your class should implement the Comparable interface). We can achieve an arbitrary sort order by utilizing the Comparator interface. When we employ a comparator, it's passed to the sorting object. First, we need to create our sorting algorithm (see Listing 4).

Now we can give this algorithm to the other implementation of Set; TreeSet. We add the following code to our main method at line 29 in Listing 4.

29
30 Set sortedSet =
31 new TreeSet
32 (new PersonComparator());
33
34 sortedSet.addAll(set);
35
36 // sorted
37 System.out.println(sortedSet);

Now the output becomes:

[000-11-1111, 222-23-1234, 000-11-1111]
[000-11-1111, 222-23-1234]
[222-23-1234, 000-11-1111]

Cool, eh? I won't go over this for each implementation. You should be able to apply this concept to any of the other sorting containers or utility methods (from Arrays or Collections). It's worth pointing out that even if you don't override either method, the TreeSet will use the Comparator and eliminate duplicates in the sorted set. Figure 2 provides a class diagram for the Set category.

 

The LinkedHashSet is a special implementation of HashSet that supports list operations without directly implementing the List interface. LinkedHashSet will maintain the insertion order of the list elements, yet still allow you to access elements via a key, such as a traditional Map. And, as the name implies, it is a Set that supports all of Set's operations.

Lists
Collection's other category is List; the implementations are ArrayList and LinkedList (see Figure 3).

 

The List interface supports the notion of direct index-based access to the entries, allows duplicates, and defines an order. Direct index-based access is realized via the get(int) method, which accepts an index as the only argument. You may even acquire a subset of the List by specifying a "from index" and a "to index", the semantics of which closely follow that of String. The element at "from index" will be included in the sublist while the element at "to index" will not.

ArrayList should be preferred if you don't require the ability to insert elements into the middle of the List (you're always adding to the end of the List) and you require random access to the elements. However, if you need to insert elements into the List and sequential access is your main concern, then LinkedList will be better.

Maps
Finally, we get to the Map category (see Figure 4). As mentioned earlier, Map is not related to any of the Collection classes. This is because the JCF authors wanted to make a clear distinction between Collections and Maps. The most notable difference is that Maps do not support index-based access semantics. What is the nth element of a Map?

 

If a Map is a Collection, what are the elements? The only reasonable answer is "Key-value pairs," but this provides a very limited (and not particularly useful) Map abstraction. You can't ask what value a given key maps to, nor can you delete the entry for a given key without knowing what value it maps to.

The workhorse of this category is HashMap. For inserting, deleting, and accessing elements, HashMap offers the best implementation. TreeMap is the sorted version and offers the ability to traverse the contents of the Map in a determined order.

As with the HashSet earlier, HashMap will require you to override Object.equals() and have a defined Object.hashCode() implementation on your own classes. And, of course, the objects you place in TreeMap should be comparable [or you must use the TreeMap(Comparator) constructor].

As with Sets, there's a special implementation of Map that supports a List-like view. LinkedHashMap provides for the same deterministic ordering as LinkedHashSet and supports all Map operations.

There's a another specialized implementation of Map, WeakHashMap, that uses weak references. By employing WeakReference, the garbage collector is able to destroy objects despite the Map's reference. If no other thread holds a reference to a key in the WeakHashMap, the garbage collector is free to collect the key-value pair.

Abstractions
The framework offers several opportunities for creating your own collection classes. The abstractions are for those instances where you want a more application-specific collection. There are several abstract classes implementing the interfaces with enough basic functionality to make your task less painful (see Figure 5).

 

In general, you won't be extending these classes unless you want to try some new algorithm or storage technique. Most likely you should turn your attention to the wrapper classes as implemented by the Collections class. Using the Decorator pattern, as these classes do, you may create highly specialized versions of the containers. There's an excellent example in the group of classes created by Piet Jonas for detecting type errors. Using Piet's classes, it's possible to have an exception thrown if an incorrect type is inserted into a collection. These classes employ the exact same design as the specialized wrappers available in the synchronization and read-only methods that I'll discuss next.

java.util.Collections API
Did you know that many of the Vector's methods are declared with the synchronized modifier? Are you aware of the cost of synchronization? While there have been advancements in many JVMs, there is still a slight overhead incurred with synchronization.

Unless several different threads might access your collection, forget about any of the thread-safe implementations. Use one of the nonthread-safe implementations, like ArrayList or HashMap. If you need index-based access, use the ArrayList. If you are more concerned about key-based access, use the HashMap.

While I may mention Vector and Hashtable from time to time, you should be aware that these two classes are now referred to as legacy code. The API has been reworked of late and all of the collection APIs are now unsynchronized. Special synchronized wrappers have been implemented (and hidden from us) for creating polymorphic, thread-safe implementations of the unsynchronized classes. You gain access to these thread-safe versions via static methods on the Collections class.

