CODEWITHSUNDEEP
javabinaryextractbit-manipulationbit
3kings
3kings

Reputation: 838

Extracting certain bit from binary data

I have a binary number that looks like this:

00000000 00000000 00000011 00001101  =  781 integer value  = integer name "packed"

How am i able to extract each of those as separate integer values using bit wise in java. Like the following:

int a = (function) = 00000000;
int b = (function) = 00000000;
int c = (function) = 00000011;
int d = (function) = 00001101;

If you can see what i am doing up there.. I want the first 8 into one integer the next into a second integer and so on.. I think you do this using bit wise in java but i am completely unsure. Any help is appreciated. Sorry but i am completely new to this kind of thing and really need help with this, Thanks! Basically (function) should equal something like this:

packed >> 5; (I know this isn't nearly right that is why i am needing help on this)...

Thanks!

Upvotes: 1

Views: 2194

Answers (3)

rgettman
rgettman

Reputation: 178253

You are right in that bitwise operations are needed here.

First, use a mask to isolate only the bits you're interested in, turning all other bits off:

value & 0xFF000000  // isolate most significant 8 bits
value & 0x00FF0000  // isolate next 8 bits
value & 0x0000FF00  // isolate next 8 bits
value & 0x000000FF  // isolate least significant  8 bits

Then shift the bits into the least significant bits to store into a, b, c, and d (an unsigned bitshift):

(value & 0xFF000000) >>> 24 // Shift bits 24-31 to bits 0-7
(value & 0x00FF0000) >>> 16 // Shift bits 16-23 to bits 0-7
(value & 0x0000FF00) >>> 8  // Shift bits 8-15 to bits 0-7
(value & 0x000000FF)        // No need to shift the last one!

Upvotes: 5

Garee
Garee

Reputation: 1284

This can be achieved by performing a bitwise-AND operation using a mask, followed by bit-shift. The mask is simply a number in which all bits that you are not interested in are set to zero. It masks the bits you do not care about.

int BYTE_ONE_MASK   = 0xFF;
int BYTE_TWO_MASK   = 0xFF00;
int BYTE_THREE_MASK = 0xFF0000;
int BYTE_FOUR_MASK  = 0xFF000000;

The bit-shift is necessary for all bytes other than the least significant one:

int a = (val & BYTE_ONE_MASK);
int b = (val & BYTE_TWO_MASK) >> 8;
int c = (val & BYTE_THREE_MASK) >> 16;
int d = (val & BYTE_FOUR_MASK) >> 24;

This can be generalised into a function such as the following:

  // Assuming the first byte is n = 0.
  public static int getByte(int val, int n) {
    int shift = n * 8;
    int mask = 0xFF << shift;
    return (val & mask) >> shift;
  }

Upvotes: 4

tckmn
tckmn

Reputation: 59273

Try something like this:

public byte getNthByte(int n, int packed) {
    // shift the desired bits into the last 8 bits
    int shifted = packed >> (8 * n);
    // and now get those bits
    int masked = 0x000000FF & shifted;
    return masked;
}

Then

int a = getNthByte(3);
int b = getNthByte(2);
int c = getNthByte(1);
int d = getNthByte(0);

Upvotes: 3

Related Questions