Add elements to a List while iterating over it. (Java) [duplicate]

Answer 1

How about

List<Foo> fooList = getFooList();
List<Foo> tempFooList = new ArrayList<Foo>()


for(Foo f : fooList)
{
   ...
   // add items that need to be added to temp
   tempFooList.add(new Foo());
   ...
}

fooList.addAll(tempFooList);

Answer 2

There might be some trick with ListIterator, but the easiest solution is probably an old style index loop. Verify performance isn't an issue (no linked lists - but ArrayList is fine).

List<Object> myList;

for(int i = 0; i < myList.size(); i++)
{
  Object current = myList.get(i); 
  // Anything you insert after i will be discovered during next iterations
}

Answer 3

You can't modify a Collection while iterating over it using an Iterator, except for Iterator.remove().

However, if you use the listIterator() method, which returns a ListIterator, and iterate over that you have more options to modify. From the javadoc for add():

The new element is inserted before the implicit cursor: ... a subsequent call to previous() would return the new element

Given that, this code should work to set the new element as the next in the iteration:

ListIterator<T> i;
i.add(e);
i.previous(); // returns e
i.previous(); // returns element before e, and e will be next

This will work except when the list starts iteration empty, in which case there will be no previous element. If that's a problem, you'll have to maintain a flag of some sort to indicate this edge case.