As per the API document, IllegalStateException need to be thrown by

decode method in CharsetDecoder when the previous step of the currrent decoding operation was an invocation neither of reset method nor of decode method with value of false for the endOfInput parameter

Also by

flush method in the same class when the previous step of the current decoding operation was an invocation neither of the reset method nor of the three argument decode method with a value of true for the endOfInput parameter.

Along with this is attached a test code wherein both the above cases are simulated to verify if the IllegalStateException does actually get thrown. We found in both the cases the Execption was not thrown by jdk b76