Collectors.toMap() keyMapper — more succinct expression?

You can use a lambda:

Collectors.toMap(p -> p.getLast(), Function.identity())

or, more concisely, you can use a method reference using :::

Collectors.toMap(Person::getLast, Function.identity())

and instead of Function.identity, you can simply use the equivalent lambda:

Collectors.toMap(Person::getLast, p -> p)

If you use Netbeans you should get hints whenever an anonymous class can be replaced by a lambda.