Welcome!

Weblogic Authors: Yeshim Deniz, Elizabeth White, Michael Meiner, Michael Bushong, Avi Rosenthal

Related Topics: Java IoT

Java IoT: Article

Java Gotchas: Instance Variables Hiding

Java Gotchas: Instance Variables Hiding

If methods with the same signatures or member variables with the same name exist in ancestor and descendant classes, the Java keyword super allows access members of the ancestor. But what if you do not use the keyword super in the descendant class? In case of methods, this is called method overriding and only the code of the descendant's method will execute. But when both classes have a member variable with the same name, it may cause a confusion and create hard to find bugs.

Recently in one of the Java online forums, a user with id cityart posted a question about a "strange behavior" of his program, and I decided to do some research on this subject.

Let's take a look at the Java program that declares a variable greeting in both super and subclasses (class A and class B). The subclass B also overrides the Object's method toString(). Please note, that the variable obj has a type of the superclass (A), but it points at the instance of the subclass (B), which is perfectly legal.


class A {
   public String greeting ="Hello";
}

class B extends A {
	public String greeting="Good Bye";
	 public String toString(){
		return greeting;
	}
}

public class VariableOverridingTest {
    public static void main(String[] args) {
      A obj = new B();
      obj.greeting="How are you";

      System.out.println(obj.greeting);
      System.out.println(obj.toString());
 }
}

If you compile and run this program, it'll print the following:

How are you
Good Bye

How come? Aren't we printing a member variable greeting of the same instance of the class B? The answer is no. If you run this program in IDE through a debugger, you'll see that there are two separate variables greeting. For example, Eclipse IDE shows these variables as greeting(A) and greeting(B). The first print statement deals with the member variable of the class A since obj has a type A, and the second print uses a method of the instance B that uses its own variable greeting.

Now, change the declaration of the variable obj to


      B obj = new B();

Run the program, and it'll print "How are you" twice.

But since you wanted the variable obj to have the type of the superclass A, you need to find a different solution. In the code below, we prohibit direct access to the variable greeting by making it private and introducing public setter and getter methods in both super and subclasses. Please note that in the following example, we override the setter and getter in the class B. This gives us a better control of which variable greeting to use.


class A {
 private String greeting ="Hello";
 public void setGreeting(String greet){greeting = greet;}
 public String getGreeting(){return greeting;}

}

class B extends A {
	 private String greeting="Good Bye";
	 public String toString(){
		return greeting;
	}
public void setGreeting(String greet){greeting = greet;}
public String getGreeting(){return greeting;}

}

public class VariableOverridingTest2 {
public static void main(String[] args) {
A obj = new B();

obj.setGreeting("How are you");

System.out.println(obj.getGreeting());
System.out.println(obj.toString());

}
}

This example is yet another illustration of how encapsulation may help you to avoid potential errors caused by multiple declarations of member variables with the same name in the inheritance hierarchy. If needed, we still can access the superclass' variable greeting from the class B by using super.getGreeting().

In Sun's Java tutorial, I found only a brief mentioning of member variables inheritance over here: http://java.sun.com/docs/books/tutorial/java/javaOO/subclass.html

Basically, you can hide a variable but override a method of a superclass. Java Language Specification describes hiding of instance variables over here: http://java.sun.com/docs/books/jls/ second_edition/html/classes.doc.html#229119

One more term to be aware of is shadowing. Here's another Sun's article that discusses hiding and shadowing: http://java.sun.com/developer/TechTips/2000/tt1010.html#tip2 What do you think of the following quote from this article: "First an important point needs to be made: just because the Java programming language allows you to do something, it doesn't always mean that it's a desirable thing to do." Well, if a feature is not desirable, why keep it in the language? Most likely, creators of the language decided to keep a separate copy of the superclass' instance variable to give developers a freedom to define their own subclasses without worrying of overriding by accident some internal members of the superclasses. But in my opinion it should be a responsibility of the superclasses to protect their members.

I'd love to see some practical examples, which would show when this feature of the Java language could be useful.

