You can monkeypatch any imported Java class, just as if it's a Python class.

monkeypatch

>>> from java.util.concurrent import ConcurrentHashMap as CHM
>>> def silly(self): return 2 * len(self)
...
>>> CHM.silly = silly
>>> x = CHM()
>>> x.silly()
0
>>> x["a"] = 42
>>> x.silly()
2