CODEWITHSUNDEEP
java
WalkThatWay
WalkThatWay

Reputation: 23

Converting from a 32-bit two's complement to decimal using java

I'm having trouble finding a way to convert from a 32-bit two's complement binary number to a decimal in java. I either get an overflow error or the number comes out nonnegative when it should be negative. 

String bin = "11111111111111111111111111111110";
long foo = Long.parseLong(bin);
System.out.println(foo);

This is one of the things I tried and I was thinking I needed to use long because it can hold up to 64-bits. Any help would be great. Thanks

Upvotes: 1

Views: 3844

Answers (3)

paxdiablo
paxdiablo

Reputation: 881293

You need to provide the base, or radix, of two if you want to parse a binary value.

long foo = Long.parseLong(bin, 2);

You can see an example on the Java doc page:

parseLong("1100110", 2) returns 102L.

Without the radix, it uses base-10, and 1032 won't fit into a long.

Keep in mind that, for a 32-character bit string, this will give you the unsigned variant. If you want the signed variant in a long, you can use:

if (foo > 0x7fffffffL) foo = -(0x100000000L - foo);

afterwards to adjust it

Or you can forcefully cast it to an integer if that's suitable:

int foo = (int)Long.parseLong(bin,2);

The requirement that Integer.parseInt() be given a signed number means that it's unsuitable as-is for 32-character binary strings with a leftmost 1-bit.

Upvotes: 3

paulsm4
paulsm4

Reputation: 121639

You're correct - you need to use "Long" in this case.

Since you're doing twos complement (not ones complement), you can use "parseLong()" with a radix of "2":

public class X {

  public static void main (String[] args) {
    String bin = "11111111111111111111111111111110";
    int i = (int)Long.parseLong(bin, 2);
    System.out.println("parseInt(" + bin + ", 2)=" + i);
  }

}
// Output:
parseInt(11111111111111111111111111111110, 2)=-2

Upvotes: 0

Ankur Singhal
Ankur Singhal

Reputation: 26067

radix is the base of the used numbering system. 

public static void main(String[] args) {
        int bits = (int) Long.parseLong("11111111111111111111111111111110", 2);
        System.out.println(bits);
    }

output

-2

Upvotes: 0

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