Welcome!

Java IoT Authors: Liz McMillan, Elizabeth White, Yakov Fain, Jonathan Fries, Flint Brenton

Related Topics: Java IoT

Java IoT: Article

Interfaces Vs Abstract Classes In Java

Interfaces Vs Abstract Classes In Java

Have you ever wondered why you should use interfaces instead of abstract classes, or vice versa? More specifically, when dealing with generalization, have you struggled with using one or the other? I'll shed some light on what can be a very confusing issue.

To start, I'll identify two pieces of the development puzzle: the behavior of an object and the object's implementation.

When designing an entity that can have more than one implementation, the goal is to describe the entity's behavior in such a way that it can be used without knowing exactly how the entity's behavior is implemented. In essence, you're separating the behavior of an object from its implementation. But is this separation best achieved by way of an interface or by way of an abstract class? Both can define methods without saying how they work. So which one do you use?

Modeling Behavior in an Abstract Class
As a rule, pure behavior is always modeled by interfaces and not in abstract classes. This example will model behavior in an abstract class to illustrate why.

Pretend you're designing a "motor" entity for an application that your sales department will use to sell motors. You're not modeling every aspect and nuance of a motor, but instead modeling what's important for the company and the process you're automating. (You find out what's important by talking to the users of a system. In this case it's your sales department. Good luck!)

Your sales department says that every motor has a horsepower rating, and this feature is the only attribute they're concerned with.

Based on this statement, you describe the following behavior of a motor:

Behavior: Someone can ask the motor for its horsepower rating, and the motor will return its rating as an integer.

At this point you don't know where the horsepower comes from, but you do know that this behavior must exist.

Translated into a method signature this behavior becomes:

public int getHorsepower()

Your company has several different types of motors, but given our particular application, this behavior is the only rule that applies to all of them. You look at both interfaces and abstract classes, but for purposes of illustration the motors will be modeled as an abstract class.

public abstract Motor{
abstract public int getHorsepower();
}
You make a handful of concrete implementations of this class, and version 1.0 of the application enters production.

Time passes and you're called to create version 2.0. While reviewing the requirements for the second version, you find that a small subset of motors is battery-powered, and that these batteries take time to recharge. The sales department wants to be able to view the time to recharge from the computer screen. From their statement, you derive a behavior:

Behavior: Someone can ask a battery-powered motor for its time to recharge and the motor will return its time as an integer.

public int getTimeToRecharge();
Translated into a method signature this behavior becomes:
public abstract BatteryPoweredMotor extends Motor{
abstract public int getTimeToRecharge();
}
The new battery-powered motors are implemented inside the application as concrete classes. It's important to note that these classes extend Battery- PoweredMotor as opposed to Motor. The changes are released as version 2 and the sales department is happy once again.

But business is changing, and soon solar-powered motors are introduced. The sales department tells you that solar-powered motors require a minimum amount of light energy to operate. This light energy is measured in lumens. The customers want to know this information. There's a fear that on cloudy days some solar-powered motors won't operate. The sales department requests that the application be changed to support the new solar-powered motors. From listening to their plight, a behavior is derived.

Behavior: Someone can ask a solar-powered battery for its lumens required to operate and the motor will return an integer.

public int getLumensToOperate();
In an Abstract class
public abstract SolarPoweredMotor extends Motor{
abstract public int getLumensToOperate();
}
Both SolarPoweredMotor and BatteryPoweredMotor extend the abstract class Motor (see Figure 1).

Throughout your application, motors are treated the same in 90% of the code. When you're checking if you have a solar- or battery-powered motor, use instanceof.

if (instanceof SolarPoweredMotor){...} if (instanceof BatteryPoweredMotor){...}
You find out that horsepower is calculated for each type of motor so the getHorsepower() method is overloaded in each of the derived abstract classes. So far, this design looks good...

That is, until you find out that the sales department wants to sell a new type of motor that has both battery and solar power! The behaviors associated with solar- and battery-powered motors haven't changed. The only difference is you have a handful of motors that exhibit both behaviors.

The Problem with Modeling Behavior in an Abstract Class
Here's where the difference between an interface and an abstract class becomes apparent.

The goal is to add these motors with as little rework as possible. After all, code related to battery- and solar-powered motors is well tested and has no known bugs.

You can make a new abstract class that's SolarBatteryPowered but then your motor won't trigger your instanceof when you check for solar- and battery-powered motors. The other option is to make the new motor extend either the SolarPowered or BatteryPowered abstract class. But if you do that, the new motor will lose the functionality of the abstract class it didn't extend. Technically your new motor needs to extend both abstract classes, but you painted yourself into a corner that can be solved only with a lot of special-case coding.

