Click here to close now.

Welcome!

Java Authors: Kevin Benedict, Tim Hinds, Lori MacVittie, Liz McMillan, Yakov Fain

Related Topics: Java

Java: 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 co-founder of two software companies: Farata Systems and SuranceBay. He authored several technical books and lots of articles on software development. Yakov is Java Champion (https://java-champions.java.net). He leads leads Princeton Java Users Group. Two of Yakov's books will go in print this year: "Enterprise Web Development" (O'Reilly) and "Java For Kids" (No Starch Press).

Comments (11) 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
Chakra Yadavalli 09/15/04 08:53:53 AM EDT

I guess the author clearly states what the problem is and how the encapsulation helps in avoiding the potential errors. I think the following line from the text of the article would be enough to red flag this for any rational developer. Or, this may make it more noticeable :-)

LOOK AT THE LINE BELOW. POTENTIAL BUG!!!

"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()."

And in most scenarios of real software development, we never have the luxury of time to track the "patient zero" who coded this sort of bug and treat him. :-)

As for the existence of debates such as this, THEY SHOULD (I mean MUST) exist for the sake of posterity. The correct solutions to these problems to be aware of such problems. If one thinks that articles such as this are "encouraging" the such "malpractices" (without reading the complete articles) then they are wrong and should be advised to use good commonsense in adopting coding techniques.

F. Libuste 09/15/04 07:53:37 AM EDT

The problem of hiding variables will never arise if you apply good practices of Oriented Object programming and NEVER make use of anything else than "private" as a modifier for class members. And yse accessors when needed.

Now as for method overriding, well...that is *exactly* what OO design is for. Making sure your objects are correctly polymorphic, behave properly and offer the proper services and proper extensibility through their exposed methods.

This whole debate should not exist in the first place, and to me, the article comes from software malpractice, and is not depicted as such, which is not so good IMHO. Correct OO design is the issue that should be addressed here, not the effects of it.

Chakra Yadavalli 09/14/04 11:07:46 AM EDT

Good pitfall. This is quite common and very had to find if you have more than two classes in the inheritance tree and the instance members are "protected" which is very common thing that we see. If we go by Bertrand Myer's Object Oriented software construction, which enforces strict encapsulation by saying no to protected variables, we will *not* run in to these sort of problems. But then again, we may be tempted to write "Train Wreck" code -- obj.getThis().getThat().getSomething()

There is another pitfall that has the disguise of this "overridding." Guess what is printed by the following code?

public class A {
public static void getInstance(){
System.out.println("class A");
}
}

public class B extends A {
public static void getInstance(){
System.out.println("class B");
}
}

public class Tester {
public staic void main(Strnig[] args) {
A obj = new B();
obj.getInstance();
}
}

This would print "class A" because, the static methods go by the name "class methods" in Java. In C++, similar code would print "class B". Java language says, static/class methods are not inherited and cannot be overriden. But allowing to define classes with the same name, though legal when looked from the namespace perspective, would result in these pitfalls. The irony with these "false" static methods AKA class methods is that, it makes your brain hurt when you look at code like the one below... Keeps you guessing why it does not throw NullPointerException.

A obj = null;
obj.getInstance();

These are the many good reasons why one should enforce, with the help of IDEs like Eclipse, the practice of qualifying instance members with "this", "super" and static members with the "type name".

Sebastian Tyrrell 09/14/04 10:39:38 AM EDT

Both David Hibbs and J.R. Titko are right that there is no problem if the design is right - in particular if you have proper encapsulation (i.e. keep the data member private and use getter and setter functions). But I still can't imagine a single legitimate usage. The closest I can come is that it means that you don't need to worry what (private) data members base classes might have, and can reuse their names for your own purposes. I think this is what David means when he says "... the capability to do this is required or else behaviours of parent classes are not encapsulated ...". I'm not convinced - a compile error at this point might save a lot of grief later.

