Java 8 Distinct by property

Question

In Java 8 how can I filter a collection using the Stream API by checking the distinctness of a property of each object?

For example I have a list of

Answer 1

Extending Stuart Marks's answer, this can be done in a shorter way and without a concurrent map (if you don't need parallel streams):

public static  Predicate distinctByKey(Function keyExtractor) {
    final Set