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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

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