bit manipulation - Obtain a specific subset of bits of an int in Java

Question

How would I obtain a specific subset, say bits 5-10, of an int in Java?

Looking for a method where one can pass in specific bit positions. I'm not sure how I would create a mask that changes given the input, or even if that is how one should go about doing it.

I know this is how one would get the front say 10 bits of an int: (I think) int x = num >> 22;

Answer 1

Say you have a number n, and want bits from i to j (i=5, j=10).

Note, that i=0 will give you the last bit

 int value = n & (((1 << (j-i)) - 1) << i );

will give you the result.

The left part is obvious: you have a value, and you will put a bitmask on it.

The value of the mask is ((1 << (j-i)) - 1) << i. It says:

• Take a 1 bit (value: 0000000000000001)
• Shift it left j-i times (value: 2^(10-5) = 2^5 = 32 = 0000000000100000)
• Deduct 1 (value: 31 = 0000000000011111) - have you seen the lowest bits reversed?
• Shift it left i times (value: 31*32=992 = 0000001111100000)

So, you have got the bitmask for bits 5 - 10 (more precisely, from 5 to 9, since 10th is not included).