CODEWITHSUNDEEP
javaoperators
mindfreak
mindfreak

Reputation: 455

Internal working of Unary Not Operator (~)

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 i.e. 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?

Upvotes: 1

Views: 871

Answers (2)

ControlAltDel
ControlAltDel

Reputation: 35011

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

Upvotes: 0

Ori Lentz
Ori Lentz

Reputation: 3688

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

Upvotes: 3

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