I want to Map a single key to array of values. Is that possible?

So, you want to do something like:

Map<String, Object[]> objectMap = new TreeMap<String, Object[]>();
Object[] values = new String[] {"A", "B", "C"};
objectMap.put("First3Letters", values);

Is that right? Yes, you can do that.

Exactly, but, I have read that: "each key can map to at most one value"

That means that each key can map to one entry in the map. The map doesn't care what's in the entry (or "value" as it is called). All it knows is that there is an object there (an array is considered an object).

Does that make sense?