Word count with Collectors.toMap mapping each word to 1, and a word repeats?

The 30 second answer

You get IllegalStateException for a duplicate key when the two-argument collector sees the same word twice. Use the three-argument toMap with a merge function like Integer sum, or groupingBy with counting, which gives Long counts. Also remember toMap throws on null values, so filter or handle those first.

**IllegalStateException** (duplicate key) when collected. Use the 3-arg form toMap(w -> w, w -> 1, Integer::sum) or groupingBy(w -> w, counting()) (gives Long). Also note toMap throws NPE on null values.

Map<String, Integer> counts = words.stream()
    .collect(Collectors.toMap(w -> w, w -> 1, Integer::sum));

Map<String, Long> counts2 = words.stream()
    .collect(Collectors.groupingBy(w -> w, Collectors.counting()));

Sources:

Collectors.toMap / groupingBy

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