由于对Java8比较陌生，我想知道为什么它允许第一个变体( merge 功能（不需要） Collectors.toMap() 使用时 List :

static <T,K,U> Collector<T,?,Map<K,U>>  toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)

一 List 允许完全重复的值。想象一个开发人员使用 stream 转换 List 至 Map java 8将运行时公开为：

Exception in thread "main" java.lang.IllegalStateException: Duplicate key ...

这个场景不需要在编译时捕获吗？afaik，hashmap用来简单地替换旧的 Entry 当一个复制品 key 是放的。如果开发人员确定数据中存在重复的值，并希望将此异常作为警告来处理，那么他不首先使用 Set 而不是 List ? 例如：-

public class Employee{
    public String name;
    public String surname;
    public Employee(String firstname, String secondName) {
        this.name=firstname;this.surname=secondName;
    }
    public boolean equals(Object o){
        return (o instanceof Employee) && ((Employee)o).name.equals(this.name) && ((Employee)o).surname.equals(this.surname);
    }
    public int hashCode(){
        return (name+surname).toCharArray().length;
    }
    public String primaryKey(){
        return this.name;
    }
    public static void main(String[] args){
        Set<Employee> set = new HashSet<>();
        ArrayList<Employee> list = new ArrayList<>();
        Employee one = new Employee("firstname","secondName");
        Employee two = new Employee("firstname","secondName");
        list.add(one);list.add(two); //allows entirely duplicate
        Map<String,Employee> hashMap = new HashMap<>();
        for(Employee employee:list){
            //replace old employees in the order maintained by list before java 8
            hashMap.put(employee.primaryKey(), employee);
            //or may be use list inside some hashmap
        }
        set.add(one);set.add(two); //we can handle duplicates just by using sets
        Map<String,Employee> dontNeedListAsValues=set.stream().collect(Collectors.toMap(Employee::primaryKey,o->o));
        Map<String, Employee> needListAsValues=list.stream().collect(Collectors.toMap(Employee::primaryKey, o -> o)); //currently ALLOWED for LIST
    }
}