问题
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!
回答1:
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
PriorityQueuewith 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.
回答2:
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.
回答3:
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
来源：https://stackoverflow.com/questions/48553286/java-priorityqueue-initializations