Java Integer.parseInt() for 32-bit signed binary string throws NumberFormatException
Integer.valueOf(String, int radix)
and Integer.parseInt(String, int radix)
will only parse numbers of value -2 147 483 648 to 2 147 483 647, i.e. the values of 32-bit signed integers.
These functions cannot interpret two's complement numbers for binary (radix = 2
), because the string being passed can be of any length, and so a leading 1 could be part of the number or the sign bit. I guess Java's developers decided that the most logical way to proceed is to never accept two's complement, rather than assume that a 32nd bit is a sign bit.
They read your input binary string as unsigned 3 549 763 730 (bigger than max int value). To read a negative value, you'd want to give a positive binary number with a -
sign in front. For example for -5
:
Integer.parseInt("1011", 2); // 11
// Even if you extended the 1s to try and make two's complement of 5,
// it would always read it as a positive binary value
Integer.parseInt("-101", 2); // -5, this is right
Solutions:
I suggest, first, that if you can store it as a positive number with extra sign information on your own (e.g. a -
symbol), do that. For example:
String binString;
if(i < 0)
binString = "-" + Integer.toBinaryString(-i);
else // positive i
binString = Integer.toBinaryString(i);
If you need to use signed binary strings, in order to take a negative number in binary two's complement form (as a string) and parse it to an int, I suggest you take the two's complement manually, convert that into int, and then correct the sign. Recall that two's complement = one's complement + 1, and one's complement is just reverse each bit.
As an example implementation:
String binString = "11010011100101010001100010010010";
StringBuilder onesComplementBuilder = new StringBuilder();
for(char bit : binString.toCharArray()) {
// if bit is '0', append a 1. if bit is '1', append a 0.
onesComplementBuilder.append((bit == '0') ? 1 : 0);
}
String onesComplement = onesComplementBuilder.toString();
System.out.println(onesComplement); // should be the NOT of binString
int converted = Integer.valueOf(onesComplement, 2);
// two's complement = one's complement + 1. This is the positive value
// of our original binary string, so make it negative again.
int value = -(converted + 1);
You could also write your own version of Integer.parseInt
for 32-bit two's complement binary numbers. This, of course, assumes you're not using Java 8 and can't just use Integer.parseUnsignedInt
, which @llogiq pointed out while I was typing this.
EDIT: You could also use Long.parseLong(String, 2)
first, then calculate the two's complement (and mask it by 0xFFFFFFFF), then downgrade the long
down to int
. Faster to write, probably faster code.
The API docs for Integer.toBinaryString(..)
explicitly state:
The value of the argument can be recovered from the returned string s by calling
Integer.parseUnsignedInt(s, 8)
.
(as of Java 8u25) I think this is a documentation error, and it should read Integer.parseUnsignedInt(s, 2)
. Note the Unsigned
. This is because the toBinaryString
output will include the sign bit.
Edit: Note that even though this looks like it would produce an unsigned value, it isn't. This is because Java does not really have a notion of unsigned values, only a few static methods to work with ints as if they were unsigned.