How to Ensure a Single Match with Java Streams in a LinkedList?

Question

How can I use Java 8 Streams to filter a LinkedList to ensure that there is exactly one matching element?

User match = users.stream()
                .filter(user -> user.getId() == 1)
                .collect(Collectors.toList());
if (match.size() != 1) {
    throw new IllegalStateException("Expected exactly one match but found: " + match.size());
} else {
    System.out.println(match.get(0).toString());
}

Answer

In Java 8 Streams, when you want to filter a list and ensure that you get exactly one result, you can use the Stream's `collect` method to gather the results and then check the size of the resulting list. If the size is not equal to one, it indicates that there was either no match or multiple matches, allowing you to throw an exception accordingly.

import java.util.LinkedList;
import java.util.stream.Collectors;

public class Main {
    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(1, "User3")); // Duplicate ID

        List<User> matches = users.stream()  
                .filter(user -> user.getId() == 1)  
                .collect(Collectors.toList());
                
        if (matches.size() != 1) {
            throw new IllegalStateException("Expected exactly one match but found: " + matches.size());
        } else {
            User match = matches.get(0);
            System.out.println(match.toString());
        }
    }

    static class User {
        int id;
        String username;

        public User(int id, String username) {
            this.id = id;
            this.username = username;
        }

        @Override
        public String toString() {
            return id + " - " + username;
        }

        public int getId() {
            return id;
        }
    }
}

Causes

Multiple elements match the filter criteria.
No elements match the filter criteria.

Solutions

Use `collect(Collectors.toList())` to gather filtered results into a list.
Check the size of the resulting list before accessing the elements.
Throw an exception if the size is not equal to one.

Common Mistakes

Mistake: Using `findAny().get()` directly without checking if there's exactly one match.

Solution: Use `collect(Collectors.toList())` and check the returned list's size.

Mistake: Assuming that the stream will always return one element for a unique criteria.

Solution: Always check the number of matches and throw an exception if required.

Helpers

Java 8 Streams
filter LinkedList
exactly one match
Java Stream example
LinkedList user filter