I have a for each loop with a Set type.
While I loop through this Set I add elements to it.
for (Object o: Set) {
//i do something and add to the set
}
I keep getting the ConcurrentModificationException. But I don't know how I would go about adding to this Set that wouldn't cause this exception. I can't create a new for each loop that wouldn't work.
Can anyone point me in the right direction?
You need to either
The simplest change is
for (Object o: set.toArray()) {
if (condition(o))
set.add(something);
}
With generics a simpler solution might be
Set<T> set =
for(T t: new HashSet<T>(set)) {
// something which might add to set
}
Note: this will prevent it iterating over elements you just added. Note2: using the concurrent sets may result in you seeing added elements or possibly not.
If you really need to see elements as you add them you need to use a list, possibly instead, or as well.
List<T> list = new ArrayList<>(set);
for(int i = 0; i < list.size(); i++) {
T t = list.get(i);
if (condition)
// if it is not a duplicate
if (set.add(something))
list.add(something);
}
This will allow you to see elements you have added in the loop in a reliable manner. Note: some care needs to be take to not create an infinite loop and run out of memory.
An alternative is to use a queue instead of a list. This might be nicer, but I suspect less efficient.
Queue<T> q = new ConcurrentLinkedQueue<>(set);
for(T t : q) {
if (condition)
// if it is not a duplicate
if (set.add(something))
q.add(something);
}
You could make a temporary Set and add your elements to this temp Set, and when you finished lopping over all elements, add the values from the temp Set to the original Set
Example:
Set<Integer> originalSet;
//stuff where you fill your original Set
Set<Integer> tempSet = new Set<Integer>();
for(Integer i : originalSet) {
tempSet.add(<integerToAdd>);
}
originalSet.addAll(tempSet);
