Welcome!

Java Authors: Liz McMillan, Sematext Blog , Elizabeth White, Charles Jolley, Torben Andersen

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 Internet of Things is not new. Historically, smart businesses have used its basic concept of leveraging data to drive better decision making and have capitalized on those insights to realize additional revenue opportunities. So, what has changed to make the Internet of Things one of the hottest topics in tech? In his session at @ThingsExpo, Chris Gray, Director, Embedded and Internet of Things, discussed the underlying factors that are driving the economics of intelligent systems. Discover how hardware commoditization, the ubiquitous nature of connectivity, and the emergence of Big Data a...
"BSQUARE is in the business of selling software solutions for smart connected devices. It's obvious that IoT has moved from being a technology to being a fundamental part of business, and in the last 18 months people have said let's figure out how to do it and let's put some focus on it, " explained Dave Wagstaff, VP & Chief Architect, at BSQUARE Corporation, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
The major cloud platforms defy a simple, side-by-side analysis. Each of the major IaaS public-cloud platforms offers their own unique strengths and functionality. Options for on-site private cloud are diverse as well, and must be designed and deployed while taking existing legacy architecture and infrastructure into account. Then the reality is that most enterprises are embarking on a hybrid cloud strategy and programs. In this Power Panel at 15th Cloud Expo (http://www.CloudComputingExpo.com), moderated by Ashar Baig, Research Director, Cloud, at Gigaom Research, Nate Gordon, Director of T...
SYS-CON Events announced today that IDenticard will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. IDenticard™ is the security division of Brady Corp (NYSE: BRC), a $1.5 billion manufacturer of identification products. We have small-company values with the strength and stability of a major corporation. IDenticard offers local sales, support and service to our customers across the United States and Canada. Our partner network encompasses some 300 of the world's leading systems integrators and security s...
SYS-CON Events announced today that Windstream, a leading provider of advanced network and cloud communications, has been named “Silver Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9–11, 2015, at the Javits Center in New York, NY. Windstream (Nasdaq: WIN), a FORTUNE 500 and S&P 500 company, is a leading provider of advanced network communications, including cloud computing and managed services, to businesses nationwide. The company also offers broadband, phone and digital TV services to consumers primarily in rural areas.

ARMONK, N.Y., Nov. 20, 2014 /PRNewswire/ --  IBM (NYSE: IBM) today announced that it is bringing a greater level of control, security and flexibility to cloud-based application development and delivery with a single-tenant version of Bluemix, IBM's platform-as-a-service. The new platform enables developers to build ap...

