of recommends looping through the removal of elements from the List and Set collections

When I was working on the project today, I needed to delete some elements in List and Set. When I used the method of edge traversal and edge deletion, the following exception was reported:

ConcurrentModificationException

In order not to forget it in the future, use a rotten pen to record it as follows:

The way the error code is written, that is, the way the above exception is written:

Set<CheckWork> set = this.getUserDao().getAll(qf).get(0).getActionCheckWorks();
for(CheckWork checkWork : set){
  if(checkWork.getState()==1){
    set.remove(checkWork);
  }
}

Note: using the above method will result in an ConcurrenModificationException exception in the newspaper. The reason is that a collection cannot be deleted by one side traversing one side.

The correct way to write it is as follows:

1. Traverse to delete List

List<CheckWork> list = this.getUserDao().getAll();
Iterator<CheckWork> chk_it = list.iterator();
while(chk_it.hasNext()){
  CheckWork checkWork = chk_it.next();
  if(checkWork.getPlanState()==1){
    chk_it.remove();
  }
}

2. Traverse to delete Set

Set<CheckWork> set = this.getUserDao().getAll().get(0).getActionCheckWorks();
		Iterator<CheckWork> it = set.iterator();
		while(it.hasNext()){
			CheckWork checkWork = it.next();
			if(checkWork.getState()==1){
				it.remove();
			}
		}