Collection Collections.synchronizedCollection(Collection);
List Collections.synchronizedList(List);
Map Collections.synchronizedMap(Map);
Set Collections.synchronizedSet(Set);
SortedMap Collections.synchronizedSortedMap(SortedMap);
SortedSet Collections.synchronizedSortedSet(SortedSet);

Notice that all of these methods accept the most general interface and return the same interface. If you make judicious use of these generalities, you'll be able to swap out implementations relatively painlessly. Now keep in mind that in theory, the implementation of collections shall be free to do whatever it wants. You don't want your code dependent upon J2SE source. If you insist on using the concrete classes, you'll have to downcast to use the results from the previous methods. Downcasting requires knowledge of implementation. Things will change over time. Try to insulate yourself from potential change points. The entire Collections Framework wreaks polymorphism, so take advantage of it, as polymorphism is a good thing.

I performed a small test to compare ArrayList, SynchronizedList, and Vector, all three of which implement the List interface. The results show that for synchronized updates, Vector is the worst performer, while SynchronizedList is much faster. Both are compared to the unsynchronized ArrayList. The test involved completing a read (get) or write (add) operation in a tight loop, 100,000 times (see Table 2).

 

Comparing Vector to SynchronizedList shows that Vector takes 138% and 40% more time than the same operations on SynchronizedList. Meanwhile, SynchronizedList takes a 12% hit over ArrayList for read operations, compared to the 167% increase for Vectors. Some people might be confused by the lack of symmetry in the numbers. If we want to compare A to B, the proper equation is (A - B)/B. Therefore if I want to compare 2 to 6, then (2 - 6)/6 gives -0.6667 or -66%. If I compare 6 to 2, then (6 - 2)/2 gives 2 or 200%. This may seem counterintuitive to saying 6 is three times as large as 2 (which is just a simple ratio, not a comparison).

All of the collections support iterator semantics. Some will bark at you if the underlying collection is altered while you are accessing the iterator by throwing a ConcurrentModification exception.

The static class Collections has many other useful methods for converting to and from certain types of collections. Of interest are those dealing with the creation of unmodifiable collections. First you create your collection and then pass it into the appropriate method and your collection is transformed into something that looks just like the original, but now it will throw an exception if anyone attempts to add or delete an object. Inner classes in Collections that simply extend the standard collection class and override the modifiers accomplish this. Now you can implement the Command pattern and employ the concept of read-only parameters and return structures. In a language that deals exclusively with object references, that's a nice-to-have feature.

List Collections.unmodifiableList(List);
Map Collections.unmodifiableMap(Map);
Set Collections.unmodifiableSet(Set);
SortedMap Collections.unmodifiableSortedMap(SortedMap);
SortedSet Collections.unmodifiableSortedSet(SortedSet);

Sorting has been taken care of with a "tuned" implementation of Merge Sort. There are routines for sorting primitives and objects. You can implement classes that have a natural order by extending Comparable. If inheritance is at a premium, use the Comparator interface. C++ programmers will feel right at home with this idiom from the STL. There are even collections that are themselves sorted. SortedTree allows you to add objects that will be sorted on the fly. The API is so flexible that you can implement the natural order.

Other utility methods in collections have to do with searching a List. The Collections class offers two binary searching methods.

Object Collections.binarySearch(List list, Object key);
Object Collections.binarySearch(List list, Object key, Comparator comparator);

These two methods, one of which employs the natural sort order of the list and the other the ad hoc, run in log(n) time where n is the number of elements in the list. However, this is true only if the list passed in implements the RandomAccess interface. Otherwise, if the list does not implement RandomAccess and is large, the search will execute an iterator-based binary search, which according to the Javadoc will "perform O(n) link traversals and O(log n) element comparisons."

Figure 6 shows the big picture with the preferred extension points highlighted. We've discussed the general categories: Collection (Set, List) and Map. We've played around a little and have seen that to take full advantage of some collections, we have to override Object.equals and Object.hashCode. Also, we went over some of the performance tradeoffs of a couple of implementations. I should mention that there are other Collection APIs available to Java programmers. There's the popular JGL and the JDSL. I haven't looked at the JGL but I have played around with the academic version of the JDSL. The JDSL gives you all those nifty data structures you talked about in your junior year algorithms class.

 

There are some new collections available in the latest JCF: LinkedHashSet, LinkedHashMap, and IdentityHashMap. In general they are highly specialized versions of the core JCF classes.

Conclusion
This article should prompt you to take another look at the Collections Framework and, if you are lucky, you'll see something that fits with your current development task. If you are really lucky, perhaps you'll see something else in the J2SE libraries that you never knew existed, collection related or not. Unfortunately, there doesn't seem to be any J2SE champion at Sun, so you'll have to make an effort to scan through the API's Javadoc every so often and perhaps even the source code as well (there are some novel snippets in there).

