Re: Array typecasting ?
me2 wrote On 04/10/07 14:10,:
On Tue, 10 Apr 2007 14:03:53 -0400, Eric Sosman wrote:
me2 wrote On 04/10/07 13:47,:
I've got an array of chars. The data in the array is really short
integers that were coerced into being bytes.
Do you understand that `byte' and `char' are not
the same thing in Java? Are you aware that `char' and
`short' are both sixteen-bit values (although with
differing ranges)?
Yes, I do. I meant byte. Its a byte array. I was thinking with my C hat
when I said and wrote char.
How can I access the array as short ints instead of bytes ?
By using a language other than Java. Java has no
magic casting wand to say "Hey, presto!" and transform
pairs of bytes (or single chars) into shorts, or into
doubles, or into BunnyRabbits.
Wow. I guess I could always do this
short myShort = myArray[1] << 8 + myArray[i+1];
.... for suitable values of "always." First, the r.h.s.
computes an int, and javac will reject the attempt to put
that value in a short; you need to insert a cast. Second,
bytes are signed types in Java and will be sign-extended
when converted to int, so a negative byte will "smear"
with twenty-four extra one-bits; you need to AND away the
unwanted bits. Third, the `+' operator binds more tightly
than `<<', so the value you're trying to calculate is
myArray[i] left-shifted by the sum of 8 and myArray[i+1];
you need some parentheses.
That is a lot of extra CPU cycles though. Isn't there another way to make
this work ? (MyArray is an array of bytes.)
.... which is the source of your trouble: You've got values
of one type (short) chopped to pieces and the pieces stuffed
into values of another type (byte).
Presumably, these shorts-as-batches-of-bytes arrived via
some kinds of byte-oriented transport: You plucked them out
of a network stream or from a disk file or something of the
kind. You now need to reassemble the chopped-up pieces into
the values they encode, and you can NOT do this by somehow
conning the compiler into believing that bytes are shorts,
true is false, and black is white.
Observe that you cannot do this even in C unless you're
willing to sacrifice all hope of portability: Not only are
there endianness issues swarming all over the place, but
alignment problems lurk in the background and sharpen their
SIGBUS scimitars. C will permit you to say "let's pretend"
about data types (although you may not get away with it);
Java rejects such make-believe games altogether.
--
Eric.Sosman@sun.com