CodingBat Java solution

Create mapAB3.java

Modify and return the given map as follows: if exactly one of the keys "a" or "b" has a value in the map (but not both), set the other to have that same value in the map.

mapAB3({"a": "aaa", "c": "cake"}) → {"a": "aaa", "b": "aaa", "c": "cake"}
mapAB3({"b": "bbb", "c": "cake"}) → {"a": "bbb", "b": "bbb", "c": "cake"}
mapAB3({"a": "aaa", "b": "bbb", "c": "cake"}) → {"a": "aaa", "b": "bbb", "c": "cake"}