The reason you're having problems is that by using abstract classes you implied not only a behavior hierarchy but a pattern of implementation as well! You modeled how the motors receive their behavior instead of just saying the motors have a specific behavior.

While the phrase "Someone can ask the motor for its horsepower rating, and the motor will return the rating as an integer" implies something about the behavior of an object, it doesn't deny any behavior. Nevertheless, when you modeled with abstract classes, you created an implementation pattern that later was found to be incorrect, even though the behavior in the hierarchy was accurate.

Modeling Behavior in an Interface
You can avoid accidentally implying an implementation pattern if you model behavior using interfaces. Let's review the behavior:

Behavior: Someone can ask the motor for its horsepower rating, and the motor will return its rating as an integer.

public interface Motor(){
public int getHorsepower();
}
Behavior: Somone can ask a battery-powered motor for its time to recharge and the motor will return its time as an integer.
public interface BatteryPoweredMotor extends Motor(){
public int getTimeToRecharge();
}
Behavior: Somone can ask a solar-powered motor for its lumens required to operate, and the motor will return its lumens as an integer.
public interface SolarPoweredMotor extends Motor{
abstract public int getLumensToOperate();
}
In this way, only behavior is modeled (see Figure 2).

Now, I'll describe the solar-battery-powered motor in question:

public DualMotor implements SolarPoweredMotor, BatteryPoweredMotor{
}
The dual-powered motor inherits behavior, not implementation (see Figure 3).

You can use abstract classes just as before, except in this case the abstract classes implement behaviors instead of defining them (see Figure 4).

Notice the two separate hierarchies. The interface defines behavior in a very pure way while the abstract class defines a pattern for implementation - including the origin of a given behavior. Notice how the bottom half of the diagram can be totally redesigned and yet the behavioral hierarchy remains intact. As long as the implementing class relies on the interfaces for behavior, the implementing class can change its parent abstract class without changing how other pieces of the code interact with it.

When to Use Abstract Classes
Now that I've fully discussed interfaces, abstract classes may seem like an evil half brother - something to be avoided. This is not the case! When you have a common implementation, abstract classes shine. Using abstract classes you can enforce an implementation hierarchy and avoid duplicate code. Using abstract classes, however, should not affect your decision to use interfaces to define your behavior.

Both parent and child abstract classes should implement interfaces that define the expected behavior if you think the implementation will change. In practice, relying on abstract classes to define behavior leads to an inheritance nightmare, while coding behavior with interfaces provides a cleaner separation of behavior and implementation. Thus it makes your code more resistant to change. If you want to modify your existing code to improve your design, I recommend reading Martin Fowler's book, Refactoring: Improving the Design of Existing Code (Addison-Wesley, 1999). He devotes an entire chapter to refactorings dealing with generalization.

More Stories By Anthony Meyer

Anthony Meyer is a
technical director and a Java developer at Flashline.com. His
experience includes the design, development and implementation of
large-scale, Java-based, Internet applications in the corporate Web
development environment. He has also created and implemented
corporate-focused reuse strategies in the
financial industry.

Comments (0)

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.


