Hasmap rehash code example

gistfile1.txt

// example with a hashmap to demonstrate how we could end up missing hashmap entries
// under concurrent modification.

package test;

import java.util.HashMap;

public class MapTestTask implements Runnable {

    private static HashMap<Object, Object> hashMap;

    private Object      value = new Object();

    public MapTestTask(HashMap<Object, Object> map) {
        this.hashMap = map;
    }

    public void run() {
                hashMap.put(Thread.currentThread().getId(), value);
                Object retrieved = hashMap.get(Thread.currentThread().getId());
                if (retrieved == null) {
                    // Can it ever Happen
                    System.out.println("just happened with key '"  + Thread.currentThread().getId() + "'. " + hashMap);
                    System.exit(1);
                }
                else
                {
                	 System.out.println("Looks good!");
                }
            }
}

package test;

import java.util.HashMap;

public class TestMap {

    public static void main(String[] args) throws InterruptedException {

        HashMap<Object, Object> map = new HashMap<Object, Object>();

        int NUM_THREADS = 1000;
        Thread[] threads = new Thread[NUM_THREADS];
        for (int i = 0; i < NUM_THREADS; i++) {
            threads[i] = new Thread(new MapTestTask(map));
        }

        for (int i = 0; i < NUM_THREADS; i++) {
            threads[i].start();
        }

        for (int i = 0; i < NUM_THREADS; i++) {
            threads[i].join();
        }
    }

// Fixes with a ConcurrentHashMap

package test;

import java.util.concurrent.ConcurrentHashMap;

public class MapTestTask implements Runnable {

    private static ConcurrentHashMap<Object, Object> hashMap;

    private Object      value = new Object();

    public MapTestTask(ConcurrentHashMap<Object, Object> map) {
        this.hashMap = map;
    }

    public void run() {
                hashMap.put(Thread.currentThread().getId(), value);
                Object retrieved = hashMap.get(Thread.currentThread().getId());
                if (retrieved == null) {
                    // Can it ever Happen
                    System.out.println("just happened with key '"  + Thread.currentThread().getId() + "'. " + hashMap);
                    System.exit(1);
                }
                else
                {
                     System.out.println("Looks good!");
                }
            }
}

package test;

import java.util.concurrent.ConcurrentHashMap;

public class TestMap {

    public static void main(String[] args) throws InterruptedException {

        ConcurrentHashMap<Object, Object> map = new ConcurrentHashMap<Object, Object>();

        int NUM_THREADS = 1000;
        Thread[] threads = new Thread[NUM_THREADS];
        for (int i = 0; i < NUM_THREADS; i++) {
            threads[i] = new Thread(new MapTestTask(map));
        }

        for (int i = 0; i < NUM_THREADS; i++) {
            threads[i].start();
        }

        for (int i = 0; i < NUM_THREADS; i++) {
            threads[i].join();
        }
    }

}

}