Spot the Bug #82

A map that refuses duplicate names

A medium Java puzzle in the streams and lambdas family. One line below contains a real bug. Try to spot it first, then read why it fails and how to fix it.

mediumStreams and lambdasbuggy line: 9

1import java.util.*;

2import java.util.stream.*;

4public class Index {

5 record User(String name, int id) {}

7 public static void main(String[] args) {

8 List<User> users = List.of(new User("ann", 1), new User("bob", 2), new User("ann", 3));

9 Map<String, Integer> byName = users.stream().collect(Collectors.toMap(User::name, User::id));

10 System.out.println(byName);

11 }

12}

Highlighted: line 9. Play this one as a puzzle without the answer.

What is wrong

The fix

.collect(Collectors.toMap(User::name, User::id, (a, b) -> b)) // or groupingBy to keep all

How to avoid it next time

Always supply a merge function to toMap unless keys are guaranteed to be unique, or use groupingBy.

Want the background? Read Collectors: groupingBy, toMap, joining in the Java concepts library.

About streams and lambdas

Streams are single-use and lazy, and lambdas can only capture effectively final variables. Side effects inside stream pipelines and parallel streams over shared state are frequent sources of surprise.

In code review: Check whether a stream variable is used twice, whether the terminal operation is present, and whether any lambda mutates outside state.

More puzzles in this family

#79: A stream used twice (easy)
#80: The pipeline that never runs (easy)
#81: A parallel stream that fills a plain list (medium)
#83: An array of three that counts as one (hard)
#84: The five smallest numbers of an endless stream (hard)

🐞 Play today's puzzle All 132 bugs ← #81 #83 →

Frequently asked questions

What is the bug in "A map that refuses duplicate names"?

Collectors.toMap throws IllegalStateException when two elements produce the same key. The message "Duplicate key ann" appears only when the data contains a duplicate, so the code works in tests with distinct names. The faulty line is line 9: Map byName = users.stream().collect(Collectors.toMap(User::name, User::id));

How do I fix it?

Replace it with: .collect(Collectors.toMap(User::name, User::id, (a, b) -> b)) // or groupingBy to keep all. Always supply a merge function to toMap unless keys are guaranteed to be unique, or use groupingBy.

How do I catch streams and lambdas in code review?

Check whether a stream variable is used twice, whether the terminal operation is present, and whether any lambda mutates outside state.

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