@ThingsExpo Stories
What a difference a year makes. Organizations aren’t just talking about IoT possibilities, it is now baked into their core business strategy. With IoT, billions of devices generating data from different companies on different networks around the globe need to interact. From efficiency to better customer insights to completely new business models, IoT will turn traditional business models upside down. In the new customer-centric age, the key to success is delivering critical services and apps wit...
The essence of data analysis involves setting up data pipelines that consist of several operations that are chained together – starting from data collection, data quality checks, data integration, data analysis and data visualization (including the setting up of interaction paths in that visualization). In our opinion, the challenges stem from the technology diversity at each stage of the data pipeline as well as the lack of process around the analysis.
In his keynote at 18th Cloud Expo, Andrew Keys, Co-Founder of ConsenSys Enterprise, will provide an overview of the evolution of the Internet and the Database and the future of their combination – the Blockchain. Andrew Keys is Co-Founder of ConsenSys Enterprise. He comes to ConsenSys Enterprise with capital markets, technology and entrepreneurial experience. Previously, he worked for UBS investment bank in equities analysis. Later, he was responsible for the creation and distribution of life ...
SYS-CON Events announced today that ContentMX, the marketing technology and services company with a singular mission to increase engagement and drive more conversations for enterprise, channel and SMB technology marketers, has been named “Sponsor & Exhibitor Lounge Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York City, New York. “CloudExpo is a great opportunity to start a conversation with new prospects, but what happens after the...
SYS-CON Events announced today that 24Notion has been named “Bronze Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York, New York. 24Notion is full-service global creative digital marketing, technology and lifestyle agency that combines strategic ideas with customized tactical execution. With a broad understand of the art of traditional marketing, new media, communications and social influence, 24Notion uniquely understands how to con...
The demand for organizations to expand their infrastructure to multiple IT environments like the cloud, on-premise, mobile, bring your own device (BYOD) and the Internet of Things (IoT) continues to grow. As this hybrid infrastructure increases, the challenge to monitor the security of these systems increases in volume and complexity. In his session at 18th Cloud Expo, Stephen Coty, Chief Security Evangelist at Alert Logic, will show how properly configured and managed security architecture can...
In his session at 18th Cloud Expo, Bruce Swann, Senior Product Marketing Manager at Adobe, will discuss how the Adobe Marketing Cloud can help marketers embrace opportunities for personalized, relevant and real-time customer engagement across offline (direct mail, point of sale, call center) and digital (email, website, SMS, mobile apps, social networks, connected objects). Bruce Swann has more than 15 years of experience working with digital marketing disciplines like web analytics, social med...
When it comes to IoT in the enterprise, namely the commercial building and hospitality markets, a benefit not getting the attention it deserves is energy efficiency, and IoT's direct impact on a cleaner, greener environment when installed in smart buildings. Until now clean technology was offered piecemeal and led with point solutions that require significant systems integration to orchestrate and deploy. There didn't exist a 'top down' approach that can manage and monitor the way a Smart Buildi...
SYS-CON Events announced today BZ Media LLC has been named “Media Sponsor” of SYS-CON's 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. BZ Media LLC is a high-tech media company that produces technical conferences and expositions, and publishes a magazine, newsletters and websites in the software development, SharePoint, mobile development and Commercial Drone markets.
WebRTC is bringing significant change to the communications landscape that will bridge the worlds of web and telephony, making the Internet the new standard for communications. Cloud9 took the road less traveled and used WebRTC to create a downloadable enterprise-grade communications platform that is changing the communication dynamic in the financial sector. In his session at @ThingsExpo, Leo Papadopoulos, CTO of Cloud9, will discuss the importance of WebRTC and how it enables companies to fo...
SYS-CON Events announced today TechTarget has been named “Media Sponsor” of SYS-CON's 18th International Cloud Expo, which will take place on June 7–9, 2016, at the Javits Center in New York City, NY, and the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. TechTarget is the Web’s leading destination for serious technology buyers researching and making enterprise technology decisions. Its extensive global networ...
With major technology companies and startups seriously embracing IoT strategies, now is the perfect time to attend @ThingsExpo 2016 in New York and Silicon Valley. Learn what is going on, contribute to the discussions, and ensure that your enterprise is as "IoT-Ready" as it can be! Internet of @ThingsExpo, taking place Nov 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 17th Cloud Expo and will feature technical sessions from a rock star conference faculty ...
The 19th International Cloud Expo has announced that its Call for Papers is open. Cloud Expo, to be held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, Big Data, Internet of Things, DevOps, Containers, Microservices and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportunity. Submit y...
Internet of @ThingsExpo, taking place November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with the 19th International Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world and ThingsExpo New York Call for Papers is now open.
There are several IoTs: the Industrial Internet, Consumer Wearables, Wearables and Healthcare, Supply Chains, and the movement toward Smart Grids, Cities, Regions, and Nations. There are competing communications standards every step of the way, a bewildering array of sensors and devices, and an entire world of competing data analytics platforms. To some this appears to be chaos. In this power panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists will discuss the vast to...
SYS-CON Events announced today that Enzu, a leading provider of cloud hosting solutions, will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. Enzu’s mission is to be the leading provider of enterprise cloud solutions worldwide. Enzu enables online businesses to use its IT infrastructure to their competitive advantage. By offering a suite of proven hosting and management services, Enzu wants companies to foc...
Customer experience has become a competitive differentiator for companies, and it’s imperative that brands seamlessly connect the customer journey across all platforms. With the continued explosion of IoT, join us for a look at how to build a winning digital foundation in the connected era – today and in the future. In his session at @ThingsExpo, Chris Nguyen, Group Product Marketing Manager at Adobe, will discuss how to successfully leverage mobile, rapidly deploy content, capture real-time d...
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, wh...
IoT generates lots of temporal data. But how do you unlock its value? How do you coordinate the diverse moving parts that must come together when developing your IoT product? What are the key challenges addressed by Data as a Service? How does cloud computing underlie and connect the notions of Digital and DevOps What is the impact of the API economy? What is the business imperative for Cognitive Computing? Get all these questions and hundreds more like them answered at the 18th Cloud Expo...
Whether your IoT service is connecting cars, homes, appliances, wearable, cameras or other devices, one question hangs in the balance – how do you actually make money from this service? The ability to turn your IoT service into profit requires the ability to create a monetization strategy that is flexible, scalable and working for you in real-time. It must be a transparent, smoothly implemented strategy that all stakeholders – from customers to the board – will be able to understand and comprehe...