Create MinHeap Using PriorityQueue from HashMap Entries-CodePudding

I am creating a Min-Heap using PriorityQueue populated from a HashMap. Where Entry objects of the Map are pairs of Integer & Integer.

And I'm implementing it with a help of Comparator. To use Value of HashMap for comparison.

My question is - How to make comparison of HashMap entries on Value of type Integer?

PriorityQueue<Map.Entry<Integer, Integer>> minHeap = new PriorityQueue<>(new Comparator<Map.Entry<Integer, Integer>>() {
    @Override
    public int compare(Entry<Integer, Integer> o1, Entry<Integer, Integer> o2) {
        return o1.getValue() - o2.getValue();
    }
});

CodePudding user response：

Defining a Comparator

Defining a Comparator using an anonymous class isn't a good approach.

Since Java 8 which was released more than 8 years ago we have an alternative, or even a bunch of alternatives like in this case.

A simple and straightforward lambda expression:

Comparator<Map.Entry<Integer, Integer>> byValueAsc =
    (entry1, entry2) -> entry1.getValue() - entry2.getValue();

Java 8 static method comparingInt() from the Comparator interface:

Comparator<Map.Entry<Integer, Integer>> byValueAsc =
    Comparator.comparingInt(Map.Entry::getValue);

And the most suitable option it this case comparingByValue() from the Entry interface :

PriorityQueue<Map.Entry<Integer, Integer>> minHeap = 
    new PriorityQueue<>(Map.Entry.comparingByValue());

Populating the Queue

To populate the queue with entries from a map, you can use a plain for loop:

Map<Integer, Integer> map = new HashMap<>();
for (Map.Entry<Integer, Integer> entry: map.entrySet()) {
    minHeap.add(entry);
}

Or by making use of forEach() from Collection interface invoked on the entry set:

PriorityQueue<Map.Entry<Integer, Integer>> minHeap = new PriorityQueue<>(Map.Entry.comparingByValue());

map.entrySet().forEach(minHeap::add);

CodePudding user response：

You could combine the comparison by first comparing the value and then the key like so:

final var queue = new PriorityQueue<>(
        Map.Entry.<Integer, Integer>comparingByValue()
                .thenComparing(Map.Entry.comparingByKey()));