Bit manipulation and output in Java

Question

If you have binary strings (literally String objects that contain only 1's and 0's), how would you output them as bits into a file?

This is for a text compressor I was working on; it's still bugging me, and it'd be nice to finally get it working. Thanks!

Answer 1

Easiest is to simply take 8 consecutive characters, turn them into a byte and output that byte. Pad with zeros at the end if you can recognize the end-of-stream, or add a header with length (in bits) at the beginning of the file.

The inner loop would look something like:

byte[] buffer = new byte[ ( string.length + 7 ) / 8 ];
for ( int i = 0; i < buffer.length; ++i ) {
   byte current = 0;
   for ( int j = 7; j >= 0; --j )
       if ( string[ i * 8 + j ] == '1' )
           current |= 1 << j;
   output( current );
}

You'll need to make some adjustments, but that's the general idea.

Answer 2

If you're lucky, java.math.BigInteger may do everything for you.

String s = "11001010001010101110101001001110";
byte[] bytes = (new java.math.BigInteger(s, 2)).toByteArray();

This does depend on the byte order (big-endian) and right-aligning (if the number of bits is not a multiple of 8) being what you want but it may be simpler to modify the array afterwards than to do the character conversion yourself.