Welcome!

Java IoT Authors: Elizabeth White, Yeshim Deniz, Zakia Bouachraoui, Liz McMillan, Pat Romanski

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.


IoT & Smart Cities Stories
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
In his general session at 19th Cloud Expo, Manish Dixit, VP of Product and Engineering at Dice, discussed how Dice leverages data insights and tools to help both tech professionals and recruiters better understand how skills relate to each other and which skills are in high demand using interactive visualizations and salary indicator tools to maximize earning potential. Manish Dixit is VP of Product and Engineering at Dice. As the leader of the Product, Engineering and Data Sciences team at D...
As you know, enterprise IT conversation over the past year have often centered upon the open-source Kubernetes container orchestration system. In fact, Kubernetes has emerged as the key technology -- and even primary platform -- of cloud migrations for a wide variety of organizations. Kubernetes is critical to forward-looking enterprises that continue to push their IT infrastructures toward maximum functionality, scalability, and flexibility. As they do so, IT professionals are also embr...
The Japan External Trade Organization (JETRO) is a non-profit organization that provides business support services to companies expanding to Japan. With the support of JETRO's dedicated staff, clients can incorporate their business; receive visa, immigration, and HR support; find dedicated office space; identify local government subsidies; get tailored market studies; and more.
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
Atmosera delivers modern cloud services that maximize the advantages of cloud-based infrastructures. Offering private, hybrid, and public cloud solutions, Atmosera works closely with customers to engineer, deploy, and operate cloud architectures with advanced services that deliver strategic business outcomes. Atmosera's expertise simplifies the process of cloud transformation and our 20+ years of experience managing complex IT environments provides our customers with the confidence and trust tha...
AI and machine learning disruption for Enterprises started happening in the areas such as IT operations management (ITOPs) and Cloud management and SaaS apps. In 2019 CIOs will see disruptive solutions for Cloud & Devops, AI/ML driven IT Ops and Cloud Ops. Customers want AI-driven multi-cloud operations for monitoring, detection, prevention of disruptions. Disruptions cause revenue loss, unhappy users, impacts brand reputation etc.
As you know, enterprise IT conversation over the past year have often centered upon the open-source Kubernetes container orchestration system. In fact, Kubernetes has emerged as the key technology -- and even primary platform -- of cloud migrations for a wide variety of organizations. Kubernetes is critical to forward-looking enterprises that continue to push their IT infrastructures toward maximum functionality, scalability, and flexibility.
Today's workforce is trading their cubicles and corporate desktops in favor of an any-location, any-device work style. And as digital natives make up more and more of the modern workforce, the appetite for user-friendly, cloud-based services grows. The center of work is shifting to the user and to the cloud. But managing a proliferation of SaaS, web, and mobile apps running on any number of clouds and devices is unwieldy and increases security risks. Steve Wilson, Citrix Vice President of Cloud,...
When Enterprises started adopting Hadoop-based Big Data environments over the last ten years, they were mainly on-premise deployments. Organizations would spin up and manage large Hadoop clusters, where they would funnel exabytes or petabytes of unstructured data.However, over the last few years the economics of maintaining this enormous infrastructure compared with the elastic scalability of viable cloud options has changed this equation. The growth of cloud storage, cloud-managed big data e...