Question

I have a value DB of 3 bytes (DB_1, DB_2, DB_3).
I need to check DB_3 for specific bits. For example I have to see if
DB_3 == 11X0XXXX
Where only bit 4, 6 and 7 should be checked. Bits marked as X can take any value and should not be checked. I am not familiar with Bit operations in Java and glad for any help!
Thanks!

Answer 1

You can use a bitwise AND (& in Java) to accomplish the masking to specific bits (the mask is the second line and will only let those bits of the first line through where the mask has a 1 [marked with arrows below the calculation]):

  11101001
& 11010000
----------
  11000000
  ↑↑ ↑

You’ll retain exactly those bits that were 1 in both operands, so essentially you set all those bits to 0 that you are not interested in.

So you just need to do

if (DB_3 & 0xD0 == 0xC0) { ... }

0xD0 is the hexadecimal form of 208 which is 11010000 in binary and you want to know whether it matches 0xC0 which is 192, or 11000000. You’ll know that all the bits you don’t care about (the Xes in your question) are already zero at this point due to the bitwise AND.

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ETA (2011-12-14 14:24): Apparently Java 7 has binary integer literals, so you can do

if (DB_3 & 0b11010000 == 0b11000000) { ... }

which makes the mask more readily apparent. Thanks, Glenn.