David Hibbs 09/14/04 09:30:26 AM EDT

Mr. Tyrrell commented that "...it is the type of the object, not the type of the pointer, that determines the behaviour. To me, that makes it a fault rather than a feature!"

In some regards, yes. The key word here though is "behaviour". Behaviour as in, what happens when a method is invoked? Direct access of fields (IMHO) is not a "behaviour" of an object.

Allowing access to member fields like this is poor style and design in any OO language.

Proper encapsulation helps this problem. This is not to say that encapsulation is a cure-all; indeed, generating getters and setters for the field in the child class (in effect, overriding them and shadowing them at the same time!) can create a whole new set of hard-to-find bugs.

The bottom line: proper design, planning, and review will avoid the pitfall, while the capability to do this is required or else behaviors of parent classes are not encapsulated -- and subject to breakage by children.

J.R. Titko 09/14/04 09:27:52 AM EDT

I have been using this example when teaching for a couple years to show what to avoid in coding Java. It is a situation set up at compile time by the compiler making the substitution of the literal for the variable. I agree its a problem in the language, but can easily be avoided by always using getters and setters to revtrieve instance level variables.

Sebastián Tyrrell 09/14/04 08:10:42 AM EDT

It seems to me from the examples that there is no way of utilising this feature without breaking the Liskov substitution principle that it is the type of the object, not the type of the pointer, that determines the behaviour. To me, that makes it a fault rather than a feature!

Sudipto Nandan 09/14/04 06:41:38 AM EDT

The article is good but can be very briefly eneded by saying that
When a method is called by a reference object, it takes into consideration the Object it is referencing and not the type of referencing object.
While, when a memeber variable is accessed by a reference object, the type of the referencing object is taken into consideration and not the object it is referencing.

MarkusH 09/14/04 05:13:39 AM EDT

This is another example how important it is, that every Developer has easy to use access to Software Audits in it's IDE, so that suspicious constructs like this don't survive until the check-in... i.e. the Audits provided by Borland Together in JBuilder and Eclipse-based IDE's

Narayanan R 09/14/04 12:43:11 AM EDT

It was interesting. It is the behavior of an object that is defined by its type (whose instance it is). I think the attributes of an object are defined by the handle used, since in Java methods are only bound at runtime.
Simple typecasting with the superclass/subclass can have obtained the desired result, as long as the typecast is valid.

Just Nell 09/13/04 09:53:13 PM EDT

Perhaps a better way to demonstrate is to print obj.greeting before setting, e.g.,

A obj = new B();
System.out.println(obj.greeting);
obj.greeting="How are you";

