I have two list both of type Long, say `ListA` and `ListB`.

I want to merge these into a `Map<Long, Long>` such that `ListA` becomes key and and `ListB` becomes value.
Here catch is I want to add these key value pair to an existing map.

I know how to parse the lists and return a new map object (using `Collectors.toMap`). My problem is to add to an existing map, may be something like:

```
Map<Long, Long> map = new HashMap<Long, Long>;
// ... entries added to map
ListA.stream().forEach(a -> {
    map.put(a, <get 0th element of ListB somehow>);
});

```

Thanks in advance!