Java casting: is the compiler wrong, or is the language spec wrong, or am I wrong?

Question

I have been reading the Java Language Spec, 3rd edition, and have found what I think is a discrepancy between the spec and the javac compiler implementation. The same discrepancies exist in the Eclipse compiler.

Section 15.16 talks about cast expressions. It says that it should be a compile time error if the argument type cannot be converted to the cast type via casting conversion (section 5.5):

It is a compile-time error if the compile-time type of the operand may never be cast to the type specified by the cast operator according to the rules of casting conversion (§5.5). Otherwise, at run-time, the operand value is converted (if necessary) by casting conversion to the type specified by the cast operator.

Section 5.5 talks about casting conversion. It gives a list of conversion types which are allowed. Specifically absent from the list is "unboxing conversion followed by widening/narrowing primitive conversion". However that exact sequence of conversions does seem to be allowed by the javac compiler (and also the Eclipse compiler). For instance:

long l = (long) Integer.valueOf(45);

... compiles just fine. (The problematic cast is the cast to long; the argument is of type java.lang.Integer, so the conversion requires unboxing to int followed by a widening primitive conversion).

Likewise, according to the JLS it should not be possible to cast from byte to char, because that (according to 5.1.4) requires a widening primitive conversion and a narrowing primitive conversion - however, this cast is also allowed by the compilers.

Can anyone enlighten me?

Edit: since asking this, I have filed a bug report with Oracle. Their response is that this is a "glitch in the JLS".

Answer 1

I think you are right, the compilers are right, and the spec is wrong....

This compiles: (Object)45 and this does not: (Long)45

The only way to make sense of the compilers' behavior (including Intellij I'm using) is if Casting Conversion is modified to agree with Assignment Conversion and Method Invocation Conversion:

• a boxing conversion (§5.1.7) optionally followed by widening reference conversion

• an unboxing conversion (§5.1.8) optionally followed by a widening primitive conversion.

plus

• widening and narrowing primitive convesion

The spec did say "casting conversions are more inclusive than assignment or method invocation conversions: a cast can do any permitted conversion other than a string conversion or a capture conversion"

Answer 2

By my reading, the cast from int to long is permitted by this clause:

A value of a primitive type can be cast to another primitive type by identity conversion, if the types are the same, or by a widening primitive conversion or a narrowing primitive conversion.

Converting int to long is a widening primitive conversion.

That just leaves the conversion from Integer to int, which is accommodated by the last bullet:

an unboxing conversion

Of course, the cast to long isn't even necessary in the example.

Consider the following four definitions:

final Integer io = Integer.valueOf(45);
final int i = io;
final long l1 = (long)i;
final long l2 = i;

Do you consider any of them surprising? Your original example doesn't look any different; it merely elides the intermediate variables.