Welcome!

Java IoT Authors: Yeshim Deniz, Pat Romanski, Ian Khan, Karyn Jeffery, Anders Wallgren

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
Complete Internet of Things (IoT) embedded device security is not just about the device but involves the entire product’s identity, data and control integrity, and services traversing the cloud. A device can no longer be looked at as an island; it is a part of a system. In fact, given the cross-domain interactions enabled by IoT it could be a part of many systems. Also, depending on where the device is deployed, for example, in the office building versus a factory floor or oil field, security ha...
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 connect your brand strategy with the right consumer. 24Notion ranked #12 on Corporate Social Responsibility - Book of List.
Fact is, enterprises have significant legacy voice infrastructure that’s costly to replace with pure IP solutions. How can we bring this analog infrastructure into our shiny new cloud applications? There are proven methods to bind both legacy voice applications and traditional PSTN audio into cloud-based applications and services at a carrier scale. Some of the most successful implementations leverage WebRTC, WebSockets, SIP and other open source technologies. In his session at @ThingsExpo, Da...
Businesses are struggling to manage the information flow and interactions between all of these new devices and things jumping on their network, and the apps and IT systems they control. The data businesses gather is only helpful if they can do something with it. In his session at @ThingsExpo, Chris Witeck, Principal Technology Strategist at Citrix, will discuss how different the impact of IoT will be for large businesses, expanding how IoT will allow large organizations to make their legacy ap...
What happens when the different parts of a vehicle become smarter than the vehicle itself? As we move toward the era of smart everything, hundreds of entities in a vehicle that communicate with each other, the vehicle and external systems create a need for identity orchestration so that all entities work as a conglomerate. Much like an orchestra without a conductor, without the ability to secure, control, and connect the link between a vehicle’s head unit, devices, and systems and to manage the ...
What does it look like when you have access to cloud infrastructure and platform under the same roof? Let’s talk about the different layers of Technology as a Service: who cares, what runs where, and how does it all fit together. In his session at 18th Cloud Expo, Phil Jackson, Lead Technology Evangelist at SoftLayer, an IBM company, spoke about the picture being painted by IBM Cloud and how the tools being crafted can help fill the gaps in your IT infrastructure.
For basic one-to-one voice or video calling solutions, WebRTC has proven to be a very powerful technology. Although WebRTC’s core functionality is to provide secure, real-time p2p media streaming, leveraging native platform features and server-side components brings up new communication capabilities for web and native mobile applications, allowing for advanced multi-user use cases such as video broadcasting, conferencing, and media recording.
In this strange new world where more and more power is drawn from business technology, companies are effectively straddling two paths on the road to innovation and transformation into digital enterprises. The first path is the heritage trail – with “legacy” technology forming the background. Here, extant technologies are transformed by core IT teams to provide more API-driven approaches. Legacy systems can restrict companies that are transitioning into digital enterprises. To truly become a lea...
SYS-CON Events announced today that Sheng Liang to Keynote at SYS-CON's 19th Cloud Expo, which will take place on November 1-3, 2016 at the Santa Clara Convention Center in Santa Clara, California.
In his session at @ThingsExpo, Kausik Sridharabalan, founder and CTO of Pulzze Systems, Inc., will focus on key challenges in building an Internet of Things solution infrastructure. He will shed light on efficient ways of defining interactions within IoT solutions, leading to cost and time reduction. He will also introduce ways to handle data and how one can develop IoT solutions that are lean, flexible and configurable, thus making IoT infrastructure agile and scalable.
Just over a week ago I received a long and loud sustained applause for a presentation I delivered at this year’s Cloud Expo in Santa Clara. I was extremely pleased with the turnout and had some very good conversations with many of the attendees. Over the next few days I had many more meaningful conversations and was not only happy with the results but also learned a few new things. Here is everything I learned in those three days distilled into three short points.
Cognitive Computing is becoming the foundation for a new generation of solutions that have the potential to transform business. Unlike traditional approaches to building solutions, a cognitive computing approach allows the data to help determine the way applications are designed. This contrasts with conventional software development that begins with defining logic based on the current way a business operates. In her session at 18th Cloud Expo, Judith S. Hurwitz, President and CEO of Hurwitz & ...
So, you bought into the current machine learning craze and went on to collect millions/billions of records from this promising new data source. Now, what do you do with them? Too often, the abundance of data quickly turns into an abundance of problems. How do you extract that "magic essence" from your data without falling into the common pitfalls? In her session at @ThingsExpo, Natalia Ponomareva, Software Engineer at Google, provided tips on how to be successful in large scale machine learning...
The Transparent Cloud-computing Consortium (abbreviation: T-Cloud Consortium) will conduct research activities into changes in the computing model as a result of collaboration between "device" and "cloud" and the creation of new value and markets through organic data processing High speed and high quality networks, and dramatic improvements in computer processing capabilities, have greatly changed the nature of applications and made the storing and processing of data on the network commonplace.
An IoT product’s log files speak volumes about what’s happening with your products in the field, pinpointing current and potential issues, and enabling you to predict failures and save millions of dollars in inventory. But until recently, no one knew how to listen. In his session at @ThingsExpo, Dan Gettens, Chief Research Officer at OnProcess, will discuss recent research by Massachusetts Institute of Technology and OnProcess Technology, where MIT created a new, breakthrough analytics model f...
The Internet of Things can drive efficiency for airlines and airports. In their session at @ThingsExpo, Shyam Varan Nath, Principal Architect with GE, and Sudip Majumder, senior director of development at Oracle, will discuss the technical details of the connected airline baggage and related social media solutions. These IoT applications will enhance travelers' journey experience and drive efficiency for the airlines and the airports. The session will include a working demo and a technical d...
Almost two-thirds of companies either have or soon will have IoT as the backbone of their business in 2016. However, IoT is far more complex than most firms expected. How can you not get trapped in the pitfalls? In his session at @ThingsExpo, Tony Shan, a renowned visionary and thought leader, will introduce a holistic method of IoTification, which is the process of IoTifying the existing technology and business models to adopt and leverage IoT. He will drill down to the components in this fra...
Digital transformation is too big and important for our future success to not understand the rules that apply to it. The first three rules for winning in this age of hyper-digital transformation are: Advantages in speed, analytics and operational tempos must be captured by implementing an optimized information logistics system (OILS) Real-time operational tempos (IT, people and business processes) must be achieved Businesses that can "analyze data and act and with speed" will dominate those t...
If you had a chance to enter on the ground level of the largest e-commerce market in the world – would you? China is the world’s most populated country with the second largest economy and the world’s fastest growing market. It is estimated that by 2018 the Chinese market will be reaching over $30 billion in gaming revenue alone. Admittedly for a foreign company, doing business in China can be challenging. Often changing laws, administrative regulations and the often inscrutable Chinese Interne...
I'm a lonely sensor. I spend all day telling the world how I'm feeling, but none of the other sensors seem to care. I want to be connected. I want to build relationships with other sensors to be more useful for my human. I want my human to understand that when my friends next door are too hot for a while, I'll soon be flaming. And when all my friends go outside without me, I may be left behind. Don't just log my data; use the relationship graph. In his session at @ThingsExpo, Ryan Boyd, Engi...