从集合创建地图的Util方法

Question

I try to create static method that creates map of instance field as key and instance itself as value. I tried something like this:

public static <T> Map<Object, T> collectionToMap(Collection<T> collection, Function<Object, T> fieldExtractor) {
        return collection.stream()
                .collect(Collectors.toMap(fieldExtractor, e -> e));
    }

But when I tried pass a function as an argument. I have compilation error, because getEmail() method non static, but toMap() static.

ProjectUtils.collectionToMap(collection, User::getEmail);

How can I write static method with similar signature and pass non static method reference? Or maybe another approach exists to do this?

Answer 1

Write a generic method collectionToMap like so.

public static <T, S> Map<S, T> collectionToMap(Collection<? extends T> collection,
    Function<? super T, ? extends S> fieldExtractor) {
    return collection.stream()
        .collect(Collectors.toMap(fieldExtractor, v -> v, (a, b) -> a));
}

Then the client code should be something like this.

ProjectUtils.collectionToMap(Collections.emptyList(), User::getEmail);