I found a mistake in my solution.
It was rather an elementary one. :)
Thank you a lot.

POINT 1: Here is an example in the task:
findNthOccurrence("TestTest", "TestTestTestTest", 3) == 8

POINT 2: Here is a result for my solution:

• testsWithOverlaps
  expected:<-1> but was:<8>

this occurs for next input data:

str = TestTestTestTest
subStr = TestTest
occurrence = 3

I can't understand why there are different results for the same task...

At the beginning the class Leetspeak is declared as an independent class instead of being an inheritor of Encoder. Just "extends" is needed.