More Stories By David McReynolds

David McReynolds has been programming for over 12 years and is currently employed by Daugherty Business Solutions as a consultant. He has an MS in computer science from Southern Polytechnic State University.

Comments (6) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
Abelardo 02/20/04 01:36:24 PM EST

I would like to nominate this article for the "best of the year"

David McReynolds 02/13/04 12:53:04 PM EST

Thanks Bill. I know I ran the code so perhaps I missed it during the cut-and-paste operation.

Bill Sommers 02/12/04 01:32:56 PM EST

This is a very nice article. I did find what appears to be an error in the source code. The Person class is missing a getSSN() method. I fixed the problem by writing this method:

public Object getSSN() {
return this.toString();
}

Troll Fiddler 02/10/04 05:13:27 AM EST

Superbly written with lovely clear diagrams. Good work. Not many authors explain things this well.

As Einstein said "if you can''t explain your work to a 10 year old, you''re a charlatan." This author is no charlatan.

Bee 02/05/04 03:07:11 PM EST

Ditto! what Shilpi said,This author is very well organized and excelent detail!

shilpi 02/05/04 03:00:42 PM EST

One of the most articulate and intelligent publishings I have read in a long time. I look forward to reading and hearing more from this author.

@ThingsExpo Stories
SYS-CON Events announced today that robomq.io will exhibit at SYS-CON's @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. robomq.io is an interoperable and composable platform that connects any device to any application. It helps systems integrators and the solution providers build new and innovative products and service for industries requiring monitoring or intelligence from devices and sensors.
Wearable technology was dominant at this year’s International Consumer Electronics Show (CES) , and MWC was no exception to this trend. New versions of favorites, such as the Samsung Gear (three new products were released: the Gear 2, the Gear 2 Neo and the Gear Fit), shared the limelight with new wearables like Pebble Time Steel (the new premium version of the company’s previously released smartwatch) and the LG Watch Urbane. The most dramatic difference at MWC was an emphasis on presenting wearables as fashion accessories and moving away from the original clunky technology associated with t...
The list of ‘new paradigm’ technologies that now surrounds us appears to be at an all time high. From cloud computing and Big Data analytics to Bring Your Own Device (BYOD) and the Internet of Things (IoT), today we have to deal with what the industry likes to call ‘paradigm shifts’ at every level of IT. This is disruption; of course, we understand that – change is almost always disruptive.
SYS-CON Events announced today that Solgenia will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY, and the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Solgenia is the global market leader in Cloud Collaboration and Cloud Infrastructure software solutions. Designed to “Bridge the Gap” between Personal and Professional Social, Mobile and Cloud user experiences, our solutions help large and medium-sized organizations dr...
The world's leading Cloud event, Cloud Expo has launched Microservices Journal on the SYS-CON.com portal, featuring over 19,000 original articles, news stories, features, and blog entries. DevOps Journal is focused on this critical enterprise IT topic in the world of cloud computing. Microservices Journal offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. Follow new article posts on Twitter at @MicroservicesE
SYS-CON Events announced today the IoT Bootcamp – Jumpstart Your IoT Strategy, being held June 9–10, 2015, in conjunction with 16th Cloud Expo and Internet of @ThingsExpo at the Javits Center in New York City. This is your chance to jumpstart your IoT strategy. Combined with real-world scenarios and use cases, the IoT Bootcamp is not just based on presentations but includes hands-on demos and walkthroughs. We will introduce you to a variety of Do-It-Yourself IoT platforms including Arduino, Raspberry Pi, BeagleBone, Spark and Intel Edison. You will also get an overview of cloud technologies s...
SYS-CON Events announced today that SafeLogic has been named “Bag Sponsor” of SYS-CON's 16th International Cloud Expo® New York, which will take place June 9-11, 2015, at the Javits Center in New York City, NY. SafeLogic provides security products for applications in mobile and server/appliance environments. SafeLogic’s flagship product CryptoComply is a FIPS 140-2 validated cryptographic engine designed to secure data on servers, workstations, appliances, mobile devices, and in the Cloud.
After making a doctor’s appointment via your mobile device, you receive a calendar invite. The day of your appointment, you get a reminder with the doctor’s location and contact information. As you enter the doctor’s exam room, the medical team is equipped with the latest tablet containing your medical history – he or she makes real time updates to your medical file. At the end of your visit, you receive an electronic prescription to your preferred pharmacy and can schedule your next appointment.
Containers and microservices have become topics of intense interest throughout the cloud developer and enterprise IT communities. Accordingly, attendees at the upcoming 16th Cloud Expo at the Javits Center in New York June 9-11 will find fresh new content in a new track called PaaS | Containers & Microservices Containers are not being considered for the first time by the cloud community, but a current era of re-consideration has pushed them to the top of the cloud agenda. With the launch of Docker's initial release in March of 2013, interest was revved up several notches. Then late last...
