Converting a String representation of bits to a byte

问题

I'm just beginning to learn about file compression and I've run into a bit of a roadblock. I have an application that will encode a string such as "program" as a compressed binary representation "010100111111011000"(note this is still stored as a String).

Encoding
g       111
r       10
a       110
p       010
o       011
m       00

Now I need to write this to the file system using a FileOutputStream, the problem I'm having is, how can I convert the string "010100111111011000" to a byte[]/bytes to be written to the file system with FileOutputStream?

I've never worked with bits/bytes before so I'm kind of at a dead end here.

回答1:

An introduction to bit-shift operators:

First, we have the left-shift operator, x << n. This will shift all the bits in x left by n bits, filling the new bits with zero:

      1111 1111 
<< 3: 1111 1000

Next, we have the signed right-shift operator, x >> n. This shifts all the bits in x right by n, copying the sign bit into the new bits:

      1111 1111 
>> 3: 1111 1111

      1000 0000
>> 3: 1111 0000

      0111 1111 
>> 3: 0000 1111

Finally, we have the zero-fill right-shift operator, x >>> n. This shifts all bits in x right by n bits, filling the new bits with zero:

       1111 1111 
>>> 3: 0001 1111

You may also find useful the bitwise-or operator, x | y. This compares the bits in each position in x and y, setting the new number's bit on if it was on in either x or y, off otherwise:

  1010 0101
| 1010 1010
  ---------
  1010 1111

You should only need the previous operators for the problem at hand, but for the sake of completeness, here are the last two:

The bitwise-and operator, x & y sets the bits in the output to one if and only if the bit is on in both x and y:

  1010 0101
& 1010 1010
  ---------
  1010 0000

The bitwise-xor operator, x ^ y sets the output bits to one if the bit is on in one number or the other but not both:

  1010 0101
^ 1010 1010
  ---------
  0000 1111

Now, applying these to the situation at hand:

You will need to use the bit-shift operators to add and manipulate bits. Start setting bits at the right side according to their string representations and shift them over. Continue until you hit the end of a byte, and then move to the next byte. Say we want to create a byte representation of "1100 1010":

Our byte    Target
---------   --------
0000 0000
            1100 1010
0000 0001   ^
            1100 1010
0000 0011    ^
            1100 1010
0000 0110     ^
            1100 1010
0000 1100      ^
            1100 1010
0001 1001        ^
            1100 1010
0011 0010         ^
            1100 1010
0110 0101          ^
            1100 1010
1100 1010           ^

I will, of course, leave it to you to apply this to your work.

回答2:

Chop your String up into lengths of 8 and call Byte#parseByte. If you set the radix to 2, it will parse the String as a binary number.

回答3:

I guess, you want to write these zeros and ones as binary values in a file. I so, you can iterate the string taking 8 signs everytime (String.substring() or smth) and create bytes with Byte(String) constructor. It's the easiest solution that comes to my mind for now.

If i'm not right about the problem, tell more about it please.

来源：https://stackoverflow.com/questions/8275474/converting-a-string-representation-of-bits-to-a-byte

标签

java

byte

bit-manipulation

bits

huffman-code