This week's book giveaway is in the Design forum.
We're giving away four copies of Design for the Mind and have Victor S. Yocco on-line!
See this thread for details.
Win a copy of Design for the Mind this week in the Design forum!
  • Post Reply
  • Bookmark Topic Watch Topic
  • New Topic

Printing an Object without casting!

 
Max White
Ranch Hand
Posts: 85
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
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
Marshal
Pie
Posts: 20997
76
C++ Chrome Eclipse IDE Firefox Browser Java jQuery Linux VI Editor Windows
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
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
 
Max White
Ranch Hand
Posts: 85
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
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
Marshal
Pie
Posts: 20997
76
C++ Chrome Eclipse IDE Firefox Browser Java jQuery Linux VI Editor Windows
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
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
Posts: 85
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
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
Marshal
Pie
Posts: 20997
76
C++ Chrome Eclipse IDE Firefox Browser Java jQuery Linux VI Editor Windows
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
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
Posts: 85
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
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
Marshal
Pie
Posts: 20997
76
C++ Chrome Eclipse IDE Firefox Browser Java jQuery Linux VI Editor Windows
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
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
Posts: 13
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator




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.
 
  • Post Reply
  • Bookmark Topic Watch Topic
  • New Topic