Message10172

As you said the BigInteger is indeed not the first Java Class that isn't compatible with Java, such as Float, Long and Short have the same issue.
Anyway, we will accept the reality if it is a feature to exchange data between Python and Java. 

Fortunately, I found a workaround to create a real BigInteger:
b1 = BigInteger.valueOf(9999) #Failed case, the result will be a long
b2 = BigInteger(str(9999))    #Success case, the result will be a BigInteger

I will downgrade the issue to minor. It can also be closed if someone thinks it is OK to accept the current behavior.