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Printing an Object without casting!

Max White
Ranch Hand

Joined: Jun 28, 2008
Posts: 85
Hi,

The code below prints - Object :2



I was expecting something in hashcode,as automatic unboxing doesn't work with Object class.
So what am i missing?
Thanks
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18874
    
  40

I was expecting something in hashcode,as automatic unboxing doesn't work with Object class.
So what am i missing?


The Integer class overrides the toString() method to return the integer value as a string -- which is what is used by the system out.

Henry


Books: Java Threads, 3rd Edition, Jini in a Nutshell, and Java Gems (contributor)
Max White
Ranch Hand

Joined: Jun 28, 2008
Posts: 85
Hi Henry,but which Integer class you are talking about?
When i iterate through the vector,objects of Object class are returned.Correct me if i am wrong.
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18874
    
  40

Max White wrote:Hi Henry,but which Integer class you are talking about?


The type of the instance that you added to the vector -- see your example for details.

Max White wrote:When i iterate through the vector,objects of Object class are returned.Correct me if i am wrong.


Polymorphism applies here. An Integer IS-A Object.... In other words, the Object that is returned is the same Integer that you added.

Henry
Max White
Ranch Hand

Joined: Jun 28, 2008
Posts: 85
I am really confused now.As far as i know if we don't use generics,we always get an Object type from a Collection and we need to cast it to the appropriate type.For example -



How does it prints the actual value at [3] and not hashcode?
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18874
    
  40

I am really confused now.As far as i know if we don't use generics,we always get an Object type from a Collection and we need to cast it to the appropriate type.


When a subclass overrides a method, then it will be the subclass' method that will be invoked, when that method is called -- and this is true regardless of the type of the reference (subclass or superclass) used to access the object.

So... it doesn't matter if it is an Object reference, or an Integer reference. The actual object is an Integer object, and it is the toString() method of the Integer class that will be called, regardless of the reference, and / or any casting.

Henry
Max White
Ranch Hand

Joined: Jun 28, 2008
Posts: 85
The actual object is an Integer object


I know that actual object is an Integer object,but how does the compiler knows?
Thanks!
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18874
    
  40

Max White wrote:
I know that actual object is an Integer object,but how does the compiler knows?
Thanks!


Actually, the compiler doesn't know -- the resolution of the method to invoke is completed at runtime.

Henry
Karthick Ravi
Greenhorn

Joined: Feb 27, 2008
Posts: 13




The above statement in your code is analagous to Object o = new Integer (2) ; Even though iterator.next() returns an object with an Object reference, it is still an Integer object. Hence the Integer version of toString executes. This is due to inheritance.




This results in a compiler error because you are equating a primitive with an object. During Autoboxing the compiler is looking for the reference type rather than the actual object you are passing through. Hence the casting.
 
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