I am porting a C++ library to Java and I need a heap data structure. Is there a standard implementation or will I need to do it myself?

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7 Answers

For Java 8, updating on an existing answer:

You can use Java Priority Queue as a Heap.

Min Heap: --> to keep the min element always on top, so you can access it in O(1).

PriorityQueue<Integer> minHeap = new PriorityQueue<Integer>();

Max Heap: --> to keep the max element always on top, the same order as above.

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

Which is the same as (Integer o1, Integer o2) -> Integer.compare(o2, o1) or - Integer.compare(o1, o2) as suggested from other answers.

And you can use:
add --> to add element to the queue. O(log n)
remove --> to get and remove the min/max. O(log n)
peek --> to get, but not remove the min/max. O(1)

Min heap:

PriorityQueue<Integer> minHeap = new PriorityQueue<Integer>();

Max heap:

PriorityQueue<Integer> maxHeap = new PriorityQueue<Integer>(new Comparator<Integer>() { @Override public int compare(Integer o1, Integer o2) { return - Integer.compare(o1, o2); } });
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In Java PriorityQueue can be used as a Heap.

Min Heap

PriorityQueue<Integer> minHeap = new PriorityQueue<>();

Max Heap

PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Comparator.reverseOrder());
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PriorityQueue uses a heap. Based on the oracle documentation at it seems likely that it is an implementation of a binary heap. I don't think there is an official implementation of a fibonacci or pairing heap, though I'd love to see either one of the two available.

No as such there isn't but you can use Priority Queue as a Heap. Its officially told by Oracle to use Priority Queue as a Heap you can also refer to this link for further clarification.

PriorityQueue<Integer> MinHeap = new PriorityQueue<>(); PriorityQueue<Integer> MaxHeap = new PriorityQueue<>(Comparator.reverseOrder());
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You can also consider TreeSet, which guarantees log(n) time for basic operations (add, remove, contains).

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From Java docs PriorityQueue which is available since 1.5 is the class to use.

This code for Min Heap creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering in which the min is at the top.

//MIN HEAP PriorityQueue<Integer> minHeap = new PriorityQueue<>(); //equivalent to PriorityQueue<Integer> minHeap = new PriorityQueue<>(11);

If you want to implement a special ordering you need to override the comparator with this constructor

PriorityQueue​(int initialCapacity, Comparator<? super E> comparator);

Since 1.8 we also have this version

PriorityQueue​(Comparator<? super E> comparator);

which helps you create the Max Heap in more elegant ways such as

//MAX HEAP PriorityQueue<Integer> maxHeap = new PriorityQueue<>((n1,n2) -> Integer.compare(n2,n1)); //equivalent to PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Comparator.reverseOrder());

For a special case check this example that shows the natural ordering for a custom object, in a scenario where we order customers based on their distance to a fictional restaurant

import java.util.List; import java.util.PriorityQueue; public class DeliveryHandler { private static final Address restaurant = new Address(5.0, 5.0); private static class Address implements Comparable<Address> { public double x, y; public Address(double x, double y) { this.x = x; this.y = y; } public double distanceToShop() { return Math.pow(restaurant.x - x, 2) + Math.pow(restaurant.y - y, 2); } @Override public int compareTo(Address other) { return Double.compare(this.distanceToShop(), other.distanceToShop()); } @Override public String toString() { return "Address {x=%s, y=%s}".formatted(x, y); } } public static void main(String[] args) { List<Address> customers = List.of( new Address(13, 14), new Address(3, 1), new Address(9, 20), new Address(12, 4), new Address(4, 4)); PriorityQueue<Address> queueServingClosest = new PriorityQueue<>(); queueServingClosest.addAll(customers); while (!queueServingClosest.isEmpty()) { System.out.println(queueServingClosest.remove()); } /* Prints Address {x=4.0, y=4.0} Address {x=3.0, y=1.0} Address {x=12.0, y=4.0} Address {x=13.0, y=14.0} Address {x=9.0, y=20.0} */ PriorityQueue<Address> queueServingFurthest = new PriorityQueue<>( (a1, a2) -> Double.compare(a2.distanceToShop(), a1.distanceToShop()) ); queueServingFurthest.addAll(customers); while (!queueServingFurthest.isEmpty()) { System.out.println(queueServingFurthest.remove()); } /* Prints Address {x=9.0, y=20.0} Address {x=13.0, y=14.0} Address {x=12.0, y=4.0} Address {x=3.0, y=1.0} Address {x=4.0, y=4.0} */ } }

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