Converting C code to Java:Pointers

Question

I know we cannot access address of an Object safely in Java and we should never try to do something like this. But i need to convert this C code to Java. Value is always of size 8 bits . The C code is like this:

void merge(int value,unsigned short* outp, int x,int y, int width)
{
    unsigned char *outc;
    outc=(unsigned char *) outp + y*width +x;
    *outc=value;
} 

To convert it into Java I have converted unsigned short* outp to short outp[] and unsigned char* outc to short outc[] . Then i can assign values in parts. But the problem i am facing is that when this function is called in a loop by some other function the width is incremented each time and only one byte is assigned to a particular cell..

I tried this Java code:

void merge(int value, int outp[], int x, int y, int width)
{

if(outp[y*width + x] >0)
   {    
        outp[y*width+x]=outp[y*width+x]<<8 +value;
   }
   else
   {
        outp[y*width+x]=value;
   }
}

How can i do this. Please help. Thanks.

Answer 1

It is not "possible" in java that does not have pointers. It has references that are irrelevant for primitive type like char .

So, in java we typically use method that returns value instead of returning the value via pointer like C programmers do:

char merge(int value,int x,int y, int width) {
    char outchar;
    /// your code...
    return outchar;
}

Answer 2

I have no idea what your code should do. To me it looks like it only sets the lower byte of a short to the given value and the shift in your java code seems out of place. Since you for some reason replaced short[] with int[] i don't know if the following will help:

void merge(int value,short[] outp, int x,int y, int width){
   int address =  y*width +x;
   short temp = outp[address];
   temp &= 0xFF00;//keep the high order bits of the old value
   temp |= (short)(value & 0x00FF);//set the low order bits
   outp[address] = temp;

}

Answer 3

Your C code looks like pointer arithmetic to find the address of a cell at a particular location in a grid, then set the value at that location.

For your Java code I would recommend a completely different approach. What data structure does your calling code use for the grid? You could use a class which models the grid, and includes a setValue(x,y) method. How the class models the grid internally is up to you - it could use a 2 dimensional array.

Answer 4

As a C programmer who tried to use Java for a project, I can certainly identify with your woes.

To resolve your problem, I would rather try to access your 2D data a 1D Java bytearray.

This will make it trivial to access the (row * col) byte and you will spare the need for this wrapper code (which will be very costly in a loop).

Whether you will allocate the array from Java or from C depends on your project but JNI calls make both solutions possible.

Good luck!