Java-按位比较和位移

Question

I need a little explanation of the following statement, What is it doing?:

int result = 154 + (153 << 8) + (25 << 16) + (64 << 24);
/* what would be the value of result after this line and how it is calculated
Why we need this? */

Answer 1

(153 << 8) is equivalent to 153 * pow(2, 8)

You are actually shifting your bits towards left..

Also: -

(153 >> 8) is equivalent to 153 / pow(2, 8)

You can guess why.. This is actually shifting bits towards right..

EG: -

3 => 0011

3 << 2 is equal to 1100 -> (Which is code for 12)

3 >> 2 is equal to 0000 -> (Which is code for 0) -> you can see - **(3 / 4 = 0)**

NOTE :- Note that the right shifting rounds off towards negative infinity ..

For EG: -

-3 >> 1 --> -2 (-1.5 is rounded to -2)

Lets see how it happens: -

In binary string representation: -

-3 ==> 11111111111111111111111111111101

-3 >> 1 ==> 11111111111111111111111111111110 (-3 / 2 = -1.5 -> rounded to -2)

(Note the left most bit is filled by the bit that was present at the left most end before shift (1 in this case))

So, the value becomes, -2 (for -3>>1, which is greater than -3 ) This happens for negative numbers.. Right shifting a negative number gives a number greater than the original number..

Which is contrary to the positive number where the left most bit will be filled by 0 ... Thus value obtained will be less than the original..

3 ==>  00000000000000000000000000000011

3 >> 1 ==> 00000000000000000000000000000001 (3 / 2 = 1.5 -> rounded to 1)

(So left most bit remains 0. So, the value is 1 (less than 3), ie, value is rounded off towards negative infinity and becomes 1 from 1.5)

Similarly you can devise results for left-shifting positive and negative number..

Answer 2

The answer is 1075419546. The left shift operator is basically addings 0s to the binary representations of the decimal numbers so this would simply to

154 + 153 * 2^8 + 25 * 2^16 + 64*2^24

So you can convert 153 to its binary representation, then add 8 zeros and convert back to decimal etc etc..

Answer 3

in practical terms, if you use the given operator on a math expression it has the same meaning as the following:

123 << n is exactly the same as 123 * 2 ^ n

For example, 2 << 2 is 2 * 2 ^ 2 which is 8 or the same as 1000

Otherwise, you are just shifting bits to the left:

3 = 11 then 3 << 2 = 1100.

Hope it makes it clear.