Variable shadowing in Java [duplicate]

Question

I am having some doubts about this Java code. The output it gives is "furry bray". My questions:

1. Why do i get this output?
2. How can I access the String object reference "name" in ZooKeeper class?
3. If it has something to do with variable shadowing, then which variable is being shadowed?

Code:

class Mammal {    String name = "furry ";    String makeNoise() { return "generic noise"; }  }  class Zebra extends Mammal {    String name = "stripes ";    String makeNoise() { return "bray"; }  }  public class ZooKeeper {    public static void main(String[] args) { new ZooKeeper().go(); }    void go() {      Mammal m = new Zebra();      System.out.println(m.name + m.makeNoise());      //Output comes as "furry bray". Please explain this.      //And how can we access the name variable, the one having "stripes " in it.      //Does it have something to do with Variable Shadowing?    }  } 

Answer 1

Variables aren't polymorphic. When you access m.name, that will always use the Mammal.name field which is part of that object, regardless of the execution-time type of the object. If you need to get access to Zebra.name, you need an expression with a compile-time type of Zebra.

The makeNoise method is called virtually though - the implementation used at execution time does depend on the type of the object.

Note that if you make all your fields private - which is generally a good idea anyway - this doesn't end up being an issue.

This is actually hiding rather than shadowing. See JLS section 8.3 for details on hiding, and section 6.4.1 for shadowing. I can't say that I always keep the differences straight...

Answer 2

Output comes as "furry bray". Please explain this.

fields in java programs are not accessed via dynamic lookup. Instead they are resolved statically while compile time. That's why you are getting furry for m.name. Whereas, methods in java programs are accessed via dynamic lookup. That's why you are getting bray for m.makeNoise().

And how can we access the name variable, the one having "stripes " in it?

And if you want to access Zebra.name , you should type cast m to 'Zebra'.This would look like this:

System.out.println(((Zebra)m).name + m.makeNoise()); 

UPDATE
The phenomena that is exhibiting here is the result of Fields Hiding rather than variable shadowing.