Question

I have some C code that I’d like to port to java. I haven’t done much C coding, but I was able to follow along up until this one function. If anyone could help me understand what is going on, it would be greatly appreciated.

int reverse_integer(int input) {
    int output = 0, i;

    for ( i=0, i<sizeof(int); i++ ) {
         output = ( input & 0x000000FF ) | output; 
         input >>= 8;
         if ( i < 3 ) {
             output <<= 8;
         }
    }

    return output;
}

The function is used as such:

char * position = //some data

/*the included comment on this next line states its the size of a string*/
int i = reverse_integer( *(int*)position ) 

Answer 1

I’ll be happy to post Java code that does the same as the C code, but only if you promise not to use it.

/** Reverses the bytes in an integer. */
   public static int reverseInteger(int input) {
      return
         (input >>> 24) |
         (input >> 8) & 0x0000ff00 |
         (input << 8) & 0x00ff0000 |
         (input << 24);
   }

Note that there’s no point in looping – an int in Java is always 4 bytes. Also note the triple-right angle bracket at the beginning the expression to perform an unsigned right-shift.

Now for the reasons not to use it:

1 – The function already exists – see Integer.reverseBytes(int)

2 – You’re going to have a hard time using that example code, since Java doesn’t let you cast an array of bytes as anything else. Java is officially big-endian (most significant byte first), so if you’re reading bytes from a file then you can use java.io.DataInputStream to extract ints, longs, etc.