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.