Casting a Reference

[Deepali Pate]

Can someone explain with an example the theroy hidden in this statement.
In contrast to converting and casting primitives, converting and casting a reference to a variable does not change the object being referred to. Once created, an object can be referred to in a variety of ways, but the object type is not changed.
You will find this in the TIP on page 105 of Java2 Exam Cram by Bill Brogden
Thx

[Jessica Sant]

I'll take a crack at it...
Notice no matter what object I cast it too -- the reference to the variable itself does not change. the object type is unchanged as well.

import java.util.*;
public class Casting
{
   public static void main( String[] args )
   {
      Collection c = new ArrayList();
      c.add( "one" );
      c.add( "two" );
      c.add( "three" );
      List list = (List)c;
      list.add( "four" );
      list.add( "five" );
      list.add( "six" );
      ArrayList arrayList = (ArrayList)list;
      arrayList.add( "seven" );
      arrayList.add( "eight" );
      arrayList.add( "nine" );
      ListIterator li = arrayList.listIterator();
      while( li.hasNext() )
      {
         System.out.println( (String) li.next() );
      }
   }
}

[Corey McGlone]

Jess is definitely on the right track. It might also help to look at the class of an object before and after casting. Think about casting primitives:

int i = 5000;
byte b = (byte)i;

In order to evaluate this expression, the JVM take the value i, which is:
00000000 00000000 00010011 10001000 = 5000
and turns it into this:
10001000 = -120
As you can see, the actual value is modified in order to be placed into the byte variable.
However, if you cast a reference variable, the object that it is referred to is not modified in any way. You may now treat it differently, but the object remains unchanged. Take a look at this code:

Collection c = new ArrayList();
System.out.println(c.getClass());
List l = (List)c;
System.out.println(l.getClass());
ArrayList a = (ArrayList)l;
System.out.println(a.getClass());
 
// Output
 
class java.util.ArrayList
class java.util.ArrayList
class java.util.ArrayList

You can see that, even though I cast the object 3 times, the type remains the same - it is an ArrayList and it will always be an ArrayList.
I hope that helps,
Corey

[Anthony Villanueva]

If it helps, you can imagine references as simply different names/labels for the same object, like,
"Padme"
"Luke Skywalker's Mom"
"Natalie Portman's character"
but you know a rose by any other name would smell as sweet...
[ Jess abusing bartender power to correct Anthony's analogy... she's Luke Skywalker's mom -- not Darth Vader's (that'd be gross) ]
[ June 20, 2002: Message edited by: Jessica Sant ]

[Anthony Villanueva]

Gack! strong is the lure of the Dark Side. My sincerest apologies to all the fans

[Deepali Pate]

Hi,
Thanks a lot for your explanations and comments. I must add here that Corey's explantion being simple hepled me most.
Thanks guys.
Have a nice weekend