Java BigInteger to represent unsigned long

Question

I'm trying to use BigInteger for representing an unsigned 64 bit (long) coming from C in java and my approach is exactly same as mentioned in the following post.

http://technologicaloddity.com/2010/09/22/biginteger-as-unsigned-long-in-java/

Can one of you please validate that this approach is correct?

Any help in this regard appreciated.

user207421 · Accepted Answer · 2014-07-14 18:11:55Z

8

Certainly, but do you really need it? There's a major performance cost. The only time you need it unsigned is when comparing, which you can fix with a small unsigned-compare method. The rest of the time you can just use a long.

edited Jul 14, 2014 at 18:11

answered Jul 23, 2012 at 3:54

user207421

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7 Comments

Mark Peters Over a year ago

+1, and you don't need to write your own method. I'm sure many libraries for doing unsigned operations exist; Guava is one: docs.guava-libraries.googlecode.com/git/javadoc/com/google/…

Aham Over a year ago

Thank you for confirming. The usecase is not very performance sensitive, so I'm not much concerned on that point.Still for a better understanding, how the case unsigned c Long.MAX will be mapped to java long if I use long as you mentioned.or may be I missed some point. @Mark, I came across the guava library, but in my case I have only one scenario where I need this operation, so didnt bring a new library.

SimonC Over a year ago

If performance isn't an issue then I would personally use the Guava UnsignedLong wrapper as it removes any ambiguity as to the number is representing.

user207421 Over a year ago

@Aham A long has 64 bits, whether signed or unsigned. The only difference is whether it compares < 0 or not. -Long.MAX_VALUE is, not just 'maps to' 0x8000000000000001, unless I have fouled up my twos-complement again.

Peter Lawrey Over a year ago

You can compare two unsigned longs with a + Long.MIN_VALUE <=> b + Long.MIN_VALUE where <=> is your choice of comparison. for == and != you can just compare a and b. Operations +, -, <<, >>> and * work the same way. / and '%' are different if the top bit is set.

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