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

@ThingsExpo Stories
The explosion of connected devices / sensors is creating an ever-expanding set of new and valuable data. In parallel the emerging capability of Big Data technologies to store, access, analyze, and react to this data is producing changes in business models under the umbrella of the Internet of Things (IoT). In particular within the Insurance industry, IoT appears positioned to enable deep changes by altering relationships between insurers, distributors, and the insured. In his session at @ThingsExpo, Michael Sick, a Senior Manager and Big Data Architect within Ernst and Young's Financial Servi...
PubNub on Monday has announced that it is partnering with IBM to bring its sophisticated real-time data streaming and messaging capabilities to Bluemix, IBM’s cloud development platform. “Today’s app and connected devices require an always-on connection, but building a secure, scalable solution from the ground up is time consuming, resource intensive, and error-prone,” said Todd Greene, CEO of PubNub. “PubNub enables web, mobile and IoT developers building apps on IBM Bluemix to quickly add scalable realtime functionality with minimal effort and cost.”
Sensor-enabled things are becoming more commonplace, precursors to a larger and more complex framework that most consider the ultimate promise of the IoT: things connecting, interacting, sharing, storing, and over time perhaps learning and predicting based on habits, behaviors, location, preferences, purchases and more. In his session at @ThingsExpo, Tom Wesselman, Director of Communications Ecosystem Architecture at Plantronics, will examine the still nascent IoT as it is coalescing, including what it is today, what it might ultimately be, the role of wearable tech, and technology gaps stil...
In the consumer IoT, everything is new, and the IT world of bits and bytes holds sway. But industrial and commercial realms encompass operational technology (OT) that has been around for 25 or 50 years. This grittier, pre-IP, more hands-on world has much to gain from Industrial IoT (IIoT) applications and principles. But adding sensors and wireless connectivity won’t work in environments that demand unwavering reliability and performance. In his session at @ThingsExpo, Ron Sege, CEO of Echelon, will discuss how as enterprise IT embraces other IoT-related technology trends, enterprises with i...
When it comes to the Internet of Things, hooking up will get you only so far. If you want customers to commit, you need to go beyond simply connecting products. You need to use the devices themselves to transform how you engage with every customer and how you manage the entire product lifecycle. In his session at @ThingsExpo, Sean Lorenz, Technical Product Manager for Xively at LogMeIn, will show how “product relationship management” can help you leverage your connected devices and the data they generate about customer usage and product performance to deliver extremely compelling and reliabl...
The Internet of Things (IoT) is causing data centers to become radically decentralized and atomized within a new paradigm known as “fog computing.” To support IoT applications, such as connected cars and smart grids, data centers' core functions will be decentralized out to the network's edges and endpoints (aka “fogs”). As this trend takes hold, Big Data analytics platforms will focus on high-volume log analysis (aka “logs”) and rely heavily on cognitive-computing algorithms (aka “cogs”) to make sense of it all.
With several hundred implementations of IoT-enabled solutions in the past 12 months alone, this session will focus on experience over the art of the possible. Many can only imagine the most advanced telematics platform ever deployed, supporting millions of customers, producing tens of thousands events or GBs per trip, and hundreds of TBs per month. With the ability to support a billion sensor events per second, over 30PB of warm data for analytics, and hundreds of PBs for an data analytics archive, in his session at @ThingsExpo, Jim Kaskade, Vice President and General Manager, Big Data & Ana...
One of the biggest impacts of the Internet of Things is and will continue to be on data; specifically data volume, management and usage. Companies are scrambling to adapt to this new and unpredictable data reality with legacy infrastructure that cannot handle the speed and volume of data. In his session at @ThingsExpo, Don DeLoach, CEO and president of Infobright, will discuss how companies need to rethink their data infrastructure to participate in the IoT, including: Data storage: Understanding the kinds of data: structured, unstructured, big/small? Analytics: What kinds and how responsiv...
Since 2008 and for the first time in history, more than half of humans live in urban areas, urging cities to become “smart.” Today, cities can leverage the wide availability of smartphones combined with new technologies such as Beacons or NFC to connect their urban furniture and environment to create citizen-first services that improve transportation, way-finding and information delivery. In her session at @ThingsExpo, Laetitia Gazel-Anthoine, CEO of Connecthings, will focus on successful use cases.
Sensor-enabled things are becoming more commonplace, precursors to a larger and more complex framework that most consider the ultimate promise of the IoT: things connecting, interacting, sharing, storing, and over time perhaps learning and predicting based on habits, behaviors, location, preferences, purchases and more. In his session at @ThingsExpo, Tom Wesselman, Director of Communications Ecosystem Architecture at Plantronics, will examine the still nascent IoT as it is coalescing, including what it is today, what it might ultimately be, the role of wearable tech, and technology gaps stil...
