how to communicate the integer value of char from C++ to Java

Ok, so I moved to this forum again, since I was told, my question doesn't belong to the JNI forum.

First of all, I do not even really understand the char type in Java.

I was hoping that for all hexadecimal numbers 0x... between 0x0000 and 0xffff there exists a character '\u...'

However, I noticed that for example there is no character

'\u000a'

and there is no character

'\u000d'

When I try to write these characters in Borland JBuilder 2006, I get the message:

illegal line end in character literal.

I am trying to communicate arbitrary numbers between 0 and 255, which are specified as unsigned chars in C++ to Java, by first making chars out of them. But it seems, I wouldn't even be able to communicate the number 10 (0x000a) or 13 (0x000d).

So it doesn't look like that the value of the C++ char, which lies between -128 and 127, is communicated to the same value of the Java char of my Java string.

Is it possible to completely abandon using Java characters and to make a file that can be converted to an array of single bytes (not two bytes like characters or more bytes)?

Thanks for your time!

BrigitAnanyaa at 2007-7-12 10:18:06 >

>

> So it doesn't look like that the value of the C++

> char, which lies between -128 and 127, is

> communicated to the same value of the Java char of my

> Java string.

>

As I told you in the other thread all that matters is the bits, not the display value.

And in C++ there is no such thing as a 'char' value with a value less than zero. That is because there is no charset in the world that has negative values. So however you are displaying it in C++ is wrong as well (and display again has nothing to do with the bits of the value.)

Also as I suggested in the other thread you should be using hex and not integer values.

> Is it possible to completely abandon using Java

> characters and to make a file that can be converted

> to an array of single bytes (not two bytes like

> characters or more bytes)?

Yes.

jschella at 2007-7-12 10:18:06 >

> Ok, so how do I write the char as hex in C++ and how

> do I read it as hex in Java?

When you display it you should use hex in both languages. Presumably you are doing that to determine that the values come across correctly.

Presuming that you haven't mangled the values in C then you just write them as chars (C 8 bit values.) In java you read them as bytes.

If you need to construct a 16 bit value then you will have to combine them.

> (I know, in Java characters never have negative

> value. but why can't they have all positive values?

Still not the point.

You are moving bits. All you should care about is the bits.

However you need to correctly display them and understand that there is a difference between a display value and the bits.

> And in C++ why is there the char type ranging from

> -128 to 127 as opposed to the unsigned char type

> ranging from 0 to 255, if there is no char with a

> negative value? My understanding is that the char

> type in C++ is the same as the byte type in Java, am

> I correct?)

>

It is a data type, not a character set.

But regardless you should only be concerned with the bits in each and not the type.

> Well, sorry, I am a mathematician, not really a

> programmer, even though I have programmed for 25

> years.

Basically in both language 8 bits will end up as unsigned values when you do the following (because the following excludes sign extension if you care.)

byteValue & 0x0ff

Notice the the above doesn't change the value (contents) of byteValue. All it does is allow you to display the bits in a non-negative form.

jschella at 2007-7-12 10:18:06 >

I found an answer to the previous question about how to communicate the integer value of an unsigned char from C++ to Java. The unsigned char array contains the r, g, b values of an image. Here is the answer, I had to receive the chars as bytes:

In Java I wrote the code:

int[] pix = new int[count]; //each element has the r, g, b values

//count = width*height

FileInputStream pixelFile = new FileInputStream("C:\\pixelvalues.dat");

DataInputStream pixelData = new DataInputStream(pixelFile);

byte[] pixelByte = new byte [3*count];

try

{

pixelData.read(pixelByte);

for (int y = 0; y < height; y++)

{

for (int x = 0; x < width; x++)

{

red = (int)pixelByte[y*pixelsw + x] & 0xff;

green = (int)pixelByte[count + y*pixelsw + x] & 0xff;

blue = (int)pixelByte[2*count + y*pixelsw + x] & 0xff;

pix[y*pixelsw + x] = 0xff000000 | ((red<<16) + (green<<8) + blue);

}

}

}

catch (IOException e)

{

}

and in C++ I wrote the code:

unsigned char *pixels = ... //the length of this array is 3*count

const char *charpixels = reinterpret_cast<char*>(pixels);

fstream file;

file.open("C:\\pixelvalues.dat", ios::out | ios::binary);

file.write(charpixels, 3*count);

file.close();

BrigitAnanyaa at 2007-7-21 21:17:43 >