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

In Java 8, duplicate values from two streams (or with same strems iterating twice) are not allowed to be merged into the Map at a time. You can use Collectors.groupingBy & Collectors.mapping to combine the results into the Map> or Map> for processing.

    List employees = new ArrayList<>();
    employees.add(new Employee("Sachin", 40));
    employees.add(new Employee("Sachin", 30));   
    employees.add(new Employee("Rahul", 30));

    Map> map = employees.stream()
                                    .collect(
                                            Collectors.groupingBy(
                                                                Employee::getAge, 
                                                                Collectors.mapping(
                                                                        Employee::getName,
                                                                        Collectors.toSet()
                                                                )
                                                            )
                                            );