Question about instance variables

[Leonardo Luiz]

Hi all,

I'm studing for SCJP exam. So I wrote this simple code and a doubt appeared.

Given:

class A {
public int number = 100;
public int getNumber() {
return number;
}
}

class B extends A {
public int number = 200;
}

class Test {
public static void main (String ... args) {
A a = new A();
A b1 = new B();
B b2 = new B();
System.out.println("a = " + a.getNumber());
System.out.println("b1 = " + b1.getNumber());
System.out.println("b2 = " + b2.getNumber());
}
}


with this output:

a = 100
b1 = 100
b2 = 100

Why is the instance variable value 200 not being used?

Thanks Ranchers,

Leonardo Luiz

[J Sato]

Instance variables are not overridden so when the getNumber() method is called it returns number from class A. You might try overriding the getNumber() method in class B and see if the output gives you what you initially expected to see. Like this:

class B extends A {
public int number = 200;
public int getNumber() {
return number;
}
}

[sky su]

Hi!
I will give you some code! I think you will know all about it !
class Test{
String s ="Hello!";
void show(){
System.out.println(s);
}
}
class TestSubClass extends Test{
String s = " Ranch friend!"
}
public class Kown{
Stirng s ="I'am Chinese!"
public static void main(String[] args){
TestSubClass t = new TestSubClass();
t.show()
}
}
It's print "Hello!" If you want print another ,you can overriding method show in subclass ! The show() will print out that where you put !
I hope my example will help you !

[Leonardo Luiz]

Thanks both of you.

I understand that �instance variables are not overridden�. Creating an overridden version of this method the sub class instance variable is used.

So, when a class has an instance variable with the same name that a super class instance variable. The methods of this class have no access to super class variable.
Ok?

[marc weber]

Originally posted by Leonardo Luiz:
... So, when a class has an instance variable with the same name that a super class instance variable. The methods of this class have no access to super class variable. Ok?

This is a case of "variable hiding." (See JLS 8.3.3.1.) The hidden variable can be accessed via super.

In the code below, x is hidden by being re-declared in the subclass. However, within the overridden method getX, both x variables can be accessed by using super to access the hidden variable.

class Super {
  String x = "SuperString";
  String getX() {
  return x;
  }
}
 
class Sub extends Super {
String x = "SubString";
String getX() {
return x + super.x;
}
public static void main(String[] args) {
Sub s = new Sub();
System.out.println(s.getX());
}
}

[Leonardo Luiz]

Variable hiding! So "super" is the key to access a variable in this case.
I will spend more time with this topic.

Thanks a lot!

[marc weber]

I just noticed that the example I linked to above (8.3.3.1) is for a hidden class (static) variable. An example of a hidden instance variable is just below that, under 8.3.3.2. The idea is the same, but a more detailed explanation is given under 8.3.3.2.

[Leonardo Luiz]

Thanks for explanation.