java - 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 an Exception with message "duplicate key ". Why is the value reported and not the key? Or even both? This is really misleading, isn't it?

Here's a little test to demonstrate the behaviour:

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class TestToMap {

    public static void main(String[] args) {

        List<Something> list = Arrays.asList(
            new Something("key1", "value1"),
            new Something("key2", "value2"),
            new Something("key3", "value3a"),
            new Something("key3", "value3b"));

        Map<String, String> map = list.stream().collect(Collectors.toMap(o -> o.key, o -> o.value));
        System.out.println(map);
    }

    private static class Something {
        final String key, value;

        Something(final String key, final String value){
            this.key = key;
            this.value = value;
        }
    }
}

Answer 1

This is reported as a bug, see JDK-8040892, and it is fixed in Java 9. Reading the commit fixing this, the new exception message will be

String.format("Duplicate key %s (attempted merging values %s and %s)", k, u, v)

where k is the duplicate key and u and v are the two conflicting values mapped to the same key.