CODEWITHSUNDEEP
javalistarraylist
Yonatan Karp-Rudin
Yonatan Karp-Rudin

Reputation: 1086

How to copy list content to anther list most efficient

I have 2 big lists that contain heavy objects and I need to join them into a single list. My first instinct was to create a new list and append the content of those 2 into it as follow:

List<Item> items = new ArrayList<>();
items.addAll(list1);
items.addAll(list2);

However, as this is an ArrayList in order to copy those 2 big lists I need to allocate new memory instead of using the existing once and copy the references.

My thought now was to use LinkedList and copy the end node of list1 into the head of list2 however as much as I can say from the documentation there is no such method.

Is there a better way to concatenate 2 lists without re-allocation of new memory? and if not, what is the best practice in such a case?

Upvotes: 0

Views: 99

Answers (3)

Stephen C
Stephen C

Reputation: 718758

This is probably the most memory / GC efficient way to do it:

List<Item> items = new ArrayList<>(list1.size() + list2.size());
items.addAll(list1);
items.addAll(list2);

The trick is to provide an accurate capacity when creating the list; see the javadoc. This saves any double copying to "grow" the target list (i.e. list).

My thought now was to use LinkedList and copy the end node of list1 into the head of list2 however as much as I can say from the documentation there is no such method.

LinkedList use more memory per list entry than ArrayList. By a factor of 4 or more.

Besides, when you use addAll from one LinkedList to another, you are creating new list node objects.

I have 2 big lists that contain heavy objects ....

The weight (size) of the objects is not relevant. A List contains references to the objects, not copies of the objects themselves.

Is there a better way to concatenate 2 lists without re-allocation of new memory?

There is no way to concatenate two java.util.List instances without allocating more memory. The best you can do is minimize the allocation.

On the other hand ... if you were prepared to implement your own linked list data structure, you could join two lists together by "splicing" them. But you the List API doesn't allow that kind of thing. (For a start, it mutilates the original lists.)

It is true that premature optimization is a bad thing. However, if you already have strong evidence that the lists involved will be large, then this is arguably not premature.

Upvotes: 3

Adithya Upadhya
Adithya Upadhya

Reputation: 2375

The lists hold references to objects, not actual objects themselves. Unless you want to create a new list which has to have a deep copy of all the objects.

  1. If you don't mind modifying one of the lists, add all contents to one of the lists and reclaim memory from other list.
list1.addAll(list2);
list2.clear();

// You could also reclaim memory using garbage collector.

list1.addAll(list2);
list2 = null;
System.gc();     // It's not guaranteed that Garbage collector will be invoked.
  1. If you want to create a new list, assign capacity while doing so and reclaim memory from other lists using the steps mentioned in step (1).

However, it mainly depends on you use case. Remember:-

Microoptimization is the root of all evil.

Upvotes: 0

Tarun
Tarun

Reputation: 3165

My thought now was to use LinkedList and copy the end node of list1 into the head of list2 however as much as I can say from the documentation there is no such method.

You can do the same using Array list as follows:

list1.addAll(list2);

And after adding list2, You can ask GC to reclaim the memory used by list2 as follows:

list2 = null;

Upvotes: 0

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