Welcome!

Java IoT Authors: Stackify Blog, Pat Romanski, Matt Lonstine, Glenda Sims, Paul Simmons

Related Topics: Java IoT

Java IoT: 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
"Space Monkey by Vivent Smart Home is a product that is a distributed cloud-based edge storage network. Vivent Smart Home, our parent company, is a smart home provider that places a lot of hard drives across homes in North America," explained JT Olds, Director of Engineering, and Brandon Crowfeather, Product Manager, at Vivint Smart Home, in this SYS-CON.tv interview at @ThingsExpo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that Conference Guru has been named “Media Sponsor” of the 22nd International Cloud Expo, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. A valuable conference experience generates new contacts, sales leads, potential strategic partners and potential investors; helps gather competitive intelligence and even provides inspiration for new products and services. Conference Guru works with conference organizers to pass great deals to gre...
The Internet of Things will challenge the status quo of how IT and development organizations operate. Or will it? Certainly the fog layer of IoT requires special insights about data ontology, security and transactional integrity. But the developmental challenges are the same: People, Process and Platform. In his session at @ThingsExpo, Craig Sproule, CEO of Metavine, demonstrated how to move beyond today's coding paradigm and shared the must-have mindsets for removing complexity from the develop...
In his Opening Keynote at 21st Cloud Expo, John Considine, General Manager of IBM Cloud Infrastructure, led attendees through the exciting evolution of the cloud. He looked at this major disruption from the perspective of technology, business models, and what this means for enterprises of all sizes. John Considine is General Manager of Cloud Infrastructure Services at IBM. In that role he is responsible for leading IBM’s public cloud infrastructure including strategy, development, and offering m...
"Evatronix provides design services to companies that need to integrate the IoT technology in their products but they don't necessarily have the expertise, knowledge and design team to do so," explained Adam Morawiec, VP of Business Development at Evatronix, in this SYS-CON.tv interview at @ThingsExpo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
To get the most out of their data, successful companies are not focusing on queries and data lakes, they are actively integrating analytics into their operations with a data-first application development approach. Real-time adjustments to improve revenues, reduce costs, or mitigate risk rely on applications that minimize latency on a variety of data sources. In his session at @BigDataExpo, Jack Norris, Senior Vice President, Data and Applications at MapR Technologies, reviewed best practices to ...
Widespread fragmentation is stalling the growth of the IIoT and making it difficult for partners to work together. The number of software platforms, apps, hardware and connectivity standards is creating paralysis among businesses that are afraid of being locked into a solution. EdgeX Foundry is unifying the community around a common IoT edge framework and an ecosystem of interoperable components.
Large industrial manufacturing organizations are adopting the agile principles of cloud software companies. The industrial manufacturing development process has not scaled over time. Now that design CAD teams are geographically distributed, centralizing their work is key. With large multi-gigabyte projects, outdated tools have stifled industrial team agility, time-to-market milestones, and impacted P&L stakeholders.
"Akvelon is a software development company and we also provide consultancy services to folks who are looking to scale or accelerate their engineering roadmaps," explained Jeremiah Mothersell, Marketing Manager at Akvelon, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
"IBM is really all in on blockchain. We take a look at sort of the history of blockchain ledger technologies. It started out with bitcoin, Ethereum, and IBM evaluated these particular blockchain technologies and found they were anonymous and permissionless and that many companies were looking for permissioned blockchain," stated René Bostic, Technical VP of the IBM Cloud Unit in North America, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Conventi...
In his session at 21st Cloud Expo, Carl J. Levine, Senior Technical Evangelist for NS1, will objectively discuss how DNS is used to solve Digital Transformation challenges in large SaaS applications, CDNs, AdTech platforms, and other demanding use cases. Carl J. Levine is the Senior Technical Evangelist for NS1. A veteran of the Internet Infrastructure space, he has over a decade of experience with startups, networking protocols and Internet infrastructure, combined with the unique ability to it...
22nd International Cloud Expo, taking place June 5-7, 2018, at the Javits Center in New York City, NY, and co-located with the 1st DXWorld Expo will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud ...
"Cloud Academy is an enterprise training platform for the cloud, specifically public clouds. We offer guided learning experiences on AWS, Azure, Google Cloud and all the surrounding methodologies and technologies that you need to know and your teams need to know in order to leverage the full benefits of the cloud," explained Alex Brower, VP of Marketing at Cloud Academy, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clar...
Gemini is Yahoo’s native and search advertising platform. To ensure the quality of a complex distributed system that spans multiple products and components and across various desktop websites and mobile app and web experiences – both Yahoo owned and operated and third-party syndication (supply), with complex interaction with more than a billion users and numerous advertisers globally (demand) – it becomes imperative to automate a set of end-to-end tests 24x7 to detect bugs and regression. In th...
"MobiDev is a software development company and we do complex, custom software development for everybody from entrepreneurs to large enterprises," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
Coca-Cola’s Google powered digital signage system lays the groundwork for a more valuable connection between Coke and its customers. Digital signs pair software with high-resolution displays so that a message can be changed instantly based on what the operator wants to communicate or sell. In their Day 3 Keynote at 21st Cloud Expo, Greg Chambers, Global Group Director, Digital Innovation, Coca-Cola, and Vidya Nagarajan, a Senior Product Manager at Google, discussed how from store operations and ...
"There's plenty of bandwidth out there but it's never in the right place. So what Cedexis does is uses data to work out the best pathways to get data from the origin to the person who wants to get it," explained Simon Jones, Evangelist and Head of Marketing at Cedexis, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that CrowdReviews.com has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5–7, 2018, at the Javits Center in New York City, NY. CrowdReviews.com is a transparent online platform for determining which products and services are the best based on the opinion of the crowd. The crowd consists of Internet users that have experienced products and services first-hand and have an interest in letting other potential buye...
SYS-CON Events announced today that Telecom Reseller has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. Telecom Reseller reports on Unified Communications, UCaaS, BPaaS for enterprise and SMBs. They report extensively on both customer premises based solutions such as IP-PBX as well as cloud based and hosted platforms.
It is of utmost importance for the future success of WebRTC to ensure that interoperability is operational between web browsers and any WebRTC-compliant client. To be guaranteed as operational and effective, interoperability must be tested extensively by establishing WebRTC data and media connections between different web browsers running on different devices and operating systems. In his session at WebRTC Summit at @ThingsExpo, Dr. Alex Gouaillard, CEO and Founder of CoSMo Software, presented ...