Reputation: 819
I have the following Java Code:
HashMap<Integer, Lesson> overflow = new HashMap<Integer, Lesson>();
HashMap<Integer, Lesson> new_lessons = this.lessons;
HashMap<Integer, Lesson> lessons = this.lessons;
for (Integer lesson : lessons.keySet()) {
if(lessons.get(lesson).getLength().equals(LessonLength.DOUBLE)){
if(lessons.containsKey(lesson + 1)){
overflow.put(lesson + 1, lessons.get(lesson + 1));
new_lessons.put(lesson+1, lessons.get(lesson));
new_lessons.get(lesson).setLength(LessonLength.ONCE);
new_lessons.get(lesson+1).setLength(LessonLength.ONCE);
}
else{
new_lessons.put(lesson+1, lessons.get(lesson));
new_lessons.get(lesson).setLength(LessonLength.ONCE);
new_lessons.get(lesson+1).setLength(LessonLength.ONCE);
}
}
}
Why is there thrown an ConcurrentModificationException?
Upvotes: 0
Views: 96
Reputation: 18423
Because new_lessons.put(lesson+1, lessons.get(lesson));
modify lessons' keySet()
which is the keySet()
you are iterate over.
HashMap<Integer, Lesson> new_lessons = this.lessons;
HashMap<Integer, Lesson> lessons = this.lessons;
doesn't create new HashMap<> objects, but simple let new_lessons
and lessons
pointing to this.lessons
Upvotes: 1
Reputation: 11006
new_lessons
and lessons
represent the same hashmap. You are altering new_lessons
while iterating over lessons
. Therefore you have a ConcurrentModificationException
.
To fix this, try creating a copy of this.lessons
.
Upvotes: 0
Reputation: 1504172
Your new_lessons
and lessons
variable have the same value - they're referring to the same object. So anything like this:
new_lessons.put(lesson+1, lessons.get(lesson));
... is modifying lessons
, which you're iterating over (via its key-set). Hence the problem.
It's possible that if you didn't change the key set (i.e. you only changed the value associated with any entry) then you could be okay, but that's clearly not the case, because if lessons
doesn't contain a key for lesson + 1
, you're adding it.
It sounds like you should probably copy the original map for new_lessons
, so that you've got two independent maps. Or more simply, just take a copy of the keys to start with:
List<Integer> keys = new ArrayList<Integer>(lessons.keySet());
for (Integer lesson : keys) {
...
}
... and get rid of your new_lessons
variable as it's basically pointless.
Upvotes: 3