Question

I have to implement Priority Queue using MultiMap. I use MultiMap from Google Collections.
The following code creates a MultiMap and adds few elements into it.

    Multimap<Integer, String> multimap = HashMultimap.create();

    multimap.put(5,"example");
    multimap.put(1,"is");
    multimap.put(1,"this");
    multimap.put(4,"some");

Now my problem is how to write the pop method?

I think that there should be a for loop and it should be iterating through MultiMap.

The lowest key should be the highest priority, so in C++ I would set a pointer to the first element and increment it. How to do it in Java?

Answer 1

The HashMultimap you’re using won’t give you any help in efficiently selecting the lowest element. Instead use a TreeMultimap (also in Google Collections) which lets you specify an ordering and iterate through the items in the list in that order. For instance:

for (Map.Entry<Integer, String> entry : multimap.entries()) {
  System.out.println("Item " + entry.getValue() + " has priority " + entry.getKey();
}

You’ll notice that this always prints out entries in priority order, so to get the first-priority element you can just do multimap.entries().iterator().next() (assuming you know the map has at least one element).

See the TreeMultimap documentation for more information.