Java: PriorityQueue initializations

Question

I am trying to understand the following line which initiates a Priority Queue:

PriorityQueue<int[]> pq = new PriorityQueue<>((a, b) -> b[1] - a[1]);

Comparing with Constructor section in the document, https://docs.oracle.com/javase/7/docs/api/java/util/PriorityQueue.html

I couldn't figure out which Constructor it uses. Could someone please share the thought?

Also, is there a document that could better explain/define syntax (a, b) -> b[1] - a[1] ... though I could guess what it means.

Thanks a lot!

It looks like a Comparator is being supplied to the queue so that items are added in the correct order. The Java 8 PriorityQueue implementation allows that form of the constructor. — Bob Dalgleish
– Bob Dalgleish, Commented Jan 31, 2018 at 23:47
You are looking in the wrong version of javadoc. It's obviously a creature from Java 8. — Andrey Tyukin
– Andrey Tyukin, Commented Jan 31, 2018 at 23:48
Whenever you see a lambda ->, it requires at least Java 1.8 to compile! — Jacob G.
– Jacob G., Commented Jan 31, 2018 at 23:49

Community · Accepted Answer · 2020-06-20 09:12:55Z

6

Your construction of the PriorityQueue uses a constructor that didn't yet exist in 1.7, which is the version of the Javadocs you linked.

It uses a constructor that takes a Comparator that was added for Java 1.8, which is matched to the lambda expression you supplied.

Creates a PriorityQueue with the default initial capacity and whose elements are ordered according to the specified comparator.

Since:

1.8

Lambda expressions were introduced with Java 1.8. Here, basically you have 2 arguments and expression that are matched to a functional interface --Comparator.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jan 31, 2018 at 23:48

rgettman

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Luiggi Mendoza · Accepted Answer · 2018-01-31 23:48:21Z

3

Since Java 8, there's a new constructor that has a Comparator for argument:

public PriorityQueue(Comparator<? super E> comparator)

Thus the initialization using a lambda is valid Java 8+ code.

answered Jan 31, 2018 at 23:48

Luiggi Mendoza

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Vinay Kumar · Accepted Answer · 2019-10-06 03:23:33Z

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In Priority Queue you will essentially put user define objects, so to do it priority queue asks you how to order these objects (because priority queues are like heap data structures -- a min/max heap ) so we give it a Comparator which has a compare method ideally denoted by ((a, b) -> b[1] - a[1]) , this method give +ve, -ve or a zero result based on b > a , b < a or b = a. By this result it decides whether it should arrange the elements in ascending or descending order.

https://github.com/openjdk-mirror/jdk7u-jdk/blob/master/src/share/classes/java/util/PriorityQueue.java

answered Oct 6, 2019 at 3:23

Vinay Kumar

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