Stream Storm15 Oct 20262 min read

Why Collectors.toMap throws "Duplicate key", and how to fix it

toMap refuses to guess which value you want when a key repeats. Give it a merge function and the crash goes away.

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A collector decides the shape of your result: a map, two groups, or one string. toMap builds a lookup table that links each key to one value. That "one" is where the crash comes from.

The setup

Three students and their test scores:

java

record Score(String name, int score) {}
 
List<Score> scores = List.of(
    new Score("Asha", 82),
    new Score("Ravi", 45),
    new Score("Meena", 67));
 
Map<String, Integer> byName = scores.stream()
    .collect(Collectors.toMap(Score::name, Score::score));
// {Asha=82, Ravi=45, Meena=67}

The crash

Asha takes the test again and scores 90. Now her name appears twice:

java

List<Score> scores = List.of(
    new Score("Asha", 82), new Score("Ravi", 45),
    new Score("Meena", 67), new Score("Asha", 90));
 
scores.stream().collect(Collectors.toMap(Score::name, Score::score));
// IllegalStateException: Duplicate key Asha (attempted merging values 82 and 90)

A map keeps one value per key, and toMap won't guess which score you want.

The fix: a merge function

Pass a third argument. It decides what happens when a key repeats:

java

Map<String, Integer> best = scores.stream()
    .collect(Collectors.toMap(Score::name, Score::score, Math::max));
// {Ravi=45, Asha=90, Meena=67}

Math::max keeps the higher score. Other common choices:

(a, b) -> a keeps the first value
(a, b) -> b keeps the last value
Integer::sum adds them up

Bonus: partitioningBy and joining

partitioningBy always gives you exactly two groups, true and false:

java

Map<Boolean, List<String>> passed = scores.stream()
    .collect(Collectors.partitioningBy(
        s -> s.score() >= 50,
        Collectors.mapping(Score::name, Collectors.toList())));

joining glues strings into one, with an optional prefix and suffix:

java

Stream.of("Asha", "Ravi", "Meena").collect(Collectors.joining(", ", "[", "]"));
// [Asha, Ravi, Meena]

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