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 (11)

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
Leading companies, from the Global Fortune 500 to the smallest companies, are adopting hybrid cloud as the path to business advantage. Hybrid cloud depends on cloud services and on-premises infrastructure working in unison. Successful implementations require new levels of data mobility, enabled by an automated and seamless flow across on-premises and cloud resources. In his general session at 21st Cloud Expo, Greg Tevis, an IBM Storage Software Technical Strategist and Customer Solution Architec...
Nordstrom is transforming the way that they do business and the cloud is the key to enabling speed and hyper personalized customer experiences. In his session at 21st Cloud Expo, Ken Schow, VP of Engineering at Nordstrom, discussed some of the key learnings and common pitfalls of large enterprises moving to the cloud. This includes strategies around choosing a cloud provider(s), architecture, and lessons learned. In addition, he covered some of the best practices for structured team migration an...
Product connectivity goes hand and hand these days with increased use of personal data. New IoT devices are becoming more personalized than ever before. In his session at 22nd Cloud Expo | DXWorld Expo, Nicolas Fierro, CEO of MIMIR Blockchain Solutions, will discuss how in order to protect your data and privacy, IoT applications need to embrace Blockchain technology for a new level of product security never before seen - or needed.
Imagine if you will, a retail floor so densely packed with sensors that they can pick up the movements of insects scurrying across a store aisle. Or a component of a piece of factory equipment so well-instrumented that its digital twin provides resolution down to the micrometer.
In his keynote at 18th Cloud Expo, Andrew Keys, Co-Founder of ConsenSys Enterprise, provided 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 settle...
BnkToTheFuture.com is the largest online investment platform for investing in FinTech, Bitcoin and Blockchain companies. We believe the future of finance looks very different from the past and we aim to invest and provide trading opportunities for qualifying investors that want to build a portfolio in the sector in compliance with international financial regulations.
No hype cycles or predictions of a gazillion things here. IoT is here. You get it. You know your business and have great ideas for a business transformation strategy. What comes next? Time to make it happen. In his session at @ThingsExpo, Jay Mason, an Associate Partner of Analytics, IoT & Cybersecurity at M&S Consulting, presented a step-by-step plan to develop your technology implementation strategy. He also discussed the evaluation of communication standards and IoT messaging protocols, data...
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 ...
In his session at 21st Cloud Expo, Raju Shreewastava, founder of Big Data Trunk, provided a fun and simple way to introduce Machine Leaning to anyone and everyone. He solved a machine learning problem and demonstrated an easy way to be able to do machine learning without even coding. Raju Shreewastava is the founder of Big Data Trunk (www.BigDataTrunk.com), a Big Data Training and consulting firm with offices in the United States. He previously led the data warehouse/business intelligence and B...
"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...
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, whic...
When shopping for a new data processing platform for IoT solutions, many development teams want to be able to test-drive options before making a choice. Yet when evaluating an IoT solution, it’s simply not feasible to do so at scale with physical devices. Building a sensor simulator is the next best choice; however, generating a realistic simulation at very high TPS with ease of configurability is a formidable challenge. When dealing with multiple application or transport protocols, you would be...
Smart cities have the potential to change our lives at so many levels for citizens: less pollution, reduced parking obstacles, better health, education and more energy savings. Real-time data streaming and the Internet of Things (IoT) possess the power to turn this vision into a reality. However, most organizations today are building their data infrastructure to focus solely on addressing immediate business needs vs. a platform capable of quickly adapting emerging technologies to address future ...
We are given a desktop platform with Java 8 or Java 9 installed and seek to find a way to deploy high-performance Java applications that use Java 3D and/or Jogl without having to run an installer. We are subject to the constraint that the applications be signed and deployed so that they can be run in a trusted environment (i.e., outside of the sandbox). Further, we seek to do this in a way that does not depend on bundling a JRE with our applications, as this makes downloads and installations rat...
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.
DX World EXPO, LLC, a Lighthouse Point, Florida-based startup trade show producer and the creator of "DXWorldEXPO® - Digital Transformation Conference & Expo" has announced its executive management team. The team is headed by Levent Selamoglu, who has been named CEO. "Now is the time for a truly global DX event, to bring together the leading minds from the technology world in a conversation about Digital Transformation," he said in making the announcement.
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 lead...
Digital Transformation (DX) is not a "one-size-fits all" strategy. Each organization needs to develop its own unique, long-term DX plan. It must do so by realizing that we now live in a data-driven age, and that technologies such as Cloud Computing, Big Data, the IoT, Cognitive Computing, and Blockchain are only tools. In her general session at 21st Cloud Expo, Rebecca Wanta explained how the strategy must focus on DX and include a commitment from top management to create great IT jobs, monitor ...
"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...
The IoT Will Grow: In what might be the most obvious prediction of the decade, the IoT will continue to expand next year, with more and more devices coming online every single day. What isn’t so obvious about this prediction: where that growth will occur. The retail, healthcare, and industrial/supply chain industries will likely see the greatest growth. Forrester Research has predicted the IoT will become “the backbone” of customer value as it continues to grow. It is no surprise that retail is ...