Re: Merging bytes

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Mon, 26 Apr 2010 20:10:52 -0400
On 26-04-2010 15:39, mikew01 wrote:

On Apr 26, 6:06 pm, rossum<> wrote:

On Mon, 26 Apr 2010 03:59:25 -0700 (PDT), mikew01
<> wrote:

I need to combine 3 number values into a 2 byte array.
The value bit sizes are:

value1 = 6 bits.
value2 = 6 bits.
value3 = 12 bits.

I understand I need to use the bit manipulation operators to do this
but would like some advice on the best approach using Java.

As Jan has pointed out you cannot fit 24 bits into 16 bits. You need
to rethink the specification. Why are you trying to compress data
like this? What other ways are there to compress the data?

Trying to do a lot of bit-twiddling will slow down your application.

Sorry there is a typo in the first post, value3 is 4 bits not 12
It's not up to me how the data is packaged unfortunately, I am dealing
with a platform that has very limited resources so every bit counts.

Probably the most efficient is just to do the bit shifting and
masking as proposed by other.

If you are willing to spend a bit more memory and CPU for the
sake of cleaner code, then I have a little record package
to write and read native structures.

See demo below.



import dk.vajhoej.record.FieldType;
import dk.vajhoej.record.Struct;
import dk.vajhoej.record.StructField;
import dk.vajhoej.record.StructWriter;

public class ThreeBitFields {
     private int value1;
     private int value2;
     private int value3;
     public int getValue1() {
         return value1;
     public void setValue1(int value1) {
         this.value1 = value1;
     public int getValue2() {
         return value2;
     public void setValue2(int value2) {
         this.value2 = value2;
     public int getValue3() {
         return value3;
     public void setValue3(int value3) {
         this.value3 = value3;
     public static void main(String[] args) throws Exception {
         ThreeBitFields o = new ThreeBitFields();
         StructWriter sw = new StructWriter();
         byte[] b = sw.getBytes();
         for(int i = 0; i < b.length; i++) {
             System.out.printf("%02X", b[i]);

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