队列的特点是先进先出。通常都把队列比喻成排队买东西，大家都很守秩序，先排队的人就先买东西。但是优先队列有所不同，它不遵循先进先出的规则，而是根据队列中元素的优先权，优先权最大的先被取出。这就很像堆的特征：总是移除优先级最高的根节点。
重点:优先级队列，是要看优先级的，谁的优先级更高，谁就先得到权限。不分排队的顺序!
上篇文章解释了堆的概念实现，现在用堆实现优先队列:
//最大堆import java.util.ArrayList;public class Heap<E extends Comparable>{private ArrayList<E> list=new ArrayList<E>();//用数组实现堆public Heap(){}public Heap(E[] objects){for(int i=0;i<objects.length;i++){add(objects[i]);}}public void add(E newObject){//添加一个元素list.add(newObject);int currentIndex=list.size()-1;while(currentIndex>0){int parentIndex=(currentIndex-1)/2;//找到该结点的父结点if(list.get(currentIndex).compareTo(list.get(parentIndex))>0){//与父节点比较//如果当前结点的值大于父结点就交换位置E temp=list.get(currentIndex);list.set(currentIndex, list.get(parentIndex));list.set(parentIndex, temp); }elsebreak;currentIndex=parentIndex;} }public E remove(){//删除并返回根结点,堆的特点是移除了根结点后还是堆if(list.size()==0) return null;E removeObject=list.get(0);list.set(0, list.get(list.size()-1));//把最后一个结点放在根结点的位置list.remove(list.size()-1);int currentIndex=0;while(currentIndex<list.size()){int leftChildIndex=2*currentIndex+1;int rightChildIndex=2*currentIndex+2;//左右孩子结点的坐标if(leftChildIndex>=list.size())break;//比较左右孩子的值，使maxIndex指向值大的结点int maxIndex=leftChildIndex;if(rightChildIndex<list.size()){if(list.get(maxIndex).compareTo(list.get(rightChildIndex))<0){maxIndex=rightChildIndex;}}//如果当前结点的值小于其左右孩子中的大的值，就交换两个结点if(list.get(currentIndex).compareTo(list.get(maxIndex))<0){E temp=list.get(maxIndex);list.set(maxIndex, list.get(currentIndex));list.set(currentIndex, temp);currentIndex=maxIndex;}elsebreak;}return removeObject; }public int getSize(){return list.size();}}
MyPriorityQueue.java
public class MyPriorityQueue<E extends Comparable> {private Heap<E> heap=new Heap<E>();//用堆实现优先队列//入队列public void enqueue(E e){heap.add(e); //这个add以后，堆会自己调整成一个新堆}//出队列public E dequeue(){return heap.remove();//这移除出之后，堆会自己调整，还是一个新堆}public int getSize(){return heap.getSize();}}
TestMyPriorityQueueMainClass.java
public class TestMyPriorityQueueMainClass {public static void main(String[] args) {// TODO Auto-generated method stubPatient p1=new Patient("John",2);Patient p2=new Patient("Tom",9);Patient p3=new Patient("Jack",4);Patient p4=new Patient("Michael",6);MyPriorityQueue<Patient> priorityQueue=new MyPriorityQueue<>();priorityQueue.enqueue(p1);priorityQueue.enqueue(p2);priorityQueue.enqueue(p3);priorityQueue.enqueue(p4);while(priorityQueue.getSize()>0){System.out.print(priorityQueue.dequeue()+" ");}}static class Patient implements Comparable{private String name;private int priority;public Patient(String name,int priority){this.name=name;this.priority=priority;}public String toString(){return name+"(priority:"+priority+")";}@Overridepublic int compareTo(Object oo) {//比较优先级// TODO Auto-generated method stubreturn this.priority-((Patient)oo).priority;}}}
测试结果：优先级高的先输出，优先级最高的就是堆的根节点
