java - Why does Arrays.asList() return its own ArrayList implementation

Question

I recently found out that there are actually 2 different ArrayList implementations in Java (better late than never I guess...).

So I was wondering why does Arrays.asList(T... a) need to return a list which can not be resized ? If they needed an unmodifiable list why add the set(int index, E element) method then ?

So my general question is why not return the java.util.ArrayList from the Arrays.asList(T... a) method ?

Also what do you gain with the java.util.Arrays.ArrayList implementation ?

Answer 1

You asked:

Also what do you gain with the java.util.Arrays.ArrayList implementation ?

It is because the Arrays$ArrayList returned by Arrays.asList is just a view on the original array. So when the original array is changed then the view is changed too.

If one would use an real ArrayList then the elements will be copied, and a change on the orignal array would not infuence the ArrayList.

The reasons to do this are quite simple:

• performance: no need to copy anyting
• memory efficent: no second array is needed