Filter Java Stream to 1 and only 1 element

Question

I am trying to use Java 8 Streams to find elements in a LinkedList. I want to guarantee, however, that there is one and only one match to the filter criteria.

Take this code:

public static void main(String[] args) {

    LinkedList<User> users = new LinkedList<>();

    users.add(new User(1, "User1"));

    users.add(new User(2, "User2"));

    users.add(new User(3, "User3"));

    User match = users.stream().filter((user) -> user.getId() == 1).findAny().get();

    System.out.println(match.toString());

}

static class User {

    @Override

    public String toString() {

        return id + " - " + username;

    }

    int id;

    String username;

    public User() {

    }

    public User(int id, String username) {

        this.id = id;

        this.username = username;

    }

    public void setUsername(String username) {

        this.username = username;

    }

    public void setId(int id) {

        this.id = id;

    }

    public String getUsername() {

        return username;

    }

    public int getId() {

        return id;

    }

}

This code finds a User based on their ID. But there are no guarantees how many Users matched the filter.

Changing the filter line to:

User match = users.stream().filter((user) -> user.getId() < 0).findAny().get();

Will throw a NoSuchElementException (good!)

I would like it to throw an error if there are multiple matches, though. Is there a way to do this?

Answer 1

Create a custom Collector

public static <T> Collector<T, ?, T> toSingleton() {

    return Collectors.collectingAndThen(

            Collectors.toList(),

            list -> {

                if (list.size() != 1) {

                    throw new IllegalStateException();

                }

                return list.get(0);

            }

    );

}

We use Collectors.collectingAndThen to construct our desired Collector by

1. Collecting our objects in a List with the Collectors.toList() collector.
2. Applying an extra finisher at the end, that returns the single element — or throws an IllegalStateException if list.size != 1.

Used as:

User resultUser = users.stream()

        .filter(user -> user.getId() > 0)

        .collect(toSingleton());

You can then customize this Collector as much as you want, for example give the exception as argument in the constructor, tweak it to allow two values, and more.