Data Hiding

[Sarah Jorden]

This Question has already been asked but I did not get satisfactory answer in this thread,

https://coderanch.com/t/265544/java-programmer-SCJP/certification/Inheritance

in the below code,

  
class Mom {
public String s ="Mom";[B]// Line 1[/B]

void test() {
System.out.println(s + " : Test Mom");
}
  
void test2() {
System.out.println(s + " : Test2 Mom");
}
}
 
class Child extends Mom {
public String s = "Child";[B]// Line 2[/B]
 
void test2() {
System.out.println(s + " : Test2 Child");
}
}
 
class InheritanceTest4 {

public static void main(String[] s) {
Mom m = new Mom();
Child b = new Child();
Mom mb = new Child();    
 
m.test();
b.test();[B]// Line 3[/B]
mb.test();[B]// Line 4[/B] 
System.out.println("m.s : " + m.s);
System.out.println("b.s : " + b.s);
System.out.println("mb.s : " + mb.s);
 
}
}
  

Here,
String variable s at Line 2 in class Child will hide the String variable s at Line 1 in class Mom, right?

Now class Child is not overriding method test() but still when we call it at Line 3 and Line 4 , shouldn't it print Child class variable s which is hiding Mom class variable s?

Why is it printing "Mom" and not "Child" ?

please correct me, I must be missing something.
[ October 03, 2007: Message edited by: Sarah Jorden ]

[vijaya bharath]

Hi,

I think i can explain better.
Let me expalin like this
When you created object for mom it will have

 String s,test and test2   

methods.
You hide test2 and String s in child through inheritance.
when you call test method through child object it calls the parent method only.parent test method doesn't come and sit in your child class.child class inherits that means it can call that method with out creating any object to that class.

Calling the inherited test method through child object is same as calling the original test method from parent object.

and in the case

Mom mb = new Child(); 

, it prints string s as child only as you hid the mom

string s

with child

string s

.

[Sarah Jorden]

  
class Mom {
public String s ="Mom";// Line 1

void test() {
System.out.println(s + " : Test Mom");
}
  
void test2() {
System.out.println(s + " : Test2 Mom");
}
}
 
class Child extends Mom {
public String s = "Child";// Line 2
 
void test2() {
System.out.println(s + " : Test2 Child");
}
}
 
class InheritanceTest4 {

public static void main(String[] s) {
Mom m = new Mom();
Child b = new Child();
Mom mb = new Child();    
 
m.test();
b.test();// Line 3
mb.test();// Line 4
 
}
}
  

Output :

Mom : Test Mom
Mom : Test Mom
Mom : Test Mom

Here, I'm asking about Line 3 and Line 4, why b.test() and mb.test() prints string "Mom" which is variable s in class Mom.

In my opinion these two lines print "Child" variable s in class Child which is hiding s variable of class Mom.

[ahmed yehia]

when you call test method through child object it calls the parent method only

When a class inherites a member its as if it has declared that member, calling method test() through a Child reference is done from Child Not from Superclass. Try replacing this method with test() in Mom class.

    
void test() {
System.out.println(s + " : Test Mom"+ " calling from class: " + this.getClass().getSimpleName());
}

[Sarah Jorden]

Hey Ahmed,

Thanks for that nice example.

Now, if the method test() is bening called from Child class then
why is it printing variable s = "Mom" of class Mom and not variable s = "Child" from class Child? even though class Child has its own variable s.

right now, I'm getting this output,

Mom : Test Mom calling from class: Mom
Mom : Test Mom calling from class: Child
Mom : Test Mom calling from class: Child

but why am I not getting this output? ,

Mom : Test Mom calling from class: Mom
Child : Test Mom calling from class: Child
Child : Test Mom calling from class: Child

please correct me, Where am I going wrong?
[ October 04, 2007: Message edited by: Sarah Jorden ]

[Sarah Jorden]

I even made a small change to method test() like this,

void test() {
    System.out.println([B]this.s[/B] + " : Test Mom" + " calling from class: " + this.getClass().getSimpleName());
}

output is same as before,

Mom : Test Mom calling from class: Mom // Line 1
Mom : Test Mom calling from class: Child // Line 2
Mom : Test Mom calling from class: Child // Line 3

Shouldn't this.s refer to the currently executing object's variable s?
In first line, we are refering to Mom object. But in second and third line, we are refering to Child object.

I'm not getting it. What am I missing here?

