Is it Possible to create a Queue for HashMap set?

Question

Right now I am trying to create a producer/consumer thread, the producer thread goes through all possible combinations of letters and creates their respective MD5 hashes. Then each combination and its hash is put into the HashMap<String,String>. Now in my consumer thread I want to be able to use the Queue<> collection on the hashmap so my consumer thread may call poll() etc thus removing values atc like a Queue but still giving me the capability of seeing both the combination and its hash when calling poll() How would I go about doing this? I have the HashMap but dont know how to 'make' or cast it as a Queue. Thanks.

Answer 1

The LinkedHashMap type is like a combination of a HashMap and a Queue - it stores key/value pairs, but also remembers the order in which they were inserted. This might be exactly the type you're looking for. There is no explicit poll() function, but if you get an iterator over the LinkedHashMap you will visit the elements in the order in which they were added. You could probably then write a function like this:

public <KeyType, ValueType> KeyType first(LinkedHashMap<KeyType, ValueType> map) {
    assert !map.isEmpty();
    return map.iterator().next();
}

which will give you back the first element. Just make sure to synchronize appropriately.

Alternatively, you could consider just storing key/value pairs inside a Queue by defining a helper class Pair and then storing Pairs in the queue.

Hope this helps!

Answer 2

You should not use a HashMap without handling the thread-safety of your code. Else, you may end with a Live-lock.

To be able to iterate your Map with the order in which keys were inserted, you can use a LinkedHashMap.

Map m = Collections.synchronizedMap(new LinkedHashMap(...));

The producer would push entries like this (nothing special):

m.put(key, object)

The consumer would poll entries like this:

while (someCondition) {
    Map.Entry nextEntry = null;

    // This block is equivalent to polling
    {
         synchronized(s) {
             Iterator i = s.iterator(); // Must be in the synchronized block
             if (i.hasNext()) {
                 nextEntry  = i.next();
                 i.remove();
             }
         }
    }

    if (nextEntry != null) {
         // Process the entry
         ...
    } else {
         // Sleep for some time
         ...
    }
    // process
}

Answer 3

I suggest you create a Queue of EntrySet -

Queue<EntrySet<String,String>> queue = new SynchronousQueue<EntrySet<String,String>>();
for (EntrySet<String,String> entry:map.entrySet()) {
   queue.add(entry);
}

You can consider using another type of queue, which lets you put the elements, and only the prdocuer waits in case of non empty such as LinkedBlockingQueue.
The producer will then be able to recompose a map based on the EntrySet objects, if needed.