Doubt from K&B book..Please elaborate....

[Vishal Chugh]

Hi ,

Our problem was of coverting a Byte to Integer

class WidenAndBox {
  static void go(Long x) { System.out.println("Long"); }
 
  public static void main(String [] args) {
    byte b = 5;
    go(b);           // must widen then box - illegal
  }
}
 

After this it has been told we are not allowed to do wideing and then boxing but reverse is true.. and the boxing followed by widening example given is as below , i am okay with example :

class BoxAndWiden {
  static void go(Object o) {
    Byte b2 = (Byte) o;        // ok - it's a Byte object
    System.out.println(b2);
  }
 
  public static void main(String [] args) {
    byte b = 5;
    go(b);       // can this byte turn into an Object ?
  }
}
 

My doubt is that is our original problem of converting from byte to Long is resolved (as was in first example ) ? as it is not rectified in second example ( in the second example we just achieved conversion from byte to Byte..)

Please put some light on it...

Thanks
Vishal







[BPSouther: Added code tags]
[ February 14, 2008: Message edited by: Ben Souther ]

[marc weber]

As you know, if boxing or unboxing is involved in method conversion, the boxing or unboxing must occur before any upcast or widening. (See JLS - 5.3 Method Invocation Conversion.)

Your second example works because the primitive byte b can be boxed to a Byte, and then the Byte reference can be upcast to type Object (because Byte is a subclass of Object).

But your first example is different because Byte is not a subtype of Long. Both of these are direct subclasses of Number. Therefore, a Byte reference cannot be upcast to type Long.
[ February 15, 2008: Message edited by: marc weber ]

[Vishal Chugh]

Hi Marc,

Thanks a lot!!!