Welcome!

Java IoT Authors: Elizabeth White, Sematext Blog, Pat Romanski, Carmen Gonzalez, Xenia von Wedel

Related Topics: Java IoT

Java IoT: Article

Are You Using Abstract Classes, Polymorphism, and Interfaces?

Are You Using Abstract Classes, Polymorphism, and Interfaces?

If the answer is no, at a minimum your project needs a code review.

Let's work on the following assignment: a company has employees and consultants. Design classes with and without the use of inheritance to represent the people who work for this company. The classes should have the following methods:

 

  • changeAddress
  • promote
  • giveDayOff
  • raiseSalary

Promotion means giving one day off and raising the salary by a specified percentage. For employees, the method raiseSalary should raise the yearly salary and, for consultants, it should increase their hourly rate.

Abstract Classes
A class is called abstract if it has at least one abstract (not implemented) method. The keyword abstract has to be placed in the definition of the method(s) and the class itself. For example, the class in Listing 1 has three concrete methods and one abstract. (The link to the source code is available below the article)

Abstract classes cannot be instantiated, but they allow you to create superclasses that implement some of the functionality, while leaving one or more methods to be implemented in subclasses.

The class Person can contain dozens of concrete methods that are the same for every person, such as changeAddress and giveDayOff, but since the process of raising a salary is different for employees and consultants, the method raiseSalary should remain abstract. Please note that even though this method is abstract, it could be called in an abstract class because by the time the concrete class is instantiated, the method will be already implemented. Since we have two types of workers, let's create subclasses Employee and Consultant and implement the method raiseSalary based on different rules (see Listings 2 and 3).

The designer of the class Person may not know the specifics of the raising salary process, but this does not stop him or her from calling the method raiseSalary. Programmers writing subclasses are forced to write an implementation of this method according to its signature declared in the abstract class. If they declare a method raiseSalary with a different argument list, this will be considered method overloading and the subclass will remain abstract. The class Promoter in Listing 4 shows how to use the classes Employee and Consultant for promoting workers.

Polymorphism
A programming language could be considered object-oriented if it supports inheritance, encapsulation, and polymorphism. The first two notions can be easily defined:

  • Inheritance lets you design a class by deriving it from an existing one. This feature allows you to reuse existing code without doing copy and paste. Java provides the keyword extends for declaring inheritance.
  • Encapsulation is the ability to hide and protect data. Java has access-level qualifiers such as public, private, and protected to control who can access class variables and methods. There is also so-called package-level protection, which is automatically engaged if you don't use any of the access-level keywords.
  • Polymorphism, though, is easier to understand through an example. Let's look at the classes Person, Employee, and Consultant from a different angle. We'll populate a Vector, mixing up the instances of classes Employee and Consultant - in real life this information usually comes from a database. For example, a program could get the person's work status from the database and instantiate an appropriate concrete class. The class Promoter (see Listing 4) will give an additional vacation day and increase the salary or hourly rate of every worker by 5%.

Please note that even though we cast every object from the collection workers to the ancestor's type Person in line 17, Listing 4, the variable pers can hold references to its descendent objects. The actual object type will be evaluated during runtime only. This feature of object-oriented languages is called runtime or late binding.

The output of the class Promoter will look as follows:

Class Person: Promoting a worker...
Class Person: Adding a day off
Class Employee:Increasing salary by 5%
Class Person: Promoting a worker...
Class Person: Adding a day off
Class Consultant: Increasing hourly rate by 5%
Class Person: Promoting a worker...
Class Person: Adding a day off
Class Employee:Increasing salary by 5%
Class Person: Promoting a worker...
Class Person: Adding a day off
Class Employee:Increasing salary by 5%

Both classes Employee and Consultant are inherited from the same base class Person. Instead of having different methods for increasing the worker's compensation based on its type, we give a polymorphic behavior to the method raiseSalary, which applies different business logic depending on the type of object from the collection. Even though it looks as if we're calling the same method promote, this is not the case. Since the actual object type is evaluated during runtime, the salary is raised properly according to this particular object's implementation of the method raiseSalary. This is polymorphism in action.

The while loop in the class Promoter will remain the same even if we add some other types of workers inherited from the class Person. For example, to add a new category of worker - a foreign contractor - we'll have to create a class Foreign- Contractor derived from the class Person and implement the method raiseSalary there. The class Promoter will keep casting all these objects to the type Person during runtime and call the method raiseSalary of the proper object.

