优先级队列的问题


package PriorityQueueTest;import java.util.*;public class PriorityQueueTest { public static void main(String[] args) {

PriorityQueue<GregorianCalendar> pq = new PriorityQueue<GregorianCalendar>();

pq.add(new GregorianCalendar(1906,Calendar.DECEMBER,9));//G. Hopper
pq.add(new GregorianCalendar(1815,Calendar.DECEMBER,10));//A. Lovelace
pq.add(new GregorianCalendar(1903,Calendar.DECEMBER,3));//J. von Neumann
pq.add(new GregorianCalendar(1910,Calendar.DECEMBER,22));//K. Zuse

System.out.println("Iterating over elements...");

for (GregorianCalendar date :pq)
System.out.println(date.get(Calendar.YEAR));

System.out.println("Removing elements...");

while (!pq.isEmpty())
System.out.println(pq.remove().get(Calendar.YEAR));
}}结果是：
Iterating over elements: 1815,1906,1903,1910
Removing elements: 1815,1903,1906,1910
刚接触优先级队列的概念，不知道它是怎么排序的，新人求教优先级队列