About Question enthuware.ocajp.i.v7.2.1159 :

[The_Nick]

I would like to add this post from stackoverflow.com
It helps to clarify the point of this question in my view.
http://stackoverflow.com/questions/1253 ... en-compare

[DesRenthuware]

Very interesting question - assumed that the answer would be an overflow (so got it wrong/right?). Remembered the min and max vals would be one out, but expected mathematical rules to take precedence (i.e. -(-x) = +x) and so would overflow.

[javanaut]

Please let me know if I am wrong, but I think the explanation for this question is a little misleading when describing two's complement.

The second step is to add 1 but it shows adding 32 1's to the complemented number, which is not correct if I am not mistaken. In other words it is like the explanation is saying to add the MAX_VALUE in binary of the number to the complemented (everything switched to the opposite) binary value.

Regards,

Javanaut

[admin]

I don't see it adding 32 1's. It shows the number that you get after adding 1 to step 1.

Step 1 (complement the bits of 1): ///11111111 11111111 11111111 11111110///
Step 2 (add 1 to step 1): ///11111111 11111111 11111111 11111111///.

HTH,
Paul.

[javanaut]

Oh yeah, I just added it on paper in binary and adding one to the complemented bits in step one does produce 32 1's. I see what the explanation is saying now.

Thanks Paul.

Regards,

Javanaut

[javanaut]

Code: Select all

// java program by javanaut :D

public class Foo {

        static public void main(String[] args) {

                Integer x = 0b11111111_11111111_11111111_11111111;
                System.out.println(x);

                Integer x1 = Integer.MAX_VALUE;
                System.out.println(x1);

                System.out.println(Integer.toBinaryString(x));
                System.out.println(Integer.toBinaryString(x1));

        }



}
 

This is the output on my machine:

Code: Select all

-1
2147483647
11111111111111111111111111111111
1111111111111111111111111111111

Hey all,

I made this program to try and experiment with some of this - binary numbers, two's complement, etc. I noticed that the first toBinaryString() method in my program prints out one more 1 that the second call to this method. Can someone please explain?

The arguments to the toBinaryString() method are -1 in x and MAX_VALUE found in x1.

Regards,

Javanaut

[admin]

That is because there is one more 1 in the binary representation of -1 than in MAX_VALUE. Please read how -1 is represented in a 32 bit int.

HTH,
Paul.

[javanaut]

Paul,

Thank-you for the reply. How odd that MAX_VALUE has only 31 bits when it is an int that is a 32 bit data-type.

This part of Java is not really on the OCAJP7 exam though?

Thank-you again for helping me understand what I was missing with the output of my program.

Regards,

Javanaut

[admin]

I didn't say MAX_VALUE has only 31 bits. I said it has 31 1s. The MSB is 0 and is probably not printed because of the way toBinaryString method is coded.

No, this is not required for the exam.
HTH,
Paul.

[javanaut]

Intense.

I had to look up MSB https://en.wikipedia.org/wiki/Most_significant_bit

I can see guess the MSB is dropped since it is a zero or something sort of like how octals are printed by Java if I am not mistaken.

Yes, I am glad this is not on the exam.

[thupten]

Has this question ever appeared on the test?
I think most would choose the Overflow exception as the answer. This is totally unexpected behavior. There probably are many programs out there with bug related to this.

[admin]

As mentioned in previous posts, this much detail about 1s and 2s complement is not required for the exam.

HTH,
Paul.

[Sergey]

I did not understand the explanation in programm, but i find this in the web:

Code: Select all

Basically, because Integer.MAX_VALUE is actually only 2147483647, so -Integer.MIN_VALUE, which would be +2147483648, actually overflows the capacity of the internal binary representation of integers. Thus, the result "loops around" back to Integer.MIN_VALUE, or -2147483648.

is it correct?

[admin]

Yes, the explanation explains the reason why "loop around" happens.