# bitwise inversion ~ operator

anand nagpal

Greenhorn

Posts: 2

posted 15 years ago

Hi all,

class test{

public static void main(String args[]){

int i = 1;

System.out.println(~i);

}

}

Output : -2

can someone kindly explain me what exactly ~ operator does ?

thanks

anand

class test{

public static void main(String args[]){

int i = 1;

System.out.println(~i);

}

}

Output : -2

can someone kindly explain me what exactly ~ operator does ?

thanks

anand

Valentin Crettaz

Gold Digger

Sheriff

Sheriff

Posts: 7610

posted 15 years ago

~ just inverts all bits of the number

In this case

i=1 => 00000000 00000000 00000000 00000001

~i => 11111111 11111111 11111111 11111110

And in binary ~1 equals -2.

If you do Integer.parseInt("11111111111111111111111111111110",2);

you'll get -2

More on binary:

2 in binary is 00000000 00000000 00000000 00000010

if you want to get the negative value of 2 you have to take the 2's complement of 2 which is just inverting all bits and adding 1.

So if you invert it you get 11111111 11111111 11111111 11111101

and then you add one and you get

11111111 11111111 11111111 11111110 which is the same pattern as above and thus ~1 = -2

The genral rule is that ~i = -i-1

HIH

------------------

Valentin Crettaz

Sun Certified Programmer for Java 2 Platform

In this case

i=1 => 00000000 00000000 00000000 00000001

~i => 11111111 11111111 11111111 11111110

And in binary ~1 equals -2.

If you do Integer.parseInt("11111111111111111111111111111110",2);

you'll get -2

More on binary:

2 in binary is 00000000 00000000 00000000 00000010

if you want to get the negative value of 2 you have to take the 2's complement of 2 which is just inverting all bits and adding 1.

So if you invert it you get 11111111 11111111 11111111 11111101

and then you add one and you get

11111111 11111111 11111111 11111110 which is the same pattern as above and thus ~1 = -2

The genral rule is that ~i = -i-1

HIH

------------------

Valentin Crettaz

Sun Certified Programmer for Java 2 Platform

SCJP 5, SCJD, SCBCD, SCWCD, SCDJWS, IBM XML

[Blog] [Blogroll] [My Reviews] My Linked In

posted 15 years ago

Hi Valentin

But I think

~1 in NOT equal to -2 in BINARY.

It is the

if we have lets say 4 bit integer size then -1 will be stored as :

i) 1 in binary = 0001

ii) 2's compliment of 1 = 1111 (invert 0001, becomes 1110, add 1, becomes 1111);

so

This is done for lot of reason, it makes airthmetic operation fasts(there are lot of algos for binary airthmetic), and it also makes -0 and +0 equal.

I just want to say that it is the implementation which makes ~1 to equal to -2, in binary it will be some different number.

CMIW

AND

------------------

Regards

Ravish

[This message has been edited by ravish kumar (edited November 18, 2001).]

Originally posted by Valentin Crettaz:

And in binary ~1 equals -2.

Hi Valentin

But I think

~1 in NOT equal to -2 in BINARY.

It is the

**implementation of negative number**which makes ~1 equal to -2, as neagtive numbers are stored as 2's compliment numbers.

if we have lets say 4 bit integer size then -1 will be stored as :

i) 1 in binary = 0001

ii) 2's compliment of 1 = 1111 (invert 0001, becomes 1110, add 1, becomes 1111);

so

*-1 will be stored as 1111*(NOTE: MSB is '1'(on) which indicates that number is negative. 2's compliment is of positive number(eg. 1))

This is done for lot of reason, it makes airthmetic operation fasts(there are lot of algos for binary airthmetic), and it also makes -0 and +0 equal.

I just want to say that it is the implementation which makes ~1 to equal to -2, in binary it will be some different number.

CMIW

AND

**thanks a lot**for the general rule. It will really help us in binary questions in SCJP.

------------------

Regards

Ravish

[This message has been edited by ravish kumar (edited November 18, 2001).]

"Thanks to Indian media who has over the period of time swiped out intellectual taste from mass Indian population." - Chetan Parekh