从Java 8中的多级映射构造列表

Question

我有一个多层次的地图如下:

 Map<String, Map<String, Student> outerMap = 
    {"cls1" : {"xyz" : Student(rollNumber=1, name="test1")}, 
     "cls2" : {"abc" : Student(rollNumber=2, name="test2")}}

现在我想从上面的地图构造一个字符串列表,如下所示:

["In class cls1 xyz with roll number 1",
 "In class cls2 abc with roll number 2"] 

我写的如下,但是这不起作用,在这个上下文中我也经历了这个帖子:Java 8 Streams - 嵌套映射到列表,但没有太多想法.

 List<String> classes = outerMap.keySet();
 List<String> studentList = classes.stream()
      .map(cls -> 
           outerMap.get(cls).keySet().stream()
           .map(student -> "In class "+ cls +
            student + " with roll number " +
            outerMap.get(cls).get(student).getRollNum() +"\n"
            ).collect(Collectors.toList());

Answer 1

您可以简单地Map.forEach将此操作用作:

List<String> messages = new ArrayList<>();
outerMap.forEach((cls, students) ->
        students.forEach((name, student) -> 
                messages.add(convertToMessage(cls, name, student.getRollNumber()))));

convertToMessageutil 在哪里:

// this could be made cleaner using 'format'
String convertToMessage(String cls, String studentName, String rollNumber) {
    return "In class" + cls + "--> " + studentName + " with roll number: " + rollNumber;
}

Answer 2

你可以这样做,

List<String> formattedOutput = outerMap
    .entrySet().stream()
    .flatMap(e -> e.getValue().entrySet().stream().map(se -> "In class " + e.getKey()
        + " " + se.getKey() + " with roll number " + se.getValue().getRollNumber()))
    .collect(Collectors.toList());

您必须使用flatMap运算符而不是map运算符.