signed to unsigned integer

[Lilou Laure]

String s="1";
      long n=Integer.parseUnsignedInt(s);
      System.out.println(~n)  // prints -2 ;

int the above code i am trying to flip the bits of an integer. However since the integer is interpreted as a signed number, its not happening correctly. I have tried alternative solutions like masking using & , or using the XOR operation to flip bits , etc. but I want to learn the useage of this new parseUnsignedInt method in  Java 8 .

So, can someone tell me how to use this method and get my desired result above ? I want to use this method to interpret my integer as an unsigned number and flip its bits accordingly as though it were an unsigned integer.

[Pierre-Yves Saumont]

What you get is the correct result given the fact that negative numbers are represented in Two's complement:

+1 = 00000 .... 001
-2 = 11111 .... 110

What result do you want to get?

[Campbell Ritchie]

Lilou Laure wrote:

String s="1";
      long n=Integer.parseUnsignedInt(s);
      System.out.println(~n)  // prints -2 ;

. . . the integer is interpreted as a signed number, its not happening correctly. . . . .

That is correct. It is the String input that has to be unsigned. Just because there is a method for parsing Strings as unsigned, that doesn't mean there has been a change in the structure of the primitive int datatype, which is as always two's complement. Have you read the details of the Integer#parseUnsignedInt() method? It returns an ordinary int. Maybe Java® is wrong not to have unsigned numbers, but it doesn't. Link explaining why unsigned numbers might be unhelpful. So the unsigned method doesn't accept negative arguments.

class UnsignedIntDemo
{
    public static void main(String... args)
    {
        for (String s : args)
        {
            System.out.println(Integer.parseUnsignedInt(s));
        }
    }
} 

Campbell's Computer wrote:java UnsignedIntDemo 0 +0 1 [phone] 2222222222 -1234567890
0
0
1
1
1234567890
-2072745074
Exception in thread "main" java.lang.NumberFormatException: Illegal leading minus sign on unsigned string -1234567890.
at java.base/java.lang.Integer.parseUnsignedInt(Integer.java:827)
at java.base/java.lang.Integer.parseUnsignedInt(Integer.java:928)
at UnsignedIntDemo.main(UnsignedIntDemo.java:7)

Notice that 2222222222 is interpreted in two's complement. If you tried Integer.parseInt("2222222222") you would suffer a number format exception.

So, ~1 has always been −2 and it still is −2.

[Rob Spoor]

If you want to read 1, flip its bits and expect a positive number, you probably should move the conversion to unsigned to later in the process:

String s = "1";
int i = Integer.parseInt(s);
long n = Integer.toUnsignedLong(~i); // method available since Java 8
System.out.println(n); // prints 4294967294

[Lilou Laure]

Rob Spoor wrote:If you want to read 1, flip its bits and expect a positive number, you probably should move the conversion to unsigned to later in the process:

String s = "1";
int i = Integer.parseInt(s);
long n = Integer.toUnsignedLong(~i); // method available since Java 8
System.out.println(n); // prints 4294967294

Yes, this is exactly what I wanted to know ! Thanks Rob Spoor !  

[Lilou Laure]

Pierre-Yves Saumont wrote:What you get is the correct result given the fact that negative numbers are represented in Two's complement:

+1 = 00000 .... 001
-2 = 11111 .... 110

What result do you want to get?

I wanted the result as posted by Rob Spoor. But thanks for your response as well.  

[Lilou Laure]

Campbell Ritchie wrote:

Lilou Laure wrote:

String s="1";
      long n=Integer.parseUnsignedInt(s);
      System.out.println(~n)  // prints -2 ;

. . . the integer is interpreted as a signed number, its not happening correctly. . . . .

That is correct. It is the String input that has to be unsigned. Just because there is a method for parsing Strings as unsigned, that doesn't mean there has been a change in the structure of the primitive int datatype, which is as always two's complement. Have you read the details of the Integer#parseUnsignedInt() method? It returns an ordinary int. Maybe Java® is wrong not to have unsigned numbers, but it doesn't. Link explaining why unsigned numbers might be unhelpful. So the unsigned method doesn't accept negative arguments.

Oh okay I get it, thanks Campbelle Ritchie  

[Rob Spoor]

Lilou Laure wrote:Yes, this is exactly what I wanted to know ! Thanks Rob Spoor !  

You're welcome