src/main/java/com/googlecode/objectify/cache/EntityMemcache.java
public static Set<Key> keysOf(final Collection<Bucket> buckets) {
		return buckets.stream().map(Bucket::getKey).collect(Collectors.toSet());
	}
public static Set<Key> keysOf(final Collection<Bucket> buckets) {
		return buckets.stream().map(Bucket::getKey).collect(Collectors.toSet());
	}
Basically a list comprehension of the keys for convenience.