The true value of the Internet of Things (IoT) lies not just in the data, but through the services that protect the data, perform the analysis and present findings in a usable way. With many IoT elements rooted in traditional IT components, Big Data and IoT isn’t just a play for enterprise. In fact, the IoT presents SMBs with the prospect of launching entirely new activities and exploring innovative areas. CompTIA research identifies several areas where IoT is expected to have the greatest impact.
Wearable devices have come of age. The primary applications of wearables so far have been "the Quantified Self" or the tracking of one's fitness and health status. We propose the evolution of wearables into social and emotional communication devices. Our BE(tm) sensor uses light to visualize the skin conductance response. Our sensors are very inexpensive and can be massively distributed to audiences or groups of any size, in order to gauge reactions to performances, video, or any kind of presentation. In her session at @ThingsExpo, Jocelyn Scheirer, CEO & Founder of Bionolux, will discuss ho...
SYS-CON Events announced today that GENBAND, a leading developer of real time communications software solutions, has been named “Silver Sponsor” of SYS-CON's WebRTC Summit, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. The GENBAND team will be on hand to demonstrate their newest product, Kandy. Kandy is a communications Platform-as-a-Service (PaaS) that enables companies to seamlessly integrate more human communications into their Web and mobile applications - creating more engaging experiences for their customers and boosting collaboration and productiv...
Roberto Medrano, Executive Vice President at SOA Software, had reached 30,000 page views on his home page - http://RobertoMedrano.SYS-CON.com/ - on the SYS-CON family of online magazines, which includes Cloud Computing Journal, Internet of Things Journal, Big Data Journal, and SOA World Magazine. He is a recognized executive in the information technology fields of SOA, internet security, governance, and compliance. He has extensive experience with both start-ups and large companies, having been involved at the beginning of four IT industries: EDA, Open Systems, Computer Security and now SOA.
From telemedicine to smart cars, digital homes and industrial monitoring, the explosive growth of IoT has created exciting new business opportunities for real time calls and messaging. In his session at @ThingsExpo, Ivelin Ivanov, CEO and Co-Founder of Telestax, shared some of the new revenue sources that IoT created for Restcomm – the open source telephony platform from Telestax. Ivelin Ivanov is a technology entrepreneur who founded Mobicents, an Open Source VoIP Platform, to help create, deploy, and manage applications integrating voice, video and data. He is the co-founder of TeleStax, a...
The industrial software market has treated data with the mentality of “collect everything now, worry about how to use it later.” We now find ourselves buried in data, with the pervasive connectivity of the (Industrial) Internet of Things only piling on more numbers. There’s too much data and not enough information. In his session at @ThingsExpo, Bob Gates, Global Marketing Director, GE’s Intelligent Platforms business, to discuss how realizing the power of IoT, software developers are now focused on understanding how industrial data can create intelligence for industrial operations. Imagine ...
Operational Hadoop and the Lambda Architecture for Streaming Data Apache Hadoop is emerging as a distributed platform for handling large and fast incoming streams of data. Predictive maintenance, supply chain optimization, and Internet-of-Things analysis are examples where Hadoop provides the scalable storage, processing, and analytics platform to gain meaningful insights from granular data that is typically only valuable from a large-scale, aggregate view. One architecture useful for capturing and analyzing streaming data is the Lambda Architecture, representing a model of how to analyze rea...
SYS-CON Events announced today that Vitria Technology, Inc. will exhibit at SYS-CON’s @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Vitria will showcase the company’s new IoT Analytics Platform through live demonstrations at booth #330. Vitria’s IoT Analytics Platform, fully integrated and powered by an operational intelligence engine, enables customers to rapidly build and operationalize advanced analytics to deliver timely business outcomes for use cases across the industrial, enterprise, and consumer segments.
When it comes to the Internet of Things, hooking up will get you only so far. If you want customers to commit, you need to go beyond simply connecting products. You need to use the devices themselves to transform how you engage with every customer and how you manage the entire product lifecycle. In his session at @ThingsExpo, Sean Lorenz, Technical Product Manager for Xively at LogMeIn, will show how “product relationship management” can help you leverage your connected devices and the data they generate about customer usage and product performance to deliver extremely compelling and reliabl...
The explosion of connected devices / sensors is creating an ever-expanding set of new and valuable data. In parallel the emerging capability of Big Data technologies to store, access, analyze, and react to this data is producing changes in business models under the umbrella of the Internet of Things (IoT). In particular within the Insurance industry, IoT appears positioned to enable deep changes by altering relationships between insurers, distributors, and the insured. In his session at @ThingsExpo, Michael Sick, a Senior Manager and Big Data Architect within Ernst and Young's Financial Servi...