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javabinarybytebitshort

Replace multiple bits in a short

How can I replace multiple bits in a short in Java?

I'm working on a encryption algorithm where I need to do the following: I got a short and need to apply a series of replacements of 4 bits. Example: if the first 4 bits are 0010, replace them with 0110, if its 1111, replace them with 1100 and so on, the same for the second 4 bits.

What is the best/fastest way to do this? At the moment I convert the short to a String and do it with String replacement but its obviously extremely slow and in my opinion the absolute wrong way.

Solution

  • Bit arithmetic, something like this:

    short s = 191;
short first = (short) (s & 0x000F);
short second = (short) ((s >> 4) & 0x000F);
short third = (short) ((s >> 8) & 0x000F);
short fourth = (short) ((s >> 12) & 0x000F);

call_the_method_to_convert_each();

s = fourth;
s = ((short) ((s << 4) | third));
s = ((short) ((s << 4) | second));
s = ((short) ((s << 4) | first));