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Stream StormPart 3 of 6

This toMap() Call Crashes Your Java Code (and the Fix) | Stream Storm Part 3

A collector decides the shape of your result. toMap builds a lookup table (and crashes on duplicate keys unless you give it a merge function), partitioningBy splits into exactly two groups, and joining glues strings together.

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The toMap crash, and the fix

java
record Score(String name, int score) {}
 
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)
 
scores.stream().collect(Collectors.toMap(Score::name, Score::score, Math::max));
// {Ravi=45, Asha=90, Meena=67}

The third argument is a merge function. It decides what happens when a key repeats; Math::max keeps the higher score.

partitioningBy and joining

java
Collectors.partitioningBy(s -> s.score() >= 50)   // always two keys: true and false
 
Stream.of("Asha", "Ravi", "Meena")
    .collect(Collectors.joining(", ", "[", "]"));  // [Asha, Ravi, Meena]

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