从 Set 初始化 Map

Question

I have a set of Student - Set<Student> students

class Student{
    String Id;
    String getId(){ return Id;} 
.....
}

I am trying to initialize a Map<String,List<StudentResult>> with the entries from set above:

Map<String,List<StudentResult>> studentResultMap = students.keySet().stream().collect(
                                                Collectors.toMap(x -> x.getId(),new ArrayList<StudentResult>()));

But this wouldn't compile - how is this to be achieved?

Answer 1

new ArrayList<StudentResult>() does not make a correct argument for a Function parameter.

You need to use:

x -> new ArrayList<StudentResult>()

---

Side note: students.keySet() wouldn't compile either, if students is a Set . You can call stream on it directly:

students.stream().collect(Collectors.toMap(x -> x.getId(), 
                                           a -> new ArrayList<>()));

Answer 2

Here lies your problem:

Map<String,List<StudentResult>> studentResultMap = students
    .stream().collect(Collectors.toMap(x -> x.getId(), new ArrayList<StudentResult>()));

You need to pass two functions to Collectors.toMap , but instead, you're passing a List instance as the second parameter

Map<String,List<StudentResult>> studentResultMap = students
    .stream().collect(Collectors.toMap(x -> x.getId(), x -> new ArrayList<StudentResult>()));