I've found a condition where a NoSuchMethodError can be thrown. Below is what I do to get it to trigger (I hope you don't mind that it's in scala). I am using hll version 1.6.0 out of maven.
val h1 = new HLL(13, 5)
val h2 = new HLL(13, 5)
h2.addRaw(2)
h1.union(h2) // <-- exception here
results in:
Exception in thread "main" java.lang.NoSuchMethodError: it.unimi.dsi.fastutil.longs.LongOpenHashSet.clone()Lit/unimi/dsi/fastutil/longs/LongOpenHashSet;
at net.agkn.hll.HLL.heterogenousUnion(HLL.java:679)
at net.agkn.hll.HLL.union(HLL.java:639)
Is anyone else running into this issue?