Polymorphism allows you to avoid using switch or if statements with the expensive operator instanceof. Listing 5 shows an ugly alternative to our while loop from the class Promoter that assumes there is no abstract method raiseSalary, but we have separate promote methods in each subclass of the Person. This code would work slower than the polymorphic version from the class Promoter, and the if statement would have to be modified every time a new type of worker is added.

Interfaces
A similar functionality could be implemented using Java interfaces. We'll keep working with a modified version of the ancestor class Person because it has such useful methods as changeAddress and giveDayOff. But this class doesn't have to be abstract anymore because the method raiseSalary will be moved to a Java interface. The method promote won't be needed; we'd rather make the method giveDayOff available to descendants of the class Person by changing the private access level to protected (see line 8 in Listing 6).

Here's the "interface way" to ensure that each person in the firm receives the proper salary raise despite the differences in payroll calculation.

Let's define an interface Payable in Listing 7. More than one class can implement this interface (see Listing 8). When the class Consultant declares that it implements interface Payable, it promises to write implementations for all methods declared in this interface - in our case it's just one method raiseSalary. Why is it so important that the class will "keep the promise" and implement all the interface's methods? In many cases interface is a description of some behavior. In our case behavior Payable means the existence of the method boolean raiseSalary(int percent). If any other class knows that Employee implements Payable, it can safely call any method declared in the Payable interface (see the interface example in Listing 9).

Let's forget for a moment about employees and consultants and switch to the Java AWT listeners and events. When a class declares that it implements the interface java.awt.Action- Listener, a JVM will call the method actionPerformed on this class whenever the user clicks on the window's button, and in some other cases as well. Try to imagine what would happen if you forgot to include the method actionPerformed in your class. The good news is that your class won't even compile if not all methods declared in the interface were implemented. The java.awt.WindowListener interface declares seven methods, and even if you are interested only in the windowClosing one, you must include six additional empty-bodied methods to compile the class (window adapters simplify this process, but they are beyond the scope of this article).

While both abstract classes and interfaces can ensure that a concrete class will have all required methods, abstract classes can also contain implemented methods, but interfaces can't.

Beside method declarations, interfaces can contain final static variables. For example, let's say we have multiple bonus-level codes used in several classes during the calculation of new salaries. Instead of redefining these constants in every class that needs them, we can create the interface shown in Listing 10.

Now a small change in the class declaration will allow us to use these bonus levels as if they were declared in the class Employee:

public class Employee
implements Payable, Bonus {
...
if (empLevel==JUNIOR_LVL){
//apply the rules for juniors
}
}

public class Consultant
implements Payable, Bonus {
...
}

Java does not allow multiple inheritance, which means a class can't have two independent ancestors, but you can use interfaces as a workaround. As you've seen in the example above, a class can implement multiple interfaces; it just needs to implement all methods declared in all interfaces. If your window needs to process button clicks and window closing events, you can declare a class as follows:

 

class MyWindow implements ActionListener, WindowListener{S}

During evolution, an Employee can obtain multiple behaviors, for example

 

class Employee extends Person
implements Payable, Transferable,
Sueable, Bonus {...}

Consultants such as myself are usually more primitive creatures and can be defined as follows:

class Consultant extends Person
implements Payable, Sueable {...}

But if a program such as Promoter is interested only in Payable functions, it can cast the object only to those interfaces it intends to use, for example:

 

Employee emp = new Employee();
Consultant con = new Consultant();
Payable person1 = (Payable) emp;
Payable person2 = (Payable) con;

Now we're ready to write a second version of the class Promoter that will use the classes Employee and Consultant defined in Listings 8 and 11.

The output of this program will look similar to the output of the class Promoter from Listing 4:

Class Employee:Increasing salary by 5%
Class Consultant: Increasing hourly rate by 5%
Class Employee:Increasing salary by 5%
Class Employee:Increasing salary by 5%

Line 18 of Listing 9 may look a little confusing: How can we call a concrete method raiseSalary on a variable of an interface type? Actually we call a method on a concrete instance of the Employee or a Consultant, but by casting this instance to the type Payable we are just letting the JVM know that we're only interested in the methods that were declared in this particular interface.

Java Technical Interviews
During the technical interviews, probably the most frequently asked question is, "What's the difference between Java abstract classes and interfaces?" While interviewing Java programmers, I also found out that only half of the job applicants could properly complete the assignment described at the beginning of this article.

During the job interview your answers should be clear and short; you won't even have a chance to use all the material presented here. Here's one version of the answer to our problem.

