java long数据类型转换为无符号值

Question

I'm porting some C++ code to Java code.

There is no unsigned datatype in java which can hold 64 bits.

I have a hashcode which is stored in Java's long datatype (which of course is signed).

long vp = hashcode / 38; // hashcode is of type 'long'

Since 38 here is greater than 2, the resulting number can be safely used for any other arithmetic in java.

The question is what if the signed bit in 'hashcode' is set to 1. I don't want to get a negative value in variable vp. I wanted a positive value as if the datatype is an unsigned one.

PS: I don't want to used Biginteger for this purpose because of performance issues.

Answer 1

Java's primative integral types are considered signed, and there isn't really anything you can do about it. However, depending on what you need it for, this may not matter.

Since the integers are all done in two's complement, signed and unsigned are exact same at the binary level. The difference is how you interpret them, and in certain operations. Specifically, right shift, division, modulus and comparison differ. Unsigned right shifts can be done with the >>> operator. As long as you don't need one of the missing operators, you can use long s perfectly well.

Answer 2

If you can use third-party libraries, you can eg use Guava's UnsignedLongs class to treat long values as unsigned for many purposes, including division. (Disclosure: I contribute to Guava.)

Answer 3

Well here is how i solved this. Right shift hashcode by 1 bit(division by 2). Then divide that right shifted number by 19(which is 38/2). So essentially i divided the number by 38 exactly like how it is done in c++. I got the same value what i got in c++