The WebRTC Summit 2014 New York, to be held June 9-11, 2015, at the Javits Center in New York, NY, announces that its Call for Papers is open. Topics include all aspects of improving IT delivery by eliminating waste through automated business models leveraging cloud technologies. WebRTC Summit is co-located with 16th International Cloud Expo, @ThingsExpo, Big Data Expo, and DevOps Summit.
SOA Software has changed its name to Akana. With roots in Web Services and SOA Governance, Akana has established itself as a leader in API Management and is expanding into cloud integration as an alternative to the traditional heavyweight enterprise service bus (ESB). The company recently announced that it achieved more than 90% year-over-year growth. As Akana, the company now addresses the evolution and diversification of SOA, unifying security, management, and DevOps across SOA, APIs, microservices, and more.
GENBAND has announced that SageNet is leveraging the Nuvia platform to deliver Unified Communications as a Service (UCaaS) to its large base of retail and enterprise customers. Nuvia’s cloud-based solution provides SageNet’s customers with a full suite of business communications and collaboration tools. Two large national SageNet retail customers have recently signed up to deploy the Nuvia platform and the company will continue to sell the service to new and existing customers. Nuvia’s capabilities include HD voice, video, multimedia messaging, mobility, conferencing, Web collaboration, deskt...
SYS-CON Media announced today that @WebRTCSummit Blog, the largest WebRTC resource in the world, has been launched. @WebRTCSummit Blog offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. @WebRTCSummit Blog can be bookmarked ▸ Here @WebRTCSummit conference site can be bookmarked ▸ Here
SYS-CON Events announced today that Cisco, the worldwide leader in IT that transforms how people connect, communicate and collaborate, has been named “Gold Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Cisco makes amazing things happen by connecting the unconnected. Cisco has shaped the future of the Internet by becoming the worldwide leader in transforming how people connect, communicate and collaborate. Cisco and our partners are building the platform for the Internet of Everything by connecting the...
Temasys has announced senior management additions to its team. Joining are David Holloway as Vice President of Commercial and Nadine Yap as Vice President of Product. Over the past 12 months Temasys has doubled in size as it adds new customers and expands the development of its Skylink platform. Skylink leads the charge to move WebRTC, traditionally seen as a desktop, browser based technology, to become a ubiquitous web communications technology on web and mobile, as well as Internet of Things compatible devices.
Docker is an excellent platform for organizations interested in running microservices. It offers portability and consistency between development and production environments, quick provisioning times, and a simple way to isolate services. In his session at DevOps Summit at 16th Cloud Expo, Shannon Williams, co-founder of Rancher Labs, will walk through these and other benefits of using Docker to run microservices, and provide an overview of RancherOS, a minimalist distribution of Linux designed expressly to run Docker. He will also discuss Rancher, an orchestration and service discovery platf...
SYS-CON Events announced today that Vitria Technology, Inc. will exhibit at SYS-CON’s @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Vitria will showcase the company’s new IoT Analytics Platform through live demonstrations at booth #330. Vitria’s IoT Analytics Platform, fully integrated and powered by an operational intelligence engine, enables customers to rapidly build and operationalize advanced analytics to deliver timely business outcomes for use cases across the industrial, enterprise, and consumer segments.
SYS-CON Events announced today that Liaison Technologies, a leading provider of data management and integration cloud services and solutions, has been named "Silver Sponsor" of SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York, NY. Liaison Technologies is a recognized market leader in providing cloud-enabled data integration and data management solutions to break down complex information barriers, enabling enterprises to make smarter decisions, faster.
@ThingsExpo has been named the Top 5 Most Influential M2M Brand by Onalytica in the ‘Machine to Machine: Top 100 Influencers and Brands.' Onalytica analyzed the online debate on M2M by looking at over 85,000 tweets to provide the most influential individuals and brands that drive the discussion. According to Onalytica the "analysis showed a very engaged community with a lot of interactive tweets. The M2M discussion seems to be more fragmented and driven by some of the major brands present in the M2M space. This really allows some room for influential individuals to create more high value inter...
SYS-CON Events announced today that Akana, formerly SOA Software, has been named “Bronze Sponsor” of SYS-CON's 16th International Cloud Expo® New York, which will take place June 9-11, 2015, at the Javits Center in New York City, NY. Akana’s comprehensive suite of API Management, API Security, Integrated SOA Governance, and Cloud Integration solutions helps businesses accelerate digital transformation by securely extending their reach across multiple channels – mobile, cloud and Internet of Things. Akana enables enterprises to share data as APIs, connect and integrate applications, drive part...