Convert a set to map in java

Question

I have a set and I want to convert it to map in order to use it later in guava's Maps.difference(). I only care about the keys in the difference.
Came up with this version:

private <T> Map<T, T> toMap(Set<T> set) {
  return set.stream().collect(Collectors.toMap(Function.identity(), Function.identity()));
}

However, I know that usually, a set has a backing field of map. This is the method I use to create the map:

public static <E> Set<E> newConcurrentHashSet() {
  return Collections.newSetFromMap(new ConcurrentHashMap<E, Boolean>());
}

Since I only need the keys I thought maybe I can get a view of this field somehow. any idea?

Answer 1

I ended up with a fairly straight-forward one line solution with Java 8 as follows:

Map<String, Foo> map = fooSet.stream().collect(Collectors.toMap(Foo::getKey, e -> e));

• fooSet is a set of objects of type Foo, i.e. Set<Foo> fooSet
• Foo has a getter called getKey which returns a String

Answer 2

Improvement of developer's answer:

Map<String, Foo> map = fooSet.stream().collect(Collectors.toMap(Foo::getKey, Function.identity()));

or if you statically import Collectors.toMap and Function.identity:

Map<String, Foo> map = fooSet.stream().collect(toMap(Foo::getKey, identity()));