instance variables

[Peter Swiss]

Are instance variables inherited? If yes, does a subclass have a local copy of the instance variable thats declared in superclass?

class A {

	String s = "-";
	protected A() {this("d"); s+= "a";}
	private A(String e){ s += "d";}
	
	
}

class B extends A {
	B() { super(); s += "b"; }
}

class C extends B {
	
	C() { s += "c"; }
	public static void main(String [] args){
		
		System.out.println(new C().s);
	}
}

This program prints: -dabc

Can someone please explain this code?? Thanks a lot.

[Riza Aktunc]

When a constructor is called, on the first line super() is called implicitly. There is just one exception. If the first line is this(...) or super(...), then super() is not called. Considering this information:

In main the constructor of C is called.
From C's constructor, B's constructor is called.
From B's constructor, A() is called.
From A(), A("d") is called.
A("d") makes s="-d"
A() makes s="-da"
B() makes s="-dab"
C() makes s="-dabc"
main prints "-dabc"

[Nitesh Nandwana]

class A {

	String s = "-";
	protected A() {this("d"); s+= "a";}
	private A(String e){ s += "d";}
	
	
}

class B extends A {
	B() { super(); s += "b"; }
}

class C extends B {
	
	C() { s += "c"; }
	public static void main(String [] args){
		
		System.out.println(new C().s);
	}
}

At first i want to say that you should start from beginning.It's important to know java code conventions.Way you coded , is it really easy to read ?

Are instance variables inherited? If yes, does a subclass have a local copy of the instance variable thats declared in superclass?

It is depend on Access modifier applied on instance variable in superclass.You need sound study of Member access modifiers topic.It's look small topic but it is a big and confusing topic.gud luck

[Javin Paul]

Yes instance variables are accessible on Sub class but as said based on access modifier , private instance variable are not accessible at all.

[Nitesh Nandwana]

Javin Paul wrote:
, private instance variable are not accessible at all.

Pauly you are right but there is also one case, about you should know that's if default access modifier is applied on superclass instance variable and subclass is not in same package then it'll not be accessible at all in subclass.

[Peter Swiss]

Riza Aktunc wrote:When a constructor is called, on the first line super() is called implicitly. There is just one exception. If the first line is this(...) or super(...), then super() is not called. Considering this information:

In main the constructor of C is called.
From C's constructor, B's constructor is called.
From B's constructor, A() is called.
From A(), A("d") is called.
A("d") makes s="-d"
A() makes s="-da"
B() makes s="-dab"
C() makes s="-dabc"
main prints "-dabc"

Riza, I know how constructor chaining works, my question was whether a subclass gets a local copy of the variable in superclass.

[Peter Swiss]

Nitesh Nandwana wrote:

At first i want to say that you should start from beginning.It's important to know java code conventions.Way you coded , is it really easy to read ?

I didnt write this code! This is exam question!!!

[Matthew Brown]

An instance of the subclass is an instance of the superclass. So yes, it has all the variables that the superclass has (although they may not all be visible to it).

[Matthew Brown]

Peter Swiss wrote:I didnt write this code! This is exam question!!!

A real exam question? Or from a mock/book?

If it's the latter, then you should quote the source. If it's the former, then posting it isn't allowed at all - it's against all the terms of the examination. Can you clarify, please? Thanks.

[Peter Swiss]

class A {  
  
    String s = "-";  
    protected A() {this("d"); s+= "a";}  
    private A(String e){ s += "d";}  
      
      
}  
  
class B extends A {  
    B() { super(); s += "b"; }  
}  
  
class C extends B {  
      
    C() { s += "c"; }  
    public static void main(String [] args){  
          
        System.out.println(new C().s);  
    }  
}  

In this line, "B() { super(); s += "b"; } " does class B have a local copy of s? Since, instance variables are inherited, s should refer to B's local copy of s.

And also on this line, "C() { s += "c"; } " s should refer to class C's local (inherited) copy of s.

But since the answer is -dabc, it seems all the changes are applied on the copy of s in superclass.




This question is from the K&B mock exam CD that comes with the textbook.

[Bear Bibeault]

The whole concept of "local copy" that you keep repeating is a red herring. For there to be any of kind of copy assumes that there are two separate instantiations: one for the superclass and one for the extending class, and of course, this is not the case.

When an object is instantiated, only a single object, containing all the members declared in the class whether inherited or explicit, is created.

[Peter Swiss]

Ok since we are saying "new C()" only C's object is created and because of constructor chaining, it changes the value of s thats declared in superclass A...

Thank you!!