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javac++java-native-interfaceuint64

Best way to convert uint64_t to jlong

I am writing a JNI code and encounter a situation where I will need convert uint64_t to jlong, and here's my current way to convert uint64_t to jlong:

jlong uint64_t_to_jlong (uint64_t value) {
  if (value > std::numeric_limits<jlong>::max()) {
    return std::numeric_limits<jlong>::max(); 
  }
  return static_cast<jlong>(value);
}

And this is obviously not a perfect approach as it can't really convert the value correctly when value > std::numeric_limits<jlong>::max(). However, directly returning static_cast<jlong>(value) doesn't sound a good approach to me either, as in the Java side we will receive negative value.

Can I know whether there exist better approach which can handle all cases smoothly and correctly?

Solution

  • As suggested in the comment by Louis, one way to handle it in Java 8 is to simply cast uint64_t to jlong in the C++ side, then use the set of unsigned operation provided by Java 8 such as Long.compareUnsigned() to do the necessary operation:

    public static int compareUnsigned(long x, long y)
Compares two long values numerically treating the values as unsigned.

    Google Guava's unsigned libraries can also achieve this in case Java 8 is not used.