From the JavadocsBigInteger isProbablePrime method for :
certainty is a measure of the uncertainty that the caller can tolerate: if the call returns true, the probability that this BigInteger simply exceeds (1 - 1/2 certainty )
, certainty, , , .. 100 , 1 - (1/2) 100 1.
Java , -, certainty ( Lucas-Lehmer).