使用迭代器和删除时得到ConcurrentModificationException错误 [英] getting ConcurrentModificationException error while using iterator and remove

问题描述

我从以下代码中获取java.util.ConcurrentModificationException，我可以找到原因。我可以成功地从csv文件中读取数据并创建一个名为其课程列表的arraylist。然后我需要将我的内容排序到一个数组列表中，每个单元格包含一个相同课程的arraylist（具有相似名称的课程）。
但是当我运行它时会产生ConcurrentModificationException并且我不明白为什么......

I am getting a java.util.ConcurrentModificationException from following code and I can find the reason. I could successfully read data form a csv file and make an arraylist called course list of it. then I need to sort my in to an array list that each of its cell contains an arraylist of identical courses (courses that have similar name). But when I run it generates ConcurrentModificationException and I do not understand why...

public class CourseLister {
    private static final String DATA = "data\\data.csv";
    File file;
    ArrayList<Course> courseList ; 

    public CourseLister(String filepath) {
        file = new File(filepath);
        courseList = new ArrayList<>();
    }

    public void readFromCsv(){
        // in this method a Csv file is written line by line , create a new object of course with some attribute such as name , number, instructor,... and is added to courseList //}

        }



    public Iterator<Course> getCourseIterator(){
        return courseList.iterator();
    }


    public ArrayList<Course> getCourseList(){
                return courseList;
    }

    public static void main(String [ ] args){

        CourseLister courseLister = new CourseLister(DATA);
        courseLister.readFromCsv();
        CourseFileSorter coursefilesoreter = new CourseFileSorter(courseLister.getCourseIterator());
        ArrayList<Course> curseList = courseLister.getCourseList();
        for (Course course : curseList) {
            System.out.println(course.getSemester());
        }
        System.out.println(curseList.size());
        coursefilesoreter.displayCategorizedList();
    }


}

这是我的CourefileSorterclass：

here is my CourefileSorterclass:

public class CourseFileSorter {

    Iterator<Course> courseItr ;

    public CourseFileSorter(Iterator<Course> courseItr) {
        this.courseItr = courseItr;
    }

    public ArrayList<ArrayList<Course>> getSourtedLists(){

        Iterator<Course> dissimilarCourseItr = null;
        ArrayList<Course> identicalCourseList = new ArrayList<Course>();
        ArrayList<Course> dissimilarCourseList = new ArrayList<Course>();
        ArrayList<ArrayList<Course>> categorizedCourseList = new ArrayList<ArrayList<Course>>();
        Course firstCourse = null;
        Course currentCourse ;
        if(courseItr.hasNext()){
        while(courseItr.hasNext()){
            firstCourse = courseItr.next();
            identicalCourseList.add(firstCourse);
            while(courseItr.hasNext()){
                currentCourse = courseItr.next();
                if(currentCourse.getCourseName().equals(firstCourse.getCourseName())){
                    identicalCourseList.add(currentCourse);
                    courseItr.remove();                 
                }
                else{
                    dissimilarCourseList.add(currentCourse);
                }
            }
            dissimilarCourseItr = dissimilarCourseList.iterator();
            courseItr = dissimilarCourseItr;
            categorizedCourseList.add(identicalCourseList);         
        }
        return categorizedCourseList;
        }
        else{
            return null;
        }
    }




}

推荐答案

将它们分类为不同的数据结构要容易得多。我看到 course 有一个 getCourseName（）方法，我假设它会返回一个字符串对象。请尝试使用 Map< String，List< Course>> 。

It would be much easier to sort them into a different data structure. I see that course has a getCourseName() method, which I assume would return a String object. Try using a Map<String, List<Course>> instead.

排序方法如下所示：

public Map<String, List<Course>> getSourtedLists(){
    Map<String, List<Course>> result = new HashMap<String, List<Course>>();
    while(courseItr.hasNext()) {
        course next = courseItr.next();
        if (!result.containsKey(next.getCourseName())) {
            result.put(next.getCourseName(), new ArrayList<Course>());
        }
        result.get(next.getCourseName()).add(next);
}

另外，你真的不想打电话给 courseItr.remove（）; 这将从基础Collection中删除 course 对象，这意味着您计划执行此操作的方式将清空来自 CourseLister 对象的 courseList 。

Also, you REALLY don't want to call courseItr.remove(); This removes the course object from the underlying Collection, meaning that the way you were planning to do it would empty out the courseList from your CourseLister object.

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