java - 使用泛型时在变量分配中出错

Question

我做了两个列表

List<LearnerEnrollment> learnerEnrollmentList = new ArrayList<LearnerEnrollment>();
List<LearnerCourseEnrollError> enrollErrorList = new ArrayList<LearnerCourseEnrollError>();

然后我制作了两个 map

Map<String, List<LearnerCourseEnrollError>> courseErrorMap = new HashMap<String, List<LearnerCourseEnrollError>>();
Map<String, List<LearnerEnrollment>> courseSuccessMap = new HashMap<String, List<LearnerEnrollment>>();

然后我制作了另一个 Map 来容纳上面的两个 Map

Map<String, Map<String, List<Object>>> courseMap = new HashMap<String, Map<String, List<Object>>>();

然后我使用下面的代码在列表中添加项目；

for (com.softech.vu360.lms.model.Course course : courseList) {

    Object result = getEnrollmentForCourse(customer, learner, course);

    if (result instanceof LearnerEnrollment) {
        LearnerEnrollment newEnrollment = (LearnerEnrollment)result;
        learnerEnrollmentList.add(newEnrollment);
    } else if (result instanceof String) { 
        String errorMessage = (String)result;
        LearnerCourseEnrollError enrollError = new LearnerCourseEnrollError(errorMessage, course);
        enrollErrorList.add(enrollError);       
    }       
}

现在我将值放入 map

courseSuccessMap.put(learner.getVu360User().getUsername(), learnerEnrollmentList);
courseErrorMap.put(learner.getVu360User().getUsername(), enrollErrorList);
courseMap.put("successfulCoursesMap", courseSuccessMap);
courseMap.put("unSuccessfulCoursesMap", courseErrorMap);

return courseMap;

但是我在这两行得到了错误

courseMap.put("successfulCoursesMap", courseSuccessMap);
courseMap.put("unSuccessfulCoursesMap", courseErrorMap);

那个

The method put(String, Map<String,List<Object>>) in the type
Map<String,Map<String,List<Object>>> is not applicable for the arguments 
(String, Map<String,List<LearnerEnrollment>>)

The method put(String, Map<String,List<Object>>) in the type
Map<String,Map<String,List<Object>>> is not applicable for the arguments 
(String, Map<String,List<LearnerCourseEnrollError>>)

为什么？

我在 map 中的列表类型是 List<Object>和 List<LearnerEnrollment>是List <Object>因为LearnerEnrollment extends Object .为什么我会收到这些错误？

如果我这样声明我的 map

Map<String, Map<String, ?>> courseMap = new HashMap<String, Map<String, ?>>();

那么就没有错误了。为什么我在第一种情况下会出错？

谢谢

Answer 1

你说:

List<LearnerEnrollment> is List<Object>

这是错误的。如果这是真的，您将能够:

List<<LearnerEnrollment> list = new ArrayList<>();
List<Object> objectList = list;
objectList.add("Now what?");

LearnerEnrollment 的类型安全列表会突然包含一个字符串。