The BPM world is going through some evolution or changes where traditional business process management solutions really have nowhere to go in terms of development of the road map. In this demo at 15th Cloud Expo, Kyle Hansen, Director of Professional Services at AgilePoint, shows AgilePoint’s unique approach to dealing with this market circumstance by developing a rapid application composition or development framework.
“In the past year we've seen a lot of stabilization of WebRTC. You can now use it in production with a far greater degree of certainty. A lot of the real developments in the past year have been in things like the data channel, which will enable a whole new type of application," explained Peter Dunkley, Technical Director at Acision, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
DevOps Summit 2015 New York, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that it is now accepting Keynote Proposals. The widespread success of cloud computing is driving the DevOps revolution in enterprise IT. Now as never before, development teams must communicate and collaborate in a dynamic, 24/7/365 environment. There is no time to wait for long development cycles that produce software that is obsolete at launch. DevOps may be disruptive, but it is essential.
"People are a lot more knowledgeable about APIs now. There are two types of people who work with APIs - IT people who want to use APIs for something internal and the product managers who want to do something outside APIs for people to connect to them," explained Roberto Medrano, Executive Vice President at SOA Software, in this SYS-CON.tv interview at Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
Nigeria has the largest economy in Africa, at more than US$500 billion, and ranks 23rd in the world. A recent re-evaluation of Nigeria's true economic size doubled the previous estimate, and brought it well ahead of South Africa, which is a member (unlike Nigeria) of the G20 club for political as well as economic reasons. Nigeria's economy can be said to be quite diverse from one point of view, but heavily dependent on oil and gas at the same time. Oil and natural gas account for about 15% of Nigera's overall economy, but traditionally represent more than 90% of the country's exports and as...
The Internet of Things is a misnomer. That implies that everything is on the Internet, and that simply should not be - especially for things that are blurring the line between medical devices that stimulate like a pacemaker and quantified self-sensors like a pedometer or pulse tracker. The mesh of things that we manage must be segmented into zones of trust for sensing data, transmitting data, receiving command and control administrative changes, and peer-to-peer mesh messaging. In his session at @ThingsExpo, Ryan Bagnulo, Solution Architect / Software Engineer at SOA Software, focused on desi...
"At our booth we are showing how to provide trust in the Internet of Things. Trust is where everything starts to become secure and trustworthy. Now with the scaling of the Internet of Things it becomes an interesting question – I've heard numbers from 200 billion devices next year up to a trillion in the next 10 to 15 years," explained Johannes Lintzen, Vice President of Sales at Utimaco, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
"For over 25 years we have been working with a lot of enterprise customers and we have seen how companies create applications. And now that we have moved to cloud computing, mobile, social and the Internet of Things, we see that the market needs a new way of creating applications," stated Jesse Shiah, CEO, President and Co-Founder of AgilePoint Inc., in this SYS-CON.tv interview at 15th Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that Gridstore™, the leader in hyper-converged infrastructure purpose-built to optimize Microsoft workloads, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Gridstore™ is the leader in hyper-converged infrastructure purpose-built for Microsoft workloads and designed to accelerate applications in virtualized environments. Gridstore’s hyper-converged infrastructure is the industry’s first all flash version of HyperConverged Appliances that include both compute and storag...
Today’s enterprise is being driven by disruptive competitive and human capital requirements to provide enterprise application access through not only desktops, but also mobile devices. To retrofit existing programs across all these devices using traditional programming methods is very costly and time consuming – often prohibitively so. In his session at @ThingsExpo, Jesse Shiah, CEO, President, and Co-Founder of AgilePoint Inc., discussed how you can create applications that run on all mobile devices as well as laptops and desktops using a visual drag-and-drop application – and eForms-buildi...
We certainly live in interesting technological times. And no more interesting than the current competing IoT standards for connectivity. Various standards bodies, approaches, and ecosystems are vying for mindshare and positioning for a competitive edge. It is clear that when the dust settles, we will have new protocols, evolved protocols, that will change the way we interact with devices and infrastructure. We will also have evolved web protocols, like HTTP/2, that will be changing the very core of our infrastructures. At the same time, we have old approaches made new again like micro-services...
Code Halos - aka "digital fingerprints" - are the key organizing principle to understand a) how dumb things become smart and b) how to monetize this dynamic. In his session at @ThingsExpo, Robert Brown, AVP, Center for the Future of Work at Cognizant Technology Solutions, outlined research, analysis and recommendations from his recently published book on this phenomena on the way leading edge organizations like GE and Disney are unlocking the Internet of Things opportunity and what steps your organization should be taking to position itself for the next platform of digital competition.
The 3rd International Internet of @ThingsExpo, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that its Call for Papers is now open. The Internet of Things (IoT) is the biggest idea since the creation of the Worldwide Web more than 20 years ago.
As the Internet of Things unfolds, mobile and wearable devices are blurring the line between physical and digital, integrating ever more closely with our interests, our routines, our daily lives. Contextual computing and smart, sensor-equipped spaces bring the potential to walk through a world that recognizes us and responds accordingly. We become continuous transmitters and receivers of data. In his session at @ThingsExpo, Andrew Bolwell, Director of Innovation for HP's Printing and Personal Systems Group, discussed how key attributes of mobile technology – touch input, sensors, social, and ...