LinkedHashMap vs HashMap != LinkedList vs ArrayList

Question

I have read that LinkedHashMap has faster iteration speed than HashMap because its elements are doubly linked to each other. Additionally, because of this, LinkedHashMap is slower when inserting or deleting elements. Presumably because these links also need to be updated.

Although I can see an analogy to LinkedList vs ArrayList, in that the elements of LinkedList are also doubly-linked, I read that it iterates slower than ArrayList, and has faster insertion and deletion times.

Why is this? Perhaps I am making a mistake somewhere?

Cheers!

Answer 1

The analogy does not work. LinkedList and ArrayList are two unrelated implementations of a List. LinkedHashMap, however, is the same data structure as a HashMap but with a LinkedList woven into it to make iteration faster and consistent.

The reason that LinkedHashMap iteration is faster than HashMap iteration is that HashMap iteration has to iterate over all of the buckets, even the empty ones. The the fact that LinkedHashMap has a list pointing to the data means that it can skip the empty buckets. The list in the LinkedHashMap is a linked list because removal times remain constant (rather than O(n) if it was some arrray-backed list).