If two classes have lots of common functionality, but some methods should be implemented differently, you could create a common abstract ancestor Person and two subclasses Employee and Consultant. The method raiseSalary must be declared abstract in the class Person while other methods should be concrete. This way we ensure that the subclasses do have the method named raiseSalary with a known signature, so we could use it in the ancestor without knowing its implementation. Java interfaces should also be considered in cases when the same method must be implemented in multiple classes - in this case we do not need to use abstract ancestors. Actually, interfaces could be your only option if a class already has an ancestor that can not be changed.

One good interview technique is to impress the interviewer by elaborating on a related topic. Discussion of abstract classes and interfaces gives you a good opportunity to show your understanding of polymorphism.

Summary
Use of abstract classes, interfaces, and polymorphism improves the design of any project by making it more readable and easily extensible. This also makes your code more compact and elegant.

More Stories By Yakov Fain

Yakov Fain is a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay. He wrote a thousand blogs (http://yakovfain.com) and several books about software development. Yakov authored and co-authored such books as "Angular 2 Development with TypeScript", "Java 24-Hour Trainer", and "Enterprise Web Development". His Twitter tag is @yfain

Comments (4) 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
sherali Inamdar 12/14/07 05:04:48 AM EST

Hi Dear Yakov Fain,
Even If a class Dont Have Abstract Method(i.e all methods may concrete), we can declare it as a Abstract Class. just appending the keyword

Taitelman mordechai 12/10/07 11:28:31 AM EST

Regarding your last paragraph: when to use Abstract class vs. Interface:
In terms of OOD the decision is more architectural:
Abstract class and interfaces are two ways to enforce an implementation in sub-classes.
However, I believe the main importance of Interfaces in Java is to overcome the multi-inheritance limitation.
The 2nd issue is agreed API between components. Two people should declare an interface if they intend to split up and re-join after several months. So what you should consider is: what is the probability this class would cross the boundaries of the component ?
Abstract class should be considered more inside the component. In many cases abstract classes are added in order to prevent circular dependencies between packages (in the same component). Furthermore, abstract class is your way to enforce some implementation details.
Beside that, in Java an interface is identical to (pure) abstract class.

punit pandey 09/04/03 11:00:08 AM EDT

It is a great article. good for newbies.

Steve Kasson 09/03/03 09:14:05 AM EDT

The source code is not available for this article. It references Listings 1 - 11, but I cannot find them anywhere.

Otherwise... excellent article.

