Click here to close now.




















Welcome!

Java IoT Authors: Sematext Blog, Mike Kavis, Liz McMillan, Dana Gardner, Elizabeth White

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
SYS-CON Events announced today that HPM Networks will exhibit at the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. For 20 years, HPM Networks has been integrating technology solutions that solve complex business challenges. HPM Networks has designed solutions for both SMB and enterprise customers throughout the San Francisco Bay Area.
For IoT to grow as quickly as analyst firms’ project, a lot is going to fall on developers to quickly bring applications to market. But the lack of a standard development platform threatens to slow growth and make application development more time consuming and costly, much like we’ve seen in the mobile space. In his session at @ThingsExpo, Mike Weiner, Product Manager of the Omega DevCloud with KORE Telematics Inc., discussed the evolving requirements for developers as IoT matures and conducted a live demonstration of how quickly application development can happen when the need to comply wit...
The Internet of Everything (IoE) brings together people, process, data and things to make networked connections more relevant and valuable than ever before – transforming information into knowledge and knowledge into wisdom. IoE creates new capabilities, richer experiences, and unprecedented opportunities to improve business and government operations, decision making and mission support capabilities.
Explosive growth in connected devices. Enormous amounts of data for collection and analysis. Critical use of data for split-second decision making and actionable information. All three are factors in making the Internet of Things a reality. Yet, any one factor would have an IT organization pondering its infrastructure strategy. How should your organization enhance its IT framework to enable an Internet of Things implementation? In his session at @ThingsExpo, James Kirkland, Red Hat's Chief Architect for the Internet of Things and Intelligent Systems, described how to revolutionize your archit...
MuleSoft has announced the findings of its 2015 Connectivity Benchmark Report on the adoption and business impact of APIs. The findings suggest traditional businesses are quickly evolving into "composable enterprises" built out of hundreds of connected software services, applications and devices. Most are embracing the Internet of Things (IoT) and microservices technologies like Docker. A majority are integrating wearables, like smart watches, and more than half plan to generate revenue with APIs within the next year.
Growth hacking is common for startups to make unheard-of progress in building their business. Career Hacks can help Geek Girls and those who support them (yes, that's you too, Dad!) to excel in this typically male-dominated world. Get ready to learn the facts: Is there a bias against women in the tech / developer communities? Why are women 50% of the workforce, but hold only 24% of the STEM or IT positions? Some beginnings of what to do about it! In her Opening Keynote at 16th Cloud Expo, Sandy Carter, IBM General Manager Cloud Ecosystem and Developers, and a Social Business Evangelist, d...
In his keynote at 16th Cloud Expo, Rodney Rogers, CEO of Virtustream, discussed the evolution of the company from inception to its recent acquisition by EMC – including personal insights, lessons learned (and some WTF moments) along the way. Learn how Virtustream’s unique approach of combining the economics and elasticity of the consumer cloud model with proper performance, application automation and security into a platform became a breakout success with enterprise customers and a natural fit for the EMC Federation.
The Internet of Things is not only adding billions of sensors and billions of terabytes to the Internet. It is also forcing a fundamental change in the way we envision Information Technology. For the first time, more data is being created by devices at the edge of the Internet rather than from centralized systems. What does this mean for today's IT professional? In this Power Panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists addressed this very serious issue of profound change in the industry.
Discussions about cloud computing are evolving into discussions about enterprise IT in general. As enterprises increasingly migrate toward their own unique clouds, new issues such as the use of containers and microservices emerge to keep things interesting. In this Power Panel at 16th Cloud Expo, moderated by Conference Chair Roger Strukhoff, panelists addressed the state of cloud computing today, and what enterprise IT professionals need to know about how the latest topics and trends affect their organization.
It is one thing to build single industrial IoT applications, but what will it take to build the Smart Cities and truly society-changing applications of the future? The technology won’t be the problem, it will be the number of parties that need to work together and be aligned in their motivation to succeed. In his session at @ThingsExpo, Jason Mondanaro, Director, Product Management at Metanga, discussed how you can plan to cooperate, partner, and form lasting all-star teams to change the world and it starts with business models and monetization strategies.