[Teo Framoe]

Hi,
Could anyone please take a look at this. I really don't get it.
Regards,
Teo

[Sarah Jorden]

me too.....

[Chen SanHau]

I think it's because of the implementation of JVM. Maybe it's the only reason to explain that.
In other language, it gets different result.

Java

class Mom {
public String s ="Mom"; 
void test() {
System.out.println(s + " : Test Mom");
}
}
class Child extends Mom {
public String s = "Child";
}
 
class Test {
public static void main(String[] s) {
Mom m = new Mom();
Child b = new Child();
Mom mb = new Child();    
 
m.test();// Mom : Test Mom
b.test();// Mom : Test Mom
mb.test();// Mom : Test Mom
System.out.println("m.s : " + m.s);// m.s : Mom
System.out.println("b.s : " + b.s);// b.s : Child
System.out.println("mb.s : " + mb.s);// mb.s : Mom
 
}
}

PHP

class MyMom {
var $str ="Mom";
public function test() {
echo $this->str . " : Test Mom. <br>";
}
}
 
class MyChild extends MyMom {
var $str = "Child";
}
 
$c1=new MyChild();
echo $c1->str . "<br>";// Child
$c1->test();// [B]Child[/B] : Test Mom. 
 
$m=new MyMom();
$m->test();// Mom : Test Mom.
 
$c2=new MyChild();
$m = $c2;
$m->test();// [B]Child[/B] : Test Mom. 

[Burkhard Hassel]

Howdy!

Sarah asked:

Shouldn't this.s refer to the currently executing object's variable s?
In first line, we are refering to Mom object. But in second and third line, we are refering to Child object.

If a class inherits a method, this does not mean it would not be like pasting in the same code. Rather it is like you use the code of the superclass with your subclass's object.

In order to avoid confusion with your Mom and Child class names, here's a short example with classes A and B1 / B2. Both B's extend A, one has the method overriden, the other has not:

class A {
String s = "A-variable";
void method() {
System.out.println(this);
System.out.println(s);
System.out.println();
}
}
 
class B1 extends A {
String s = "B1-variable";
// no override
}
 
class B2 extends A {
String s = "B2-variable";
@Override // pasted, same method as in A
void method() {
System.out.println(this);
System.out.println(s);
System.out.println();
}
}
 
class User {
public static void main(String[] args) {
B1 b1 = new B1(); 
b1.method();
B2 b2 = new B2();
b2.method();
}
}

The output is
B1@82ba41
A-variable

B2@130c19b
B2-variable

You see the difference if the method is overridden with pasted code (same code as in the superclass) and if the method is just inherited.

In B1, there is no override and the object uses the superclass's method. Because of this the method does not see that variable s has a new value in the B1 class. It prints "A-variable".
In B2, there is an override, so here the method of the subclass is called, and therefore "B2-variable" is printed.


this will always return a reference the currently executing object, but this does not mean it will invoke a Child-method in all cases, because not in all cases the method is overridden.


When accessing the variables directly, there is no polymorphism at work. In this case only the type of the variable counts, so if you have
Mom variable = new Child();
and you print variable.s, it sees only Mom's s.
With variables, there is no polymorphism, only with methods.


Generally it is normal that you get some knots in your brain when you study this...



Yours,
Bu.

[Teo Framoe]

Knots untied
Thanks a lot Burkhard!

Teo

[Sarah Jorden]

thanks a lot Burkhard.



I was waiting for a clear answer like this for last two days.

[Yogesh Baraskar]

thanks a lot Burkhard..
Very well explained.

To make it a simple rule:
1. If method is overridden by a sub-class and subclass has the same variable name as declared in Super class, then any call (whether it is on subclass reference or super class reference ) to that method will display the value of variable declared in subclass.
2. If method is not overridden then in that case ,when you call a method of super class it will always take the variable declared in super class(reason super class method doesnt have any visibility to any methods or variables declared in child class).

Please correct me if my above statements are wrong.

Regards
Yogesh

[ahmed yehia]

Originally posted by Yogesh Baraskar:

1. If method is overridden by a sub-class and subclass has the same variable name as declared in Super class, then any call (whether it is on subclass reference or super class reference ) to that method will display the value of variable declared in subclass.

No, in that case each method call will reference its own variable.

[Matthias Wendel]

If the explanation of Burkhard is adequate, then the lightball hints in netbeans indicating "Field hides another field" in lines 11 and 16 of Burkhard's code are rather misleading, or am I still missing something?
Yours,
Matthias