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Explain the code...

sharmistha mohapatra
Greenhorn

Joined: Dec 12, 2008
Posts: 18
I was just trying out a code as below

public class Flip{
public static void main(String argv[]){
System.out.println(~4);
}
}

And i get the result as -5.

Kindly explain me how does it happen?
Maneesh Godbole
Saloon Keeper

Joined: Jul 26, 2007
Posts: 10535
    
    9

The tilde "~" inverts the bits ( 0 to 1 and 1 to 0)
More here
http://java.sun.com/docs/books/tutorial/java/nutsandbolts/op3.html

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sharmistha mohapatra
Greenhorn

Joined: Dec 12, 2008
Posts: 18
Thanks for the reply!!!

But still under confusion,as where does this invertion come into action...
If i replace the number in the code to 5 or 6 or 7,the output gives a +1 to the number with a negative sign.

For eg: if i replace it with say 6 now...
I get the output as -7
Vijitha Kumara
Bartender

Joined: Mar 24, 2008
Posts: 3860

sharmistha mohapatra wrote:

If i replace the number in the code to 5 or 6 or 7,the output gives a +1 to the number with a negative sign.

For eg: if i replace it with say 6 now...
I get the output as -7


Yes, That's an easy way to calculate the result without trying it in two's complement way (which is what happens internally).


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fred rosenberger
lowercase baba
Bartender

Joined: Oct 02, 2003
Posts: 11499
    
  16

There are two thing you need to understand

1) the '~' inverts all the bits. Since a '4' is represented as

00000000 00000000 00000000 00000100

when you invert the bits you get

11111111 11111111 11111111 11111011

2) now you need to understand how java interprets this value. google or search this forum for "two's complement". basically, you look a the left-most digit, and if it's a 1, you know your value will be negative. you then invert all the digits and add one, so you get



which is 5. and we know our answer is negative, so "11111111 11111111 11111111 11111011" = -5.


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