Initialize a Priority Queue with custom comparator, which is inside a HashMap

Question

Assume that I have a HashMap, where the value is of type PriorityQueue like:

HashMap<Integer, PriorityQueue<Integer>> someMap = new HashMap<>();

But how do I initialize this HashMap, if I need the PriorityQueue to have a custom comparator?

The actual comparator is much more complex, But for simplicity let's assume that I need the PriorityQueue to sort by reverse order, which I can do by:

PriorityQueue<Integer> queue = new PriorityQueue<>(Comparator.reverseOrder());

Where and How should I define the comparator for the value in HashMap?

score 1 · Answer 1 · answered Dec 16 '22 at 11:49

The Answer in the question itself. You put the comparator inside the Priority Queue instance so.

HashMap<Integer, PriorityQueue<Integer>> someMap = new HashMap<>();
PriorityQueue<Integer> queue = new PriorityQueue<>(Comparator.reverseOrder());
someMap.put(1,queue);

that for initialization and when you put data in your queue you can use the same queue to put data.

Alexander Ivanchenko · Accepted Answer · 2022-12-16T16:34:11.917

The Comparator which PriorityQueue uses internally doesn't change its type anyhow.

A PriorityQueue<Integer> which uses the natural ordering, or Comparator.reverseOrder(), or for instance Comparator.nullsFirst(Comparator.reverseOrder()) (or any other Comparator) is still a PriorityQueue<Integer>. And you can store all these Queues in a map of type Map<Integer,PriorityQueue<Integer>>

You don't need to do anything special while instantiating the Map, just simply invoke the constructor new HashMap<>() as you did.

Map<Integer, Queue<Integer>> someMap = new HashMap<>();
        
Queue<Integer> pq1 = new PriorityQueue<>();
Queue<Integer> pq2 = new PriorityQueue<>(Comparator.reverseOrder());
Queue<Integer> pq3 = new PriorityQueue<>(Comparator.nullsFirst(Comparator.reverseOrder());
        
someMap.put(1, pq1);
someMap.put(2, pq1);
someMap.put(3, pq1);

Note

That generic type parameters like <Integer,Queue<Integer>> are meant to allow the compiler to verify that you're using the appropriate types. It is a compile-type mechanism for preventing problems like Heap-pollution, and it has nothing to do with how the memory would be allocated at Runtime.
You might be interested in reading What does it mean to "program to an interface"? to learn what are the benefits of using high-level abstractions like Map and Queue.

Initialize a Priority Queue with custom comparator, which is inside a HashMap

2 Answers2