collections - How to add element in List while iterating in java?

Question

Say I have a List like:

List<String> list = new ArrayList<>();
list.add("a");
list.add("h");
list.add("f");
list.add("s");

While iterating through this list I want to add an element at the end of the list. But I don't want to iterate through the newly added elements that is I want to iterate up to the initial size of the list.

for (String s : list)
     /* Here I want to add new element if needed while iterating */

Can anybody suggest me how can I do this?

Answer 1

You can't use a foreach statement for that. The foreach is using internally an iterator:

The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException.

(From ArrayList javadoc)

In the foreach statement you don't have access to the iterator's add method and in any case that's still not the type of add that you want because it does not append at the end. You'll need to traverse the list manually:

int listSize = list.size();
for(int i = 0; i < listSize; ++i)
  list.add("whatever");

Note that this is only efficient for Lists that allow random access. You can check for this feature by checking whether the list implements the RandomAccess marker interface. An ArrayList has random access. A linked list does not.