Object Reference Conversion

[Brian Nguyen]

Hello,
Could some one please explain to me the following rules of object reference conversion:
- If Old Type is an interface and New Type is a class, then the New Type must be Object.
- If Old Type is an array and New Type is a class, then New Type must be Object.
Thank you very much.
Brian

[Corey McGlone]

Look at the JLS, §5.5 Casting Conversion. It outlines all of the details about casting conversions. If you have any more questions, just let me know.
Corey

[Anthony Villanueva]

Yes on both counts.

public class Test
{   
public static void main(String args[])
{
Impl i  = new Impl();
A a = i;
Object o = a;
}
}
interface A
{
}
class Impl implements A
{
}

 
public class Test
{   
static Object [] objArray;
static int [] intArray;

public static void main(String args[])
{
Object array1 = objArray;
Object array2 = intArray;
}
}

The new object reference must be a supertype of the old object reference. No class can be considered a supertype of an interface except for Object.
(If we have interface/classes A and B, and B is a subclass or subinterface of A either directly or though some intervening "layers" of classes/interfaces, then A is a supertype of B.)
Object arrays, and arrays of primitives are considered of type Object.
[ June 27, 2002: Message edited by: Anthony Villanueva ]