Java primitive long overflow

Question

A C# system is generating data and providing data to another Java system using SBE(Simple Binary Encoding) by primitiveType="uint64" (long).

In Java, when data is received as primitive long it is overflowing and in few cases leading to –ve numbers.

How to handle this situation ?

This shouldn't be happening, as C#'s uint64 should be analogous to Java's long. Can you include some data which is causing this problem? — Tim Biegeleisen
– Tim Biegeleisen, Commented Mar 5, 2019 at 10:41
@TimBiegeleisen Presumably the difference is that uint64 is unsigned, whereas Java's long is signed. So any number in the top half of uint64's range will end up negative when it is interpreted as a long. — khelwood
– khelwood, Commented Mar 5, 2019 at 10:42
unsigned or signed does not really make a difference, you're just receiving 64 bits of data which you interpret as a number. What are you using this number for? — Lino
– Lino, Commented Mar 5, 2019 at 10:46

Joop Eggen · Accepted Answer · 2019-03-05 10:58:13Z

1

You can receive the unsigned long as long, as "long" as one does not calculate with the long. You can even display that long as unsigned using Long.toUnsignedString(n).

Otherwise store it in a BigInteger. Ideally loaded as 8 big-endian bytes.

// Some snippets, unordered, showing useful constructs.
long n = ...;
n = Long.reverseBytes();
byte[] bigendian = new byte[8];
ByteBuffer buf = ByteBuffer.wrap(bigendian); // .order(ByteOrder.LITTLE_ENDIAN);
buf.putLong(n);

// Creating a BigInteger from unsigned long's bytes, requiring big-endian order.
BigInteger num = new BigInteger(1, bigendian); // 1 = positive

answered Mar 5, 2019 at 10:58

Joop Eggen

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2 Comments

biziclop Over a year ago

Even if you calculate with the long, you can usually get away with using methods like Long.divideUnsigned(). (The other basic arithmetic operations are identical in two's complement.)

Joop Eggen Over a year ago

@biziclop Thanks, I almost added divideUnsigned/remainderUnsigned myself. I find it remarkable that numbers > 2^63 are actually used.

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