Shift operators

Question

I have this code:

class Test
{
    public static void main(String args[])
    {
        System.out.println((-9 >>> 1)); //O/P  2147483643
        System.out.println((-9 >> 1));  //O/P -5
    }
}

Correct me if I am wrong in understanding the following:

The bit pattern for -9 is

10000000 00000000 00000000 00001001

If shifted to >> right with 1 position then

1100000 00000000 00000000 00000100

This above bit pattern is not equal to -5. How do these 2 operators work on negative numbers? I do understand that if they are used on positive numbers, then the behavior is same.

Answer 1

The bit pattern for -9 is not what you think it is.

It's

11111111 11111111 11111111 11110111

Therefore, shifting it by one (not keeping or keeping the sign bit) :

01111111 11111111 11111111 11111011 // 2147483643

11111111 11111111 11111111 11111011 // -5

Answer 2

The bit pattern for -9 is not

10000000 00000000 00000000 00001001

public static void main(String[] args) {
    System.out.printf("%d = %s%n", -9, Integer.toBinaryString(-9));
    System.out.printf("%d = %s%n", -9 >>> 1,Integer.toBinaryString(-9 >>> 1));
    System.out.printf("%d = %s%n", -9 >> 1, Integer.toBinaryString(-9 >> 1));
}

We see the full story is

-9 = 11111111111111111111111111110111
2147483643 = 1111111111111111111111111111011
-5 = 11111111111111111111111111111011

Answer 3

Your are having a confusion about how -9 is represented. A simple way is given below,

Write the bit pattern for 9
Reverse all the bit
add binary +1 (Binary addition)

you will get binary of -9

00000000 00000000 00000000 00001001 //9
11111111 11111111 11111111 11110110 //Reverse all the bit
11111111 11111111 11111111 11110111 // +1 will give -9

I hope that, you might have studied in your childhood days..