最佳实践：如何从Java中的方法返回地图的键集

Question

My function returns a keyset of a HashMap member.

From the JavaDoc :

The set is backed by the map, so changes to the map are reflected in the set, and vice-versa.

I want to avoid this behavior.

I want to return an unmodifiable set, perhaps a copy of the set.

I do not want changes to the set or map to be reflected in one another.

Are there any best practices to deal with such a situation?

Answer 1

Personally, I'd go with Google's Guava - especially for I want to return an unmodifiable set, perhaps a copy of the set. - if the map grows, ImmutableSet won't exhibit growth:

ImmutableSet.copyOf(map.keySet());

Or if you want to use the standard Java API - NOTE the resulting Set will grow as your Map grows:

Collections.unmodifiableSet(map.keySet());

Using either of the above is considered good practice given your requirements.

Answer 2

The standard approach is to return a copy, typically using a copy constructor :

Here's a utility method you can use:

public static <K, V> Set<Map.Entry<K, V>> copyKeys(Map<K, V> map) {
    return new HashSet<Map.Entry<K, V>>(map.keySet());
}

Answer 3

You can use standard Collections.unmodifiableSet which is designed exactly for that. Attempts to modify the wrapped Set will end up in UnsupportedOperationException