Duplicate with HashSet using method reference

Question

I want to know if my List<T> has duplicates elements.

I have seen the method below :

public static <T> boolean areAllUnique(List<T> list){
    return list.stream().allMatch(new HashSet<>()::add);
}

It works and I'm surprised why ? Because it seems a new HashSet<> is created everytime (so basically the method should always return true even if duplicates)

If I write differently the method above, it no longer works :

public static <T> boolean areAllUnique(List<T> list){
    return list.stream().allMatch(t -> {
        return new HashSet<>().add(t);
    });
}

I'm surprised the 1st method works while the other does not. Because for me they look the same

See [What is the equivalent lambda expression for System.out::println](https://stackoverflow.com/q/28023364/2711488)… — Holger, Jan 15 '20 at 11:00
Also [this Q&A](https://stackoverflow.com/q/37413106/2711488) — Holger, Jan 15 '20 at 11:07
might be obvious, but `new HashSet<>(list).size() == list.size()` will work also. — Eugene, Jan 15 '20 at 12:24
@Holger I have realized that right after I posted; thought about deleting or not — Eugene, Jan 15 '20 at 18:59

score 6 · Accepted Answer · answered Jan 15 '20 at 09:30

new HashSet<>()::add is a method reference referencing a specific instance of a HashSet.

It's equivalent to creating an instance of a HashSet outside that method, storing a reference to it in a variable set, and using the method reference set::add instead. i.e. it will always operate on the same HashSet instance.

The lambda expression behaves differently, since the body of the lambda expression is executed each time allMatch() has to apply the Predicate to an element of the Stream. And each time the body is executed, a new HashSet instance is created.

Duplicate with HashSet using method reference

1 Answers1