Why does Collectors.toMap report value instead of key on Duplicate Key error?

Question

This is really a question about a minor detail, but I\'m under the impression to get something wrong here. If you add duplicate keys using Collectors.toMap-method it throws

Answer 1

If you have this problem and are forced to use Java 8 in your project, then you will have to create your own collector to get a meaningful error message.

Below you can find an example of a collector which gives you a meaningful message similar to the one you would get with Java 9:

private static class Something {
    private final String k;
    private final String v;

    public Something(String k, String v) {
        this.k = k;
        this.v = v;
    }
}

public static void main(String[] args) throws IOException, ParseException {
    List list = Arrays.asList(
            new Something("key1", "value1"),
            new Something("key2", "value2"),
            new Something("key3", "value3a"),
            new Something("key3", "value3b"));

    Collector> eloquentCollector =
            Collector.of(HashMap::new, (map, something) -> putUnique(map, something.k, something.v),
                    (m1, m2) -> { m2.forEach((k, v) -> putUnique(m1, k, v)); return m1; });

    Map map = list.stream().collect(eloquentCollector);
    System.out.println(map);
}

private static  void putUnique(Map map, K key, V v1){
    V v2 = map.putIfAbsent(key, v1);
    if(v2 != null) throw new IllegalStateException(
            String.format("Duplicate key '%s' (attempted merging incoming value '%s' with existing '%s')", key, v1, v2));
}

If you run this code, you will see the following error message pointing to the duplicate key and merge values:

Duplicate key 'key3' (attempted merging values value3a and value3b)

You should get the same exception message if you use in the example above parallelStream(), like e.g:

Map map = list.parallelStream().collect(eloquentCollector);

Credits to Holger for pointing out that the original answer was not OK for parallel streams.