Question

I am searching for a list data structure in Java which allows cheap appending of long lists. I tried LinkedList but I found in the documentation of addAll, that an iterator is used to append the two lists. This means that the list, which gets appended, gets cloned during the operation. The iterator walks though the whole list returning every single element. Is there any collection available which omits the iteration while appending two lists?

Answer 1

You can use Guava’s Iterables.concat method to create concatenated Iterable View..

Iterable<T> combined = Iterables.concat(list1, list2);

• This does not copy elements from one list to another..
• So, it does not change any of your lists..
• Also, this does not create a new list (It creates an Iterable which is not a list)

Basically it creates an Iterable through which you can iterate over the two lists back to back (It iterates elements from list1 then from list2..

NOTE: – If you want a list as a concatenation of the two lists, then this might not help you much.. Because, it does not create a list, but an Iterable.. For that case, you have no other choice than Iterating over your lists and copy each of your reference..

From the docs: –

It Combines two iterables into a single iterable. The returned iterable
has an iterator that traverses the elements in a, followed by the
elements in b. The source iterators are not polled until necessary.
The returned iterable’s iterator supports remove() when the
corresponding input iterator supports it.

You also have a var-args version of this method.. See Docs.. This can take any number of lists, and returns Iterables that can iterate over those lists in order.. So, you can do like this..

Iterable<T> combined = Iterables.concat(list1, list2, list3, list4, ...);

This link –> google-guava-libraries-essentials might also be of interest to you..