一元非运算符的内部工作（〜）

Question

public class bitwise_operator {

    public static void main(String[] args) {

        int var1 = 42;
        int var2 = ~var1;
        System.out.println(var1 + " " + var2); 
   }
}

The above code produces 42 -43 as the output.

As far as my understanding goes, Unary Not operator ( ~ ), inverts all of the bits of its operand.

Now, 42 in binary is 00101010 . On using ~ operator, we get inverted value of 42 ie 11010101

If you convert the preceding binary value, the output should be something else and not -43

Tried my luck with different numbers to observe the pattern and found that, the output is 1 number more than the initial value supplied with a leading (-) sign before it, as seen in the above case.

For eg..,

if num is 45           // Output is 45 -46
if num is 1001        // Output is 1001 -1002

Could someone please explain how the Unary Not Operator (~) works internally to output such results?

Answer 1

You are using a signed integer value which is in 2's complement.

Your result is correct: 11010101 is in fact -43:

-2^7 + 2^6 + 2^4 + 2^2 + 2^0 = -128 + 64 + 16 + 4 + 1 = -128 + 85 = -43

Answer 2

This is what's known as two's complement, and it is how integers and all fixed point numbers in Java, C, C++ etc work

-x = ~x + 1

So for example -1 (0xFFFF) negated bitwise (0x0) plus 1 = 0x1