print same values?

[srikanth reddy]

public class TestClass
{
      public static void main(String[] args) throws Exception
      {
            int a = Integer.MIN_VALUE;
            int b = -a;
            System.out.println( a+ "   "+b);
      }
}  

why does the o/p prints same -ve values?? instead of one +ve and other -ve

[Sandeep Chhabra]

Hi,

-Integer.MIN_VALUE= Integer.MAX_VALUE+1
and
Integer.MAX_VLAUE+x= Integer.MIN_VALUE-(x-1)


so
-Integer.MIN_VALUE= Integer.MAX_VALUE+1
-(-2147483648)=2147483648 (which is 1 greater then MAX_VALUE)

hence result is :

Integer.MAX_VLAUE+x= Integer.MIN_VALUE-(x-1)
-2147483648-(1-1)
=> -2147483648-0
=> -2147483648

Hence both the values are same.

I know the way I Have expalined is bit difficult...
But this is the way it works.

[Thomas Drew]

Here a short program to illustrate what Sandeep is saying.

 public static void main(String[] args) {
       int a = Integer.MIN_VALUE;
System.out.println("MIN_VALUE FOR int   a = " + Integer.toBinaryString(a)+   " or " +  a);
System.out.println("Two Complements of  a =" + Integer.toBinaryString(~a) +  " or "  + ~a);
        System.out.println("Two Complements + 1 a = " + Integer.toBinaryString(-a) + " or " + -a);
        
    }
  

results of running program:

MIN_VALUE FOR int a = 10000000000000000000000000000000 or -2147483648
Two Complements of a =1111111111111111111111111111111 or 2147483647
Two Complements + 1 a = 10000000000000000000000000000000 or -2147483648

[Barry Gaunt]

Thomas, when you flip the bits you get ones-complement. Twos complement is the ones-complement + 1.

[Roel De Nijs]

now 2 complements is mentioned, got a question about it:

all numeric types in java are signed, so the leftmost bit is indicating the sign (1 = negative, 0 is positive). so far so good. but can someone explain the two's complement method to calculate the value of a bit representation

1100 0010 = ?
0100 0010 = ?

and whats the bit representation of 20 and -20 ?

Thanks!

[Mandy Hou]

That is right

[Henry Wong]

Originally posted by Roel De Nijs:
but can someone explain the two's complement method to calculate the value of a bit representation

As already mentioned, a twos complement of a number is to flip all the bits and add one. The question you might be asking is why would anyone design such a silly way to represent negative numbers... believe it or not, it is because it makes the math easier !

To add and subtract numbers, with twos complement, you just ignore the negative bit, treat everything as unsigned numbers, and everything works... for example (16 bits only) ...

 
    1 = 0000 0000 0000 0001
   -1 = 1111 1111 1111 1111 (flip all bits and add one)
  ---------------------------
    0 = 0000 0000 0000 0000

If you add one to negative one it will roll over and get you a zero -- and this will work for all cases.

Henry

[Mani vannan]

Refer Java Language Specfication [JLS]. It has good explanation and examples for this kind of scenario.

[Mani vannan]

Refer Java Language Specfication [JLS]. It has good explanation and examples for this kind of scenario.

[Roel De Nijs]

So if i get it right:
- to know the bit representation of a negative number, you take the bit representation for the positive number, flip all bits and add 1
- if you have the bit representation of a negative number and want to know the positive one, just do the same: flip all bits and add 1