Question

I have two methods, the first needs a Map<ItemA, ItemB> the second a Map<ItemA ,ItemB[]>. How can I create a generic Map that covers both cases?

Answer 1

You cannot. How is this going to work? If the first method does a map.get(key) and gets an ItemB[] instead of an ItemB, it’ll complain with a ClassCastException.

The whole point of generic collections is to separate these two cases.

You could make a wrapper that wraps a Map<ItemA, ItemB> as a Map<ItemA, ItemB[]>, returning an array with just one element for everything. But this may cause trouble with array manipulation semantics.

Maybe just copy everything:

 // upgrade from Map to MultiMap
 HashMap<ItemA, ItemB[]> map1Copy = new HashMap<ItemA, ItemB[]>(map1.size());
 for (Entry<ItemA, ItemB> e: map1.entrySet()){
    map1Copy.put(e.getKey(), new ItemB[]{ e.getValue()});
 }

The other direction (converting ItemB[] into a single ItemB) does not work in general.