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Q: 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 Person object and I want to remove people with the same name,
persons.stream().distinct();Will use the default equality check for a Person object, so I need something like,
persons.stream().distinct(p -> p.getName());Unfortunately the distinct() method has no such overload. Without modifying the equality check inside the Person class is it possible to do this succinctly?
A1: Consider distinct to be a stateful filter. Write function that returns a predicate that also maintains state about what it's seen previously, and returns whether the given element was seen for the first time:
public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
Map<Object,Boolean> seen = new ConcurrentHashMap<>();
return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}Then you can write:
persons.stream().filter(distinctByKey(p -> p.getName());
A2: An alternative would be to place the persons in a map using the name as a key:
persons.collect(toMap(Person::getName, p -> p, (p, q) -> p)).values();Note that the Person that is kept, in case of a duplicate name, will be the first encontered.