java VM doesn't cast an array of references according to casting conversion

Name: dkC59003 Date: 07/08/98



Compiler version "1.2beta4" (build JDK-1.2beta4-K, green threads, sunwjit)

JLS, 5.5, p.69-70 says:
"If R is a class representing an array type RC[]-that is, an array of components of type RC:

If T is a class type, then T must be Object, or a run-time exception is thrown.

        If T is an interface type, then a run-time exception is thrown unless T is the interface
type Cloneable, the only interface implemented by arrays (this case could slip past the
compile-time checking if, for example, a reference to an array were stored in a variable
of type Object).
 
If T is an array type TC[], that is, an array of components of type TC, then a run-time
exception is thrown unless one of the following is true:
                TC and RC are the same primitive type.

                TC and RC are reference types and type RC can be cast
to TC by a recursive application of these run-time rules for casting." ( !!! )

Java VM does not allow cast from array where elements are one reference type (here Object, TC in JLS),
to array where elements are its subclass type (here String, RC in JLS).

In the example below java VM should not throw an exception according
to JLS while trying to cast "Object[] o" to "String[] s", because type
of all objects of an array are the same type as type in a cast expression.

An example and VM diagnostics follow:
-------------------------------------------------------
public class conv08305 {
       public static void main(String argv[]) {

          boolean b1 = true;
          String s[] = {null,null};
          Object o[] = {"1","2"};
          try {
             s = (String[]) o; //narrowing conversion
          }catch(ClassCastException e) {
             b1 = false;
          }
          if ( b1 ) System.out.println("Match JLS"); /*STATUS_PASSED*/
          System.out.println("b1=" + b1);
          /*STATUS_FAILED*/
       }
}

> uname -a
SunOS novo35 5.5.1 Generic_103640-12 sun4m sparc SUNW,SPARCstation-20
> javac conv08305.java
> java conv08305
b1=false
>
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