java - 如何向哈希集添加值？

Question

我正在做一道练习题，要求我将一个值添加到 HashMap 中。但我不明白为什么我一直在 courseName.add(student);

行收到错误消息

这是我的代码:

public class StudentDatabase {

    // add instance variables
    private Map<String, HashSet<Integer>> dataContent = new LinkedHashMap<String, HashSet<Integer>>();

    // Prints a report on the standard output, listing all the courses and all the
    // students in each.  If the map is completely empty (no courses), prints a message
    // saying that instead of printing nothing.
    public void report() {
        if (dataContent.isEmpty()) {
            System.out.println("Student database is empty.");
        } else {
            for (String key : dataContent.keySet()) {
                System.out.println(key + ":" + "\n" + dataContent.get(key));
            }
        }
    }

    // Adds a student to a course.  If the student is already in the course, no change
    // If the course doesn't already exist, adds it to the database.
    public void add(String courseName, Integer student) {
        if (dataContent.containsKey(courseName)) {
            courseName.add(student);
        } else {
            Set<Integer> ids = new HashSet<Integer>();
            ids.add(student);
            dataContent.put(courseName, ids);
        }
    }
}

Answer 1

好的，这个结构:

if (dataContent.containsKey(courseName)) {
    courseName.add(student);
}

完全古怪。你想要的是:

if (dataContent.containsKey(courseName)){
    Set<Integer> studentsInCourse = dataContent.get(courseName);
    studentsInCourse.add(student);
}

应该修复它。