Reputation: 17
So I have been having a go with using the method reference in Java 8 (Object::Method). What I am attempting to do, which I have done before but have forgotten (last time I used this method reference was about 4 months ago), is find the amount of players that != online using the Method Reference.
public static Set<Friend> getOnlineFriends(UUID playerUUID)
{
Set<Friend> friends = new HashSet<>(Arrays.asList(ZMFriends.getFriends(playerUUID)));
return friends.stream().filter(Friend::isOnline).collect(Collectors.toSet());
}
public static Set<Friend> getOfflineFriends(UUID playerUUID)
{
Set<Friend> friends = new HashSet<>(Arrays.asList(ZMFriends.getFriends(playerUUID)));
return friends.stream().filter(Friend::isOnline).collect(Collectors.toSet());
As you can see I managed to so it when the player (friend) is online but I cannot figure out how to filter though the Set and collect the offline players. I'm missing something obvious, but what is it?!?!
Thanks, Duke.
Upvotes: 0
Views: 1549
Reputation: 298529
In you code
public static Set<Friend> getOnlineFriends(UUID playerUUID)
{
Set<Friend> friends = new HashSet<>(Arrays.asList(ZMFriends.getFriends(playerUUID)));
return friends.stream().filter(Friend::isOnline).collect(Collectors.toSet());
}
you are creating a List
view to the array returned by ZMFriends.getFriends(playerUUID)
, copy its contents to a HashSet
, just to call stream()
on it.
That’s a waste of resources, as the source type is irrelevant to the subsequent stream operation. You don’t need to have a Set
source to get a Set
result. So you can implement your operation simply as
public static Set<Friend> getOnlineFriends(UUID playerUUID)
{
return Arrays.stream(ZMFriends.getFriends(playerUUID))
.filter(Friend::isOnline).collect(Collectors.toSet());
}
Further, you should consider whether you really need both, getOnlineFriends
and getOfflineFriends
in your actual implementation. Creating utility methods in advance, just because you might need them, rarely pays off. See also “You aren’t gonna need it”.
But if you really need both operations, it’s still an unnecessary code duplication. Just consider:
public static Set<Friend> getFriends(UUID playerUUID, boolean online)
{
return Arrays.stream(ZMFriends.getFriends(playerUUID))
.filter(f -> f.isOnline()==online).collect(Collectors.toSet());
}
solving both tasks. It still wastes resource, if the application really needs both Set
s, as the application still has to perform the same operation twice to get both Set
s. Consider:
public static Map<Boolean,Set<Friend>> getOnlineFriends(UUID playerUUID)
{
return Arrays.stream(ZMFriends.getFriends(playerUUID))
.collect(Collectors.partitioningBy(Friend::isOnline, Collectors.toSet()));
}
This provides you both Set
s at once, the online friends being associated to true
, the offline friends being associated to false
.
Upvotes: 2
Reputation: 5591
There are 2 ways I can think of:
friends.stream().filter(i -> !i.isOnline()).collect(Collectors.toSet());
But I guess that's not what you want, since it's not using a method reference
. So maybe something like this:
public static <T> Predicate<T> negation(Predicate<T> predicate) {
return predicate.negate();
}
...
friends.stream().filter(negation(Friend::isOnline)).collect(Collectors.toSet());
Upvotes: 0