Converging digital disruptions is creating a major sea change - Cisco calls this the Internet of Everything (IoE). IoE is the network connection of People, Process, Data and Things, fueled by Cloud, Mobile, Social, Analytics and Security, and it represents a $19Trillion value-at-stake over the next 10 years. In her keynote at @ThingsExpo, Manjula Talreja, VP of Cisco Consulting Services, discussed IoE and the enormous opportunities it provides to public and private firms alike. She will share what businesses must do to thrive in the IoE economy, citing examples from several industry sectors.
There will be 150 billion connected devices by 2020. New digital businesses have already disrupted value chains across every industry. APIs are at the center of the digital business. You need to understand what assets you have that can be exposed digitally, what their digital value chain is, and how to create an effective business model around that value chain to compete in this economy. No enterprise can be complacent and not engage in the digital economy. Learn how to be the disruptor and not the disruptee.
Akana has released Envision, an enhanced API analytics platform that helps enterprises mine critical insights across their digital eco-systems, understand their customers and partners and offer value-added personalized services. “In today’s digital economy, data-driven insights are proving to be a key differentiator for businesses. Understanding the data that is being tunneled through their APIs and how it can be used to optimize their business and operations is of paramount importance,” said Alistair Farquharson, CTO of Akana.
Business as usual for IT is evolving into a "Make or Buy" decision on a service-by-service conversation with input from the LOBs. How does your organization move forward with cloud? In his general session at 16th Cloud Expo, Paul Maravei, Regional Sales Manager, Hybrid Cloud and Managed Services at Cisco, discusses how Cisco and its partners offer a market-leading portfolio and ecosystem of cloud infrastructure and application services that allow you to uniquely and securely combine cloud business applications and services across multiple cloud delivery models.
The enterprise market will drive IoT device adoption over the next five years. In his session at @ThingsExpo, John Greenough, an analyst at BI Intelligence, division of Business Insider, analyzed how companies will adopt IoT products and the associated cost of adopting those products. John Greenough is the lead analyst covering the Internet of Things for BI Intelligence- Business Insider’s paid research service. Numerous IoT companies have cited his analysis of the IoT. Prior to joining BI Intelligence, he worked analyzing bank technology for Corporate Insight and The Clearing House Payment...
"Optimal Design is a technology integration and product development firm that specializes in connecting devices to the cloud," stated Joe Wascow, Co-Founder & CMO of Optimal Design, in this SYS-CON.tv interview at @ThingsExpo, held June 9-11, 2015, at the Javits Center in New York City.
SYS-CON Events announced today that CommVault has been named “Bronze Sponsor” of SYS-CON's 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. A singular vision – a belief in a better way to address current and future data management needs – guides CommVault in the development of Singular Information Management® solutions for high-performance data protection, universal availability and simplified management of data on complex storage networks. CommVault's exclusive single-platform architecture gives companies unp...
Electric Cloud and Arynga have announced a product integration partnership that will bring Continuous Delivery solutions to the automotive Internet-of-Things (IoT) market. The joint solution will help automotive manufacturers, OEMs and system integrators adopt DevOps automation and Continuous Delivery practices that reduce software build and release cycle times within the complex and specific parameters of embedded and IoT software systems.
"ciqada is a combined platform of hardware modules and server products that lets people take their existing devices or new devices and lets them be accessible over the Internet for their users," noted Geoff Engelstein of ciqada, a division of Mars International, in this SYS-CON.tv interview at @ThingsExpo, held June 9-11, 2015, at the Javits Center in New York City.
Internet of Things is moving from being a hype to a reality. Experts estimate that internet connected cars will grow to 152 million, while over 100 million internet connected wireless light bulbs and lamps will be operational by 2020. These and many other intriguing statistics highlight the importance of Internet powered devices and how market penetration is going to multiply many times over in the next few years.