-Steve

@ThingsExpo Stories
Data is the fuel that drives the machine learning algorithmic engines and ultimately provides the business value. In his session at Cloud Expo, Ed Featherston, a director and senior enterprise architect at Collaborative Consulting, discussed the key considerations around quality, volume, timeliness, and pedigree that must be dealt with in order to properly fuel that engine.
IoT solutions exploit operational data generated by Internet-connected smart “things” for the purpose of gaining operational insight and producing “better outcomes” (for example, create new business models, eliminate unscheduled maintenance, etc.). The explosive proliferation of IoT solutions will result in an exponential growth in the volume of IoT data, precipitating significant Information Governance issues: who owns the IoT data, what are the rights/duties of IoT solutions adopters towards t...
Businesses and business units of all sizes can benefit from cloud computing, but many don't want the cost, performance and security concerns of public cloud nor the complexity of building their own private clouds. Today, some cloud vendors are using artificial intelligence (AI) to simplify cloud deployment and management. In his session at 20th Cloud Expo, Ajay Gulati, Co-founder and CEO of ZeroStack, will discuss how AI can simplify cloud operations. He will cover the following topics: why clou...
As data explodes in quantity, importance and from new sources, the need for managing and protecting data residing across physical, virtual, and cloud environments grow with it. Managing data includes protecting it, indexing and classifying it for true, long-term management, compliance and E-Discovery. Commvault can ensure this with a single pane of glass solution – whether in a private cloud, a Service Provider delivered public cloud or a hybrid cloud environment – across the heterogeneous enter...
Everyone knows that truly innovative companies learn as they go along, pushing boundaries in response to market changes and demands. What's more of a mystery is how to balance innovation on a fresh platform built from scratch with the legacy tech stack, product suite and customers that continue to serve as the business' foundation. In his General Session at 19th Cloud Expo, Michael Chambliss, Head of Engineering at ReadyTalk, discussed why and how ReadyTalk diverted from healthy revenue and mor...
The many IoT deployments around the world are busy integrating smart devices and sensors into their enterprise IT infrastructures. Yet all of this technology – and there are an amazing number of choices – is of no use without the software to gather, communicate, and analyze the new data flows. Without software, there is no IT. In this power panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, Dave McCarthy, Director of Products at Bsquare Corporation; Alan Williamson, Principal...
The 20th International Cloud Expo has announced that its Call for Papers is open. Cloud Expo, to be held June 6-8, 2017, at the Javits Center in New York City, 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 your speaking proposal ...
You have great SaaS business app ideas. You want to turn your idea quickly into a functional and engaging proof of concept. You need to be able to modify it to meet customers' needs, and you need to deliver a complete and secure SaaS application. How could you achieve all the above and yet avoid unforeseen IT requirements that add unnecessary cost and complexity? You also want your app to be responsive in any device at any time. In his session at 19th Cloud Expo, Mark Allen, General Manager of...
"IoT is going to be a huge industry with a lot of value for end users, for industries, for consumers, for manufacturers. How can we use cloud to effectively manage IoT applications," stated Ian Khan, Innovation & Marketing Manager at Solgeniakhela, in this SYS-CON.tv interview at @ThingsExpo, held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA.
Successful digital transformation requires new organizational competencies and capabilities. Research tells us that the biggest impediment to successful transformation is human; consequently, the biggest enabler is a properly skilled and empowered workforce. In the digital age, new individual and collective competencies are required. In his session at 19th Cloud Expo, Bob Newhouse, CEO and founder of Agilitiv, drew together recent research and lessons learned from emerging and established compa...
Bert Loomis was a visionary. This general session will highlight how Bert Loomis and people like him inspire us to build great things with small inventions. In their general session at 19th Cloud Expo, Harold Hannon, Architect at IBM Bluemix, and Michael O'Neill, Strategic Business Development at Nvidia, discussed the accelerating pace of AI development and how IBM Cloud and NVIDIA are partnering to bring AI capabilities to "every day," on-demand. They also reviewed two "free infrastructure" pr...
Financial Technology has become a topic of intense interest throughout the cloud developer and enterprise IT communities. Accordingly, attendees at the upcoming 20th Cloud Expo at the Javits Center in New York, June 6-8, 2017, will find fresh new content in a new track called FinTech.
Information technology is an industry that has always experienced change, and the dramatic change sweeping across the industry today could not be truthfully described as the first time we've seen such widespread change impacting customer investments. However, the rate of the change, and the potential outcomes from today's digital transformation has the distinct potential to separate the industry into two camps: Organizations that see the change coming, embrace it, and successful leverage it; and...
"Dice has been around for the last 20 years. We have been helping tech professionals find new jobs and career opportunities," explained Manish Dixit, VP of Product and Engineering at Dice, in this SYS-CON.tv interview at 19th Cloud Expo, held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.
Extracting business value from Internet of Things (IoT) data doesn’t happen overnight. There are several requirements that must be satisfied, including IoT device enablement, data analysis, real-time detection of complex events and automated orchestration of actions. Unfortunately, too many companies fall short in achieving their business goals by implementing incomplete solutions or not focusing on tangible use cases. In his general session at @ThingsExpo, Dave McCarthy, Director of Products...
"At ROHA we develop an app called Catcha. It was developed after we spent a year meeting with, talking to, interacting with senior citizens watching them use their smartphones and talking to them about how they use their smartphones so we could get to know their smartphone behavior," explained Dave Woods, Chief Innovation Officer at ROHA, in this SYS-CON.tv interview at 19th Cloud Expo, held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events has announced today that Roger Strukhoff has been named conference chair of Cloud Expo and @ThingsExpo 2017 New York. The 20th Cloud Expo and 7th @ThingsExpo will take place on June 6-8, 2017, at the Javits Center in New York City, NY. "The Internet of Things brings trillions of dollars of opportunity to developers and enterprise IT, no matter how you measure it," stated Roger Strukhoff. "More importantly, it leverages the power of devices and the Internet to enable us all to im...
We are always online. We access our data, our finances, work, and various services on the Internet. But we live in a congested world of information in which the roads were built two decades ago. The quest for better, faster Internet routing has been around for a decade, but nobody solved this problem. We’ve seen band-aid approaches like CDNs that attack a niche's slice of static content part of the Internet, but that’s it. It does not address the dynamic services-based Internet of today. It does...
"ReadyTalk is an audio and web video conferencing provider. We've really come to embrace WebRTC as the platform for our future of technology," explained Dan Cunningham, CTO of ReadyTalk, in this SYS-CON.tv interview at WebRTC Summit at 19th Cloud Expo, held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.
20th Cloud Expo, taking place June 6-8, 2017, at the Javits Center in New York City, NY, 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 strategy.