Java 8 Stream Collecting Set

Question

To better understand the new stream API I'm trying to convert some old code, but I'm stuck on this one.

 public Collection<? extends File> asDestSet() {     HashMap<IFileSourceInfo, Set<File>> map = new HashMap<IFileSourceInfo, Set<File>>();     //...     Set<File> result = new HashSet<File>();     for (Set<File> v : map.values()) {         result.addAll(v);     }     return result; } 

I can't seem to create a valid Collector for it:

 public Collection<? extends File> asDestSet() {     HashMap<IFileSourceInfo, Set<File>> map = new HashMap<IFileSourceInfo, Set<File>>();     //...     return map.values().stream().collect(/* what? */); } 

Answer 1

Use flatMap:

return map.values().stream().flatMap(Set::stream).collect(Collectors.toSet()); 

The flatMap flattens all of your sets into single stream.