Question

I am not clear on a point in the documentation of List.

It says:

i) Note that these operations may execute in time proportional to the
index value for some implementations (the LinkedList class, for
example).
ii) Thus, iterating over the elements in a list is typically
preferable to indexing through it if the caller does not know the
implementation.

Note that I put the (i) and (ii) in the quote.

Point (i) is pretty obvious due to the way we access a linked list vs the random access of an array.

I can not understand point (ii) though.
What do we gain by prefering an iterator if we don’t know the implementation?
I mean if the implementation is a LinkedList is there any difference in the performance than accessing via the index?
I imagine not, since the Iterator would be manipulating a LinkedList anyway.
So there would be no difference.

So what is the meaning of the recommendation of (ii) in the doc?

Answer 1

The iterator of a linked list can just have a pointer to the next node in the list, and go to the next node each time next() is called. It doesn’t start from the beginning every time. Whereas if you use an index and call get(i), the linked list has to iterate from the beginning until the ith element at each iteration.

What you missed is that the iterator implementation of an ArrayList and the one of a LinkedList are completely different.