Java 8 Optional and flatMap - what is wrong?

Question

Some piece of code:

public class Player {
    Team team;
    String name;
}

public class Team {
    List<Player> players;
}

public class Demo {

    @Inject
    TeamDAO teamDAO;

    @Inject
    PlayerDAO playerDAO;

    List<String> findTeamMatesNames(String playerName) {
        Optional<Player> player = Optional.ofNullable(playerDAO.get(playerName));

        return player.flatMap(p -> teamDAO.findPlayers(p.team))
            .map(p -> p.name)
            .orElse(Collections.emptyList());
    }
}

Why am I not able to do this? In flatMap method I am getting error "Type mismatch: cannot convert from List to Optional"

My goal is:

If optional is present I want to get list of items based on this optional object property
If optional is not present I want to return empty list

It's not the point what are you asking for. In flatMap I'am getting: "Type mismatch: cannot convert from List<Player> to Optional<Object>" — Dariusz Mydlarz
– Dariusz Mydlarz, Commented Oct 8, 2014 at 9:11

Holger · Accepted Answer · 2014-10-08 10:51:35Z

10

You can use map to perform the desired operation. The map operation will not take place if the Optional is empty but leave again an empty Optional. You can provide the fallback value afterwards:

player.map(p -> teamDAO.findPlayers(p.team)).orElse(Collections.emptyList())

The mapping from a List of Player to a List of Player’s name Strings can’t be performed by an Optional; that’s a Stream task:

Optional<Player> player = Optional.ofNullable(playerDAO.get(playerName));
return player.map(p -> teamDAO.findPlayers(p.team)
                           .stream().map(tp -> tp.name).collect(Collectors.toList()))
             .orElse(Collections.emptyList());

answered Oct 8, 2014 at 10:51

Holger

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2 Comments

Dariusz Mydlarz Over a year ago

I didn't that I have to perform next operations inside first map function. Thanks!

Holger Over a year ago

@dmydlarz: it is also possible to do it afterwards, e.g.

player.map(p -> teamDAO.findPlayers(p.team)).orElse(Collections.emptyList()) .stream().map(tp -> tp.name).collect(Collectors.toList())

has the same semantics but it would imply an unnecessary collect operation performed on the empty stream in case of an empty Optional.

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