Is the error reporting in Collectors.toMap() broken?

Question

List<String> strings = Arrays.asList("3","55","3");
Map<String,Integer> map = strings
    .stream()
    .collect(Collectors.toMap(s ->s, s -> s.length()));

returns

java.lang.IllegalStateException: Duplicate key 1

where I would expect Duplicate key 3

Answer 1

This was fixed in Java 9. Now the error message is correct:

java.lang.IllegalStateException: Duplicate key 3 (attempted merging values 1 and 1)

Answer 2

Seems, like this was a bug in JDK 8 but is no longer the case as of JDK 9. Reason one being that I cannot replicate it on JDK 9 and reason two this link provided by @Zircon derives about the issue and it being fixed as of JDK 9.

Seems like there have been several posts about this issue, another link being:

https://bugs.openjdk.java.net/browse/JDK-8040892

which itself is a duplicate of few other posts.

Answer 3

Yes it's a bug but there is another way of converting this into a map which is by using identity function:

List<String> strings = Arrays.asList("3","55","3");
Map<String, List<String>> map = strings.stream()
                .collect(Collectors.toMap(Function.identity(), Arrays::asList));

By doing this you will get correct error which is

java.lang.IllegalStateException: Duplicate key [3]

For array with unique values

List<String> strings = Arrays.asList("3","55","4");

result would be

{55=[55], 3=[3], 4=[4]}