Stats of HashSet

Question

I wonder if there is a best way to get the aggregated statistics of objects in a HashSet . Is there any good approach rather than iterating the whole Set ?

import java.lang.Thread.State;
import java.util.HashSet;
import java.util.Set;

public class Stats {

    static class Item {

        private State state;

        public State getState() {
            return state;
        }

        public void setState(State state) {
            this.state = state;
        }

        public Item(State state){
            this.state = state;
        }
    }

    public static void main(String[] args) {

        Set<Item> items = new HashSet<Stats.Item>();
        items.add(new Item(State.NEW));
        items.add(new Item(State.BLOCKED));
        items.add(new Item(State.BLOCKED));

        /*
         * printout here
         */
            // New=1
            // Blocked=2
    }
}

Answer 1

No,but you can write your own implementation of Set interface and in add method check item state and increments counters. Then you can retrive stats.

Answer 2

No their isn't. However with a self-made implementation or a wrapper you might be able to calculate statistics while inserting into the hash set.

Answer 3

If memory serves, doing something like this with a Set implementation is presented as an example in Bloch's Effective Java. Chosen precisely because if you want stats on the collection, you have to roll your own.

A search here reveals the reference I am thinking of, where he discusses composition over inheritance: How is having a wrapper class equals composition as described Joshua Bloch?

I admit this is not strictly an answer to the question, but I think it adds to the conversation and shows an example of how (and why) to make your own instrumented classes for your own purposes.