筛选可选 <List<Object> &gt;在Java8中

Question

I am trying to Filter an Optional<List<Object>> in Java8. In the below example, I trying to filter the list, without collecting the full list (players). Is this possible?

public List<Player> getPlayers(int age, Team team) {
    Optional.ofNullable(team).map(Team::getPlayers); 
    // needs to filter players older than 20 years, without collecting it as a list.
}

class Team {

    String name;
    List<Player> players;
    public String getName() {
        return name;
    }

    public void setName(final String name) {
        this.name = name;
    }

    public List<Player> getPlayers() {
        return players;
    }

    public void setPlayers(final List<Player> players) {
        this.players = players;
    }

}


class Player {

    String playerName;
    String age;

    public String getPlayerName() {
        return playerName;
    }

    public void setPlayerName(final String playerName) {
        this.playerName = playerName;
    }

    public String getAge() {
        return age;
    }

    public void setAge(final String age) {
        this.age = age;
    }
}

Answer 1

It's a bad idea to have null lists . It's better to simply have empty lists, but always non-null. That way you don't have to check for null all the time, you can just iterate straight away.

If you do that then you can just call stream() directly without any of this Optional business:

team.getPlayers().stream()
    .filter(p -> p.getAge() > 20)

Answer 2

With the updated signature of the method, what you seem to be looking for is:

public List<Player> getPlayers(Team team, int age) {
    return Optional.ofNullable(team).map(Team::getPlayers)
            .orElse(Collections.emptyList())
            .stream()
            .filter(a -> Integer.parseInt(a.getAge()) > 20)
            .collect(Collectors.toList());
}