CONVERSIONS AND PROMOTIONS

Casting Conversion

5.5

If a cast to a reference type is not a compile time error, there are two cases:

The cast can be determined to be correct at compile time. A cast from the

compile time type

S

 to compile time type

T

 is correct at compile time if and

only if

S

 can be converted to

T

 by assignment conversion ( 5.2).

The cast requires a run time validity check. If the value at run time is

null

,

then the cast is allowed. Otherwise, let

R

 be the class of the object referred to

by the run time reference value, and let

T

 be the type named in the cast opera 

tor. A cast conversion must check, at run time, that the class

R

 is assignment

compatible with the type

T

, using the algorithm specified in  5.2 but using the

class

R

 instead of the compile time type

S

 as specified there. (Note that

R

 can 

not be an interface when these rules are first applied for any given cast, but

R

may be an interface if the rules are applied recursively because the run time

reference value refers to an array whose element type is an interface type.)

This modified algorithm is shown here:

N

If

R

 is an ordinary class (not an array class):

O

If

T

 is a class type, then

R

 must be either the same class ( 4.3.4) as

T

 or a

subclass of

T

, or a run time exception is thrown.

O

If

T

 is an interface type, then

R

 must implement ( 8.1.4) interface

T

, or a

run time exception is thrown.

O

If

T

 is an array type, then a run time exception is thrown.

N

If

R

 is an interface:

O

If

T

 is a class type, then

T

 must be

Object

 ( 4.3.2,  20.1), or a run time

exception is thrown.

O

If

T

 is an interface type, then

R

 must be either the same interface as

T

 or a

subinterface of

T

, or a run time exception is thrown.

O

If

T

 is an array type, then a run time exception is thrown.

N

If

R

 is a class representing an array type

RC[]

 that is, an array of compo 

nents of type

RC

:

O

If

T

 is a class type, then

T

 must be

Object

 ( 4.3.2,  20.1), or a run time

exception is thrown.

O

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 ref 

erence to an array were stored in a variable of type

Object

).

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