Java create Map of single value using stream collector groupingBy

Cosaic :

I know the groupingBy return a Map<K,List<V>>. But If I know that each key has a unique value, how do I get a Map whose value is V instead of List<V>?

For example:

Map<String, Transaction> transactions =
transactions.stream().collect(groupingBy(Transaction::getID));

where ID is unique.

Patrick Parker :

using Collectors.toMap:

Map<String, Transaction> transactions = transactions.stream()
        .collect(Collectors.toMap(Transaction::getID, Function.identity()));