Why doesn't Java ship with a CopyOnWriteMap?

Question

The JDK ships with CopyOnWrite* implementations for Set and List, but none for Map and I've often lamented this fact. I know there are other collections implementations out there that have them, but it would be nice if one shipped as standard. It seems like an obvious omission and I'm wondering if there was a good reason for it. Anyone any idea why this was left out?

Answer 1

I guess this depends on your use case, but why would you need a CopyOnWriteMap when you already have a ConcurrentHashMap?

For a plain lookup table with many readers and only one or few updates it is a good fit.

Compared to a copy on write collection:

Read concurrency:

Equal to a copy on write collection. Several readers can retrieve elements from the map concurrently in a lock-free fashion.

Write concurrency:

Better concurrency than the copy on write collections that basically serialize updates (one update at a time). Using a concurrent hash map you have a good chance of doing several updates concurrently. If your hash keys are evenly distributed.

If you do want to have the effect of a copy on write map, you can always initialize a ConcurrentHashMap with a concurrency level of 1.

Answer 2

The easiest implementation of a set would usually be to use an underlying map. They even have a Collections.newSetFromMap() method [maybe from 1.6 only].

What they should have done was have a CopyOnWriteMap and the CopyOnWriteSet being equivalent to Collections.newSetFromMap(new CopyOnWriteMap()).

But as you can see the CopyOnWriteArraySet is actually backed by an array not a map. And wouldn't Collections.newSetFromMap(ConcurrentHashMap()) be acceptable for your usecase?