How expensive is calling size() on List or Map in Java?

Question

How expensive is calling size() on List or Map in Java? or it is better to save size()'s value in a variable if accessed frequently?

Answer 1

Implement it, then test it. If it is slow, take a closer look.

"Premature optimisation is the root of all evil." - D. Knuth

Also: You should not require certain implementation features, especially if they are black-boxed. What happens if you replace that list with a concurrent list at a later date? What happens if Oracle decides to rewrite List? Will it still be fast? You just don't know.

Answer 2

for ArrayList the implementation is like

   public int size() {
       return lastIndex - firstIndex;
   }

So not over head

You can check the source code for detailed info for your required Impl